Introduction

Fleming (1825) created the genus Skenea for Helix serpuloides Montagu, 1808 and Clark, (1851a: 472 Skeneadae as nomen nudum, 1851b: 44ff with diagnosis) designated Skenea serpuloides as the type species for the vetigastropod family Skeneidae, a concept stood for more than 120 years. Keen and McLean 1971) summed up the diagnosis of the family as follows: ‘No other family combines the features of rhipidoglossate radula, lack of nacre, and multispiral operculum’. Some authors have speculated that there may be hundreds of described and many more undescribed species of Skeneidae (e.g. Marshall 1988; Kano et al. 2009). However, with increasing data, it is clear that many small snails share these characters, especially inhabitants of deep water or of chemosynthetic habitats like sunken wood or hydrothermal vents and these have proved to belong to different subgroups (e.g. Marshall 1988; Warén and Bouchet 1993, 2001; Hasegawa 1997). It is now obvious that the accepted diagnosis is not sufficient to reflect a monohyletic clade. Indeed, most recent authors assumed the family to be polyphyletic (Marshall 1988; Hickman and McLean 1990; Warén 1992; Warén and Bouchet 1993; Hickman 1998, 2013; Kano et al. 2009). Accordingly, several authors have named these gastropods informally ‘skeneimorph’ or ‘skeneiform’ (Warén 1992; Kano 2008; Hickman 1998, 2013). Herein, we follow this tradition, calling gastropods having the features described above skeneimorph, and restrict Skeneidae for those species forming a clade including the type species, Skenea serpuloides (Montagu, 1808) (Table 1). The respective taxon has been assigned both, family status as Skeneidae (e.g. Clark 1851a, b; Wenz 1938; Fretter and Graham 1977; Keen and McLean 1971; Marshall 1988; Hickman and McLean 1990; Warén 1991, 1992, 1993; Warén and Bouchet 1993, 2001; Williams and Ozawa 2006; Williams 2012) ocae (e.g. Thiele 1929; Bouchet et al. 2005; Williams et al. 2008; Nye et al. 2013).

Table 1 Size and volume of the inner organs of Skenea serpuloides (specimens SMSN 98643/98644)
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Several molecular studies revealed Skeneidae as part of Turbinidae (Williams and Ozawa 2006; Williams et al. 2008; Kano 2008; Aktipis and Giribet 2012), but the most recent ones (Williams 2012; Nye et al. 2013) again resolved Skeneidae as a separate clade, a point of view we followed herein. Skeneidae and Turbinidae are incorporated together with the families Calliostomatidae, Liotiidae, Margaritidae, Solariellidae and Trochidae as Trochoidea (Williams et al. 2008; Williams 2012). In contrast, the ‘trochoid’ Phasianelloidea and Angarioidea as well as Seguenzioidea (see Kano 2008; Kano et al. 2009) are currently accepted as separate vetigastropod clades.

Concerning the content of Skeneidae sensu stricto, Warén (1991, 1992) first assumed the presence of a propodial penis as diagnostic and included, aside from Skenea Clark, 1851, also the genera Lissospira Bush, 1897; Pseudorbis Monterosato, 1884; Lodderena Iredale, 1924 (but see below); Dikoleps Høisæter, 1968 and Skeneoides Warén, 1992. Later on also Dillwynella Dall, 1889 and Protolira Warén & Bouchet, 1993 from the hot-vent habitat were integrated (Marshall 1988; Warén and Bouchet 1993). About 130 genus names have been assigned to Skeneidae (Hickman 2013). The current (access 5 December 2015) WoRMS-webpage still lists more than 40 genera among Skeneidae, the Worldwide Mollusc Species Data Base only 32, but in both cases, many of these ‘skeneimorphs’ are still doubtful members of Skeneidae. Recently Hickman (2013) assigned Conjectura Finlay, 1926; Conradia A. Adams, 1860; Crossea A. Adams, 1865 and Crosseola Iredale, 1924 to the newly erected family Crosseolidae Hickman 2013, but according to A. Warén (pers. comm.) Conjectura belongs to Tornidae and has a taenioglossate radula.

Recent studies using molecular and micromorphological methods excluded a good number of genera originally assigned to Skeneidae: Among the vetigastropods Akritogyra Warén, 1992; Anekes Bouchet & Warén, 1979; Granigyra Dall, 1889; Ventsia Warén & Bouchet, 1993; Xyloskenea Marshall, 1988 and Adeuomphalus Seguenza, 1876 that were transferred to Seguenzioidea (Kano 2008; Kunze et al. 2008, 2016; Kano et al. 2009). Bathyxylophila Marshall, 1988 is now classified among Scissurelloidea (Kano 2008; Kunze et al. 2008). Some genera have been assigned to Neomphalina, these being Leptogyra Bush, 1897 and Leptogyropsis Marshall, 1988 (Heß et al. 2008; Kunze et al. 2008). Hyalogyra Marshall, 1988; Hyalogyrina Marshall, 1988 and Xenoskenea Warén & Gofas, 1993 are now included in the heterobranch Hyalogyrinidae (Warén and Bouchet 1993; Warén et al. 1993; Kunze et al. 2008; Haszprunar et al. 2011). Whereas a number of recent reports and descriptions on skeneimorphs (e.g. Aartsen and van Bogi 1988; Ponder 1990; Rubio-Salazar 1991; Rubio and Rodriguez-Babio 1991; Rubio and Rolán 1991, 2013a, b, 2015; Warén 1991, 1992, 1993, 1996; Engl 1996, 2001; Moolenbeek 1996; La Perna 1998; Rubio et al. 1998a, 2004, 2015; Rolán and Ryall 2000; Carrozza and van Aartsen 2001; de Barros et al. 2002; Redfern and Rolán 2005; Hoffman et al. 2008, 2010; Kunze 2011; de Lima et al. 2011; Romani et al. 2015) provided extensive SEM-data on shell, protoconch and radula, there is a severe lack of anatomical and molecular data (Tables 2 and 3). Indeed, head–foot data are scarce and skeneid anatomy is known from only two species (Table 3). Because the only known diagnostic character of Skeneidae is the presence of a propodial penis (Warén 1992; Kunze et al. 2008; Hickman 2013), most species are only tentatively assigned to Skeneinae.

Table 2 Molecular data (GenBank accession numbers) on Skeneidae and possibly related forms (marked by *)
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Table 3 Previous data on the soft bodies of Skeneidae
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Based on serial semi-thin sectioning, 3D reconstructions and their interactive embedding in PDFs, we provide the first detailed anatomical description with histological data for a skeneid, although some preliminary data have been published by Brückner et al. (2004), and Kunze et al. (2008). Together with the SEM-data of the head-foot recently provided by Rubio and Rolán (2013a), the anatomical data of the type species of Skeneidae allow us to define the family morphologically. To show variability of characters we also provide relevant data on the internal anatomy of three other skeneids.

Material and methods

  1. 1.

    S. serpuloides (Montagu, 1808): Collected on maërl off Roscoff, Bretagne, France, by E.C. Rodriguez-Babio (see Rodriguez-Babio and Thiriot-Quievreux 1975: 172) and determined by Anders Warén (Naturhistoriska Riksmuseet, Stockholm: SMNH). Section series SMNH: 98643, 98644, 98645, and 98646 were used for histological examination. 3D reconstructions were compiled based on of the section series: SMNH-98643 and 98644 body and inner organs; SMNH-98643 and 98645 body and tentacles. Measurements of the organ systems (size and volume) were taken for both specimens (Table 1). Both have almost the same size and arrangement of organs, but the retraction grade is different. Respiratory and circulatory systems were better preserved in SMNH-98643, while in SMNH-98644 the nervous system and tentacles are in a better preserved condition. Because the epipodial tentacles were poorly preserved in the other series, this part was described and reconstructed based on SMNH-98645. In addition, five sections series are based on specimens collected in 2005 by GH on the surface of ‘amphioxus-sand’ dredged at 48° 43′ 532 N, 3° 50′ 712 W, 20–25 m (Zoologische Staatssammlung Munich, molluscan section, SNSB-ZSM-Moll 20140452 to 20140456). The shell (1.5 mm) and protoconch of S. serpuloides are described and figured via SEM by several authors (Rodriguez-Babio and Thiriot-Quievreux 1975; Fretter and Graham 1977; van Aartsen et al. 1984; Rubio-Salazar 1991; Warén 1991).

  2. 2.

    Skenea profunda Friele, 1879 is a deep-water species, originally described from sunken wood in 2400 m west of Svalbad (Friele 1879). The present sample is the host of an epibiotic nematode described by Holovachov et al. (2011) discovered at a large wood fall found during an expedition with RV ‘G.O. Sars’ in the northeast East Atlantic in June 2007 at 2830 m depth using the ROV ‘Bathysaurus’ at 73° 833.19′ N 08° 816.900′ E (Swedish Museum of Natural History in Stockholm, SMNH 99624). SEM images of shell, protoconch and radula were provided by Warén (1991).

  3. 3.

    Dillwynella lignicola Marshall, 1988 represents the genus Dillwynella Dall, 1889, which is supposed to be closely related to Skenea (Kano 2008) and has been included in several molecular analyses concerning phylogeny (Williams and Ozawa 2006; Kano 2008; Williams et al. 2008; Aktipis and Giribet 2012; Nye et al. 2013). Paratypes (M.75290, BS931, 42° 47.1′–48.2′ S, 175° 45.6′–47.2′ W, NE of Chatham Is., 1174–1180 m, 22 July 1984, FV OtagoBuccaneer, stn B01/102/84 (773)). Marshall (1988: 953–955, Figs. 1a–e, j, 9a, 10c–e, Table 1) provided SEM photos of the shell, protoconch and radula, description and a sketch of the external morphology of the head-foot (two section series in the ZSM-Molluscan section).

  4. 4.

    Dillwynella voightae Kunze, 2011. Paratypes (North Atlantic, Gulf of Mexico, Louisiana, USA (27° 44.090′ N, 91° 14.490′ W), natural wood fall, 610 m depth). Kunze (2011) provided data on shell, radula and external morphology (section series have in Field Museum of Natural History, Chicago).

  5. 5.

    Lodderena minima Tenison-Woods, 1887, the type species of Lodderena Iredale, 1924, is still included in Skeneidae by most current authors, although the available molecular data (Table 3) on a not named Lodderena species, do not support this assumption. Shells have been depicted and an actual description of proto- and teleoconch is provided at http://seashellsofnsw.org.au/Skeneidae/Pages/Lodderena_minima.htm and http://www.gastropods.com/8/Shell_16378.shtml The specimen under investigation (DLG706, shelly beach at low tide, collected at 2001-11-14 by rock washing, Manly Beach, Sidney, NSW, Australia, collected by Daniel Geiger) were originally preserved in seawater-buffered formalin and embedded in Epon resin. A resin block and two section series have been deposited in the ZSM-Molluscan section, and the remaining specimens of the original sample is still housed in the Swedish Museum of Natural History.

Specimens of S. serpuloides, S. profunda and D. voightae were preserved and stored in 70 % ethanol. Shells were decalcified by postfixation with Bouin’s fluid (picric acid, acetic acid and concentrated (i.e. 36 %) formaldehyde in a ratio of 15:1:5) and after dehydration in a graded ethanol series the bodies were embedded in araldite resin. Semi-thin section series were performed with ‘Ralph’-glass knifes with the glue-method according to Ruthensteiner (2008), stained with a 1:1 mixture of methylene blue and Azur II for approx. 5 s at 80 °C (Richardson et al. 1960) and sealed with araldite resin.

Specimens of D. lignicola were originally frozen in situ, later on preserved in alcohol causing a suboptimal histological condition in particular of the external epithelia. The shell was again decalcified by postfixation with Bouin’s fluid and after dehydration in a graded ethanol series the body was embedded in paraplast. Histological section series (á 5 μm) were done with a regular microtome, and the sections were stained by Hainhain’s Azan trichrome method or by the more robust haematoxylin-eosin protocol (Mulisch and Welsch 2015). Unfortunately, many sections are folded or partly disrupted so that only certain characteristics could be verified.

The sections of S. serpuloides were photographed with a digital camera (Olympus Camedia 5060) mounted on a light microscope (Olympus CX 41, objective Plan C 10×). The digital images were pre-processed in Adobe Photoshop: RGB-images set to greyscale, contrast and brightness adjustment, unsharp masking, size reduction by resampling. Thereafter, the images were imported in AMIRA Resolve RT 4.2 (TFG Template Graphics Software, Inc., USA). In AMIRA the 3D reconstructions were performed after manual alignment and segmentation (for details, see Heß et al. 2008; Haszprunar et al. 2011), snapshots of the surface-model were taken and morphometric measurements performed. Morphometric data are affected by the retraction grade of the specimen, anyway morphometric values are given here, to get an overview and an information basis about size dimensions in the examined species. The 3D-PDF was created with Adobe Acrobat 3D according to Ruthensteiner and Heß (2008).

Specimens of S. profunda and D. voightae were fixed and treated as with S. serpuloides, but the analyses were restricted to observe the histological sections in order to detect any differences from the type species.

Results

The anatomical and histological data of S. serpuloides, S. profunda and the two Dillwynella species are nearly identical; thus, we provide a common description mainly based on S. serpuloides, from which measurements were taken (Table 1) for future comparison.

Head

Recently, Rubio and Rolán (2013b) provided an excellent series of SEM-photos on the external morphology of S. serpuloides. We agree on their results and add the histological details of the respective structures.

Caused by the retraction of the animal into the shell, the foot and the tentacles of S. serpuloides were compressed. The visceropallium is located on the right side of the median plane and is comprise of 1.5 whorls (height 350 μm; Fig. 1a). The operculum is circular and multispiral (SEM-photos by Warén 1992: Fig. 4b and Rubio and Rolán 2013b: Figs. 1 and 2) and located on the dorso-posterior portion of the foot. The snout is long and broad, distinct oral lappets are however lacking.

Fig. 1
figure 1

3D reconstructions of the body and tentacles of Skenea serpuloides. a Latero-frontal view on the right side, mantle roof transparent. b Latero-frontal view on the left side, mantle roof transparent. c Frontal view, mantle roof transparent. d Right side view onto epipodial tentacle and epipodial sense organ, mantle roof removed. a, b SMNH-98645; c, d SMNH-98644. ct/ct’ right/left cephalic tentacle; e/e’ right/left eye; es right eyestalk; eso/eso’ right/left epipodial sense organ; et1-3/et’1-3 right/left first, second, third epipodial (anterior to posterior) tentacle; h head; m mouth opening; mp metapodium; n/n’ right/left neck lobe; o operculum; p penis; pp propodium; sn snout

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The paired cephalic tentacles are long (contracted 350 μm) with a circular profile and a diameter of 30–40 μm (Fig. 1a–c). The tentacles are filled with longitudinal and diagonal muscles forming a complex grid. The distal parts of the cephalic tentacles are provided with sensory papillae (not shown in the 3D reconstructions, but beautifully depicted by Rubio and Rolán 2013b).

An eye lobe is located dorsal to each eye at the base of the cephalic tentacle. While the left one is small (length 30 μm, diameter 5 μm) and inconspicuous, the right eye lobe is large (length 140 μm, diameter 40 μm; Fig. 1a), smooth and oriented parallel (latero-dorsal) to the right cephalic tentacle. Both sides bear a ciliated neck lobe (Fig. 1a–c), originating on the level of the mouth cavity and leading backward. The right one is 130-μm long, while the left one has a length of 90 μm (diameter of both neck lobes 25 μm). They are innervated by the right/left pedal ganglion.

Marshall (1988: 954) described the head conditions of D. lignicola: ‘Two small, smooth, narrowly tapered right suboptic tentacles. One smooth, bifid left suboptic tentacle, below which is a tight cluster of 2 or 3 tentacles’.

Foot and shell muscles

The epipodium bears three pairs of epipodial tentacles (Fig. 1a–c: et1–3). The tentacles are strongly retracted (length 90–140 μm, diameter 25–50 μm). All of these tentacles are distally covered with sensory papillae (Fig. 3d; also Rubio and Rolán 2013b). The most anterior pair of epipodial tentacles (Fig. 1: et1) is short, situated on the level of the pedal ganglia, and slightly anterior to the operculum. The second and third pair is located beyond the operculum. The epipodial sense organs (ESOs) are small, smooth knobs with the sensory epithelium lying distally (Fig. 3d). They are attached ventrally to the first pair of epipodial tentacles in both, S. serpuloides and D. voightae, whereas in S. profunda ESOs are attached to the second epipodial tentacle. In D. lignicola, the large epipodial tentacles have strongly ciliated edges, there are 3 on left side and 3 or 4 on the right side (Marshall 1988).

In all Skeneidae investigated, the large foot has a broad sole and a prominent propodium. In S. serpuloides, the right, frontal edge of the propodium forms a thick, smooth, cyclindrical-shaped penis, which is 160-μm long in the given state of retraction (height 250 μm, width 100 μm). The epithelium appears strongly folded suggesting a high potential for extension. A deep channel (seminal groove) is formed by a fold and leads from a latero-proximal to a dorsal position along the entire penis. The apical channel of the penis is ciliated, and the interior of the penis has large haemocoel spaces (Fig. 3b). Whereas the penis of S. profunda is identical, D. voightae and D. lignicola both have a quite small propodial penis at the right frontal edge of the foot with just an outer ciliated rim.

The pedal gland is mainly situated in the dorsal part of the propodium, but reaches far back into the foot. The cells of the gland are quite voluminous (Fig. 2b, c, g). The pedal gland opens via a ciliated channel at the dorso-anterior tip of the propodium.

Fig. 2
figure 2

3D reconstructions of inner organs of Skenea serpuloides. a,b Right side view of body with all inner organs (2 specimens), body surface transparent. c Dorsal view of vascular and excretory system with ctenidium, body surface transparent. d Dorsal view of the digestive system, body surface transparent. eRight side view of the reproduction system, body surface transparent. f Frontal view onto the nervous system. g Dorsal view onto the nervous system. h Left side view onto the nervous system and oesophagus. a, d–h Specimen SMNH-98644; b, c specimen SMNH-98643. a auricle, B/B’ right/left buccal ganglion, b bursa copulatrix, bc buccal commissure, C/C’ right/left cerebral ganglion, cc cerebral commissure, cn/cn’ right/left cephalic tentacle nerve, con ctenidial-osphradial nerve, Ct/Ct’ right/left cephalic tentacle ganglion, cte ctenidium, dg digestive gland, e/e’ right/left eye, hg hypobranchial gland, i intestine, k/k’ right/left kidney, ln/ln’ right/left labial nerve, ns nervous system, O osphradial ganglion, o operculum, od oviduct, oe/oe* anterior/posterior oesophagus, ov ovary, P/P’ right/left pedal ganglion, pc pedal commissure, pe pericardium, pg anterior pedal gland, Pl/Pl’ right/left pleural ganglion, pn/pn’ right/left pedal nerves, r radula and radular sac, s/s’ right/left statocyst, Sb suboesophageal ganglion, Sp supraoesophageal ganglion, sr seminal receptacle, st stomach, t testis, u urogenital duct, V visceral ganglion, vd vas deferens, ve ventricle, vi visceral loop, vs visceral nerve, 1 right cerebro-pedal connective, 2 right cerebro-pleural connective, 3, pleuro-suboesophageal connective, 4 pleuro-supraoesophageal connective, 5/5’ right/left pedal nerves. The interactive 3D-model of S. serpuloides can be accessed by clicking into Fig. 2 (Adobe Reader Version 7 or higher required). Click letter A for a 3D model of specimen SMNH-98644 or letter B of specimen SMNH-98643. Rotate model by dragging with left mouse button pressed, shift model: same action + ctrl, zoom: use mouse wheel (or change default action for left mouse button). Select or deselect (or change transparency of) components in the model tree, switch between prefab views or change surface visualization (e.g. lightning, render mode, crop etc.)

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There are two shell muscles made up of smooth muscle fibres, which spread into the foot and form the muscular wall of the head. The right shell muscle has its attachment zone at the line where the posterior oesophagus begins. The attachment area of the left shell muscle is situated beneath the osphradial ganglion.

Mantle cavity

The mantle rim itself is smooth in S. serpuloides and S. profunda, but papillate like the cephalic tentacles in D. voightae (no data on D. lignicola).

The mantle cavity of S. serpuloides extends around a quarter whorl of the body. The epithelium of the mantle roof is very thin and underlain by haemolymph sinuses. The central portion of the mantle roof is largely occupied by the rectum, which performs two semi-circular loops. The osphradium and ctenidium are located at the anterior left site (Fig. 2c). To the right the hypobranchial gland, the left kidney and the rectum are present. The centre of the posterior end of the mantle roof hosts the seminal receptacle (interactive Fig. 2). At the posterior left end of the mantle cavity, the seminal receptacle opens dorsally into the mantle cavity, while the urogenital opening emerges medio-ventrally into the mantle cavity. Cilia are present at the bottom of the mantle cavity opposite to the seminal receptacle opening, and a ciliary tract runs along the right neck towards the base of the penis. The opening of the penis channel is ciliated (Fig. 3b).

Fig. 3
figure 3

Histological details of Skenea serpuloides. a Overview: 3D reconstruction with the relative location of the section planes. b Propodial penis with penis channel. c Transversal section of the anterior soft body with propodeal penis. d Epipodial tentacle and epipodial sense organ (ESO). e Transversal section of the soft body with ganglia. f Statocysts with statoconia. g Section of the soft body with cephalic tentacle and sensory papillae. h Ctenidium with skeletal rods and bursicles. i Transversal section of the soft body with seminal receptacle opening to the mantle cavity and kidneys. j Transversal section of the soft body with seminal receptacle and gonoducts. k Heart and bursa copulatrix. l Transversal section of the posterior soft body with testis, ovary and egg details. a–c, e, i, j, l Specimen SMNH-98644; d, f, g specimen SMNH-98645; h, k specimen SMNH-98643). a auricle, b bursa copulatrix, bl bursicle lumen, bu bursicle, C/C’ right/left cerebral ganglion, ci cilia, cp penis channel, cs ctenidial sinus, ct right cephalic tentacle, cte ctenidium, dg digestive gland, e right eye, eg egg, es right eyestalk, eso’ left epipodial sense organ, et1’ left (most anterior) epipodial tentacle, f foot, i intestine and pallial rectal loops, k/k’ right/left kidney, m mantle, mc mantle cavity, nc nucleolus, nu nucleus, o operculum, oe/oe* anterior/posterior oesophagus, ov ovary, P/P’ right/left pedal ganglion, p penis, pa parasite, pe pericardium, pg anterior pedal gland, ph pharynx, Pl/Pl’ right/left pleural ganglia, r radula and radular sac, rc radular cartilage, s/s’ right/left statocyst, sc statoconia, sk ctenidial skeletal rods, sp sensory papillae, spe sperm, sr seminal receptacle, st stomach, t testis, ud urinogenital duct, vd vas deferens, ve ventricle, vi visceral loop, vl vitteline layer of egg

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The single (left) monopectinate ctenidium has eight leaflets. The ctenidial axis, which is equipped with a skeletal rod, is attached along its entire length to the mantle roof. The leaflets are positioned slightly obliquely to the axis with a length of 110 up to 170 μm; the fourth one is the longest. The epithelium of the ctenidial leaflets is built up by cuboidal to high-prismatic and densely ciliated cells. A zone with flat epithelium is lacking; accordingly, there is no distinct respiration area at the ctenidium. Also, the ctenidial leaflets are supported by paired skeletal rods. A bursicle occupies the anterior, efferent part of each leaflet, showing a slit-like channel about 30-μm long. In the larger D. lignicola, the monopectinate ctenidium consists of 12 leaflets, each with a (small but distinct) respiratory area in the proximal part.

The single (left) hypobranchial gland occupies the central right side of the mantle cavity, between the gill and left kidney (interactive Fig. 2). Three different parts can be distinguished histologically: the cells of the anterior part stain darkly, showing granular secretion. Posteriorly, the largest section follows with large, voluminous cells. The third portion runs parallel at first, continuing posteriorly and consists of two secreting cell types, one apocrine and the other one mesocrine.

The opening of the seminal receptacle is situated at the left dorsal end of the mantle cavity (Figs. 2e and 3i–k), while the urogenital opening is placed more ventrally.

In several specimens of S. serpuloides, we found a parasite (Fig. 3e), which was located in the right portion of the mantle cavity. The cuticularized parasite was attached anterio-laterally of the hypobranchial gland to the epithelium of the mantle cavity. Its anatomy was highly simplified, but histology suggests it is a crustacean (probably copepod).

Circulatory and excretory system

The monotocardian heart is located just behind the mantle cavity on the right side of the visceral body (Fig. 2c) and consists of a single (left) auricle and a (thicker) ventricle, both being surrounded by a pericardium (Fig. 3k). The irregular shape of the heart results from being wedged between the intestine, seminal receptacle and both kidneys. The wall of the pericardium is very thin. The auricle and ventricle are both oval (only preserved in specimen SMNH-98643). The auricle is located anterio-dorsally and it is connected ventrally with the ventricle. The heart is always close to the intestine: In S. serpuloides and D. voightae, the heart does not completely encircle the rectum, whereas this is the case in S. profunda (no data on D. lignicola).

Unfortunately, the preservation of the blood sinuses was not sufficient for complete reconstruction. Small blood sinuses from the gill leaflets merge into the efferent gill sinus, which proceeds on the left part of the mantle roof and leads backwards into the auricle. A second efferent sinus from the left kidney opens into the auricle at its ventral side. The anterior aorta emerges posterior-ventrally from the ventricle and runs in the same direction.

All skeneids studied have two kidneys, both located at the right side of the animal (Fig. 2c). The left kidney lies ventro-laterally left of the rectum (Figs. 2a and 3j: k’). It borders the rear part of the mantle cavity behind the ctenidium, to the right of the midline, and runs in a slight curve latero-caudally. The left kidney is a longish, papillary tube with a diameter of about 50 μm (Fig. 3j: k’). Its anterio-dorsal part lies close to the pericardium and is connected to the pericardium via a ciliated renopericardial duct. The nephroporus is located in the anterior part of the left kidney and is equipped with a sphincter muscle.

The right kidney is placed ventrally at the same level as the heart. It ramifies with irregular lobes between the viscera and has a large lumen (Fig. 3i, j: k). The epithelial cells are small and weakly stained. A renopericardial duct connects the right kidney with the ventral side of the pericardium. Anterior-ventrally, both the oviduct and the vas deferens open into the distal part of the right kidney. From here, the very short urogenital duct opens into the mantle cavity.

Genital system

The hermaphroditic genital apparatus of all species investigated consists of separate ovary with oviduct and testis with vas deferens and is also equipped with a seminal receptacle, a bursa copulatrix and a propodial penis (Fig. 2e; see also external morphology: Fig. 1a–c). Ovary and testis are fully mature in the examined specimens with sperm and eggs in all stages of development.

The large ovary together with the digestive gland occupies the uppermost whorl of the shell. The various developmental stages of the eggs (Fig. 3l) are not sorted by size. Yolky, mature eggs have diameters of about 150 μm, each is covered by an irregularly shaped vitelline layer (up to 30-μm thick). Yolk granules are small; the nucleus has a diameter of about 40 μm and contains a prominent nucleolus. The anterio-ventral end of the ovary is continued by the oviduct, which is a compressed tube (about 100-μm long) being situated between body wall and testis. The oviduct entirely lacks glandular cells or cilia and opens into the right kidney.

The testis is located anterio-ventrally of the ovary. It has a long cylindrical, curved shape, orientated horizontally in the animal (Figs. 2e and 3l). The anterior part of the testis is filled with fibrous spermatids, whereas the posterior testis contains spermatogonia (diameter 4 μm) stuffed with small granules. Anteriorly, the testis merges into the vas deferens. This tube is very short and opens into the right kidney close to the opening of the oviduct.

The large and oval seminal receptacle is located posteriorly in the ventral portion of the mantle roof (Fig. 2e). The opening into the mantle cavity is at the anterio-ventral end of the seminal receptacle, containing sperm cells in its frontal part. Sperm cells form a thick, unordered cluster in the posterior part close to a distinct bursa copulatrix (Fig. 3k). The latter is located anterio-proximally of the seminal receptacle and is filled with a sperm mass, which is in progress of disintegration. It is connected with the seminal receptacle via a narrow channel. S. profunda and the two Dillwynella species lack a bursa copulatrix.

Alimentary tract

The blunt snout is retracted far inside the mantle cavity (Fig. 1a–c). The mouth opening lies fronto-dorsally and has a small fold on each side. The mouth opening is approx. 50-μm long and then merges into a straight channel, which enlarges to the buccal cavity with the radula (Fig. 3e–g). The pharynx is short and has small pouches laterally and dorsally. Salivary glands could not be detected.

The delicate, paired jaws are fused dorsally and consist of small rod-like elements. The radula of S. serpuloides is located latero-dorsal of the jaws, is of the rhipidoglossate type (see the SEM photos provided by Ponder 1990; Rubio-Salazar 1991 and Warén 1991, 1992), and lacks a radular caecum. Due to contraction of the body, the radula is s-shaped and has an overall length of 400 μm. The radular diverticulum occupies around 210 μm of this length, is not bifid and is thickened at its posterior end. Contrary to the smooth shell muscles and head musculature, all buccal muscles are cross striated. The odontohore has two pairs of slender radular cartilages. The drop-shaped anterior pair of cartilages is located anterio-ventral of the radula, and the left and right cartilages contact each other closely proximally (interactive Fig. 2). The cartilage cells are largest at the ventral side (max. diameter 14 μm), getting smaller dorsally (Fig. 3e). The second pair of radular cartilages is located posterior-dorsal of the other pair. It is quite small and the separation from the anterior pair is so inconspicuous, that they could not be individualized in the reconstructions. A subradular organ is lacking.

After about 80 μm, the dorsal pharynx passes into the large anterior oesophagus (see interactive Fig. 2), the epithelium of which bears long cilia. The dorsal food channel shows a slight, but not complete, torsion of approximate 45°. The posterior part of the anterior oesophagus shows many papillae (in particular in the larger D. lignicola), forms two blind, glandular pouches (about 40-μm long) and is continued by the posterior oesophagus, which is a 200-μm long, quite thin (diameter 50 μm) tube with star-shaped lumen leading straight backwards along the ventral wall of the body (Fig. 3i, j). The oesophagus opens medially into the ventral wall of the stomach between the openings of the digestive glands.

The oval stomach is slightly curved (Figs. 2d and 3l), and its epithelium is quite thin in the anterior part (10 μm). The cells have long cilia (8 μm) and are also entirely covered with microvilli. In the posterior part, around the opening of the digestive glands, the epithelium changes to high prismatic, ciliated cells (height 30 μm, width 2 μm). A gastric shield covers the posterior part of the stomach. There are two digestive glands, each with a separate opening to the stomach. Together with the ovary, the digestive glands occupy the last whorl of the soft body, but they reach distally further out then the ovary (Fig. 2a, b, d). The digestive glands form lobes with a curved lumen. The darkly stained epithelial cells (Fig. 3j, l) suggest intense secretion.

The intestine emerges in the most anterior part of the stomach. The epithelium is ciliated and a longitudinal deep rim is present. First the intestine leads 130 μm straight forward and then to the right side (Fig. 2d). Afterwards it loops 180° backwards to the left and passes the heart. In S. serpuloides and D. voightae, the heart it encircling the rectum only partly (interactive Fig. 2b), while in S. profunda and many other vetigastropods, the rectum is encircled completely (no clear data in D. lignicola). Then the rectum loops 180° forward to the right and leads 450 μm in the same direction. This is followed by a narrow 180° turn to the left side. After 160 μm, the intestine performs a last 180° loop to the left and finally it leads straight forward (260 μm). The anus opens into the right part of the mantle cavity at the level of the anterior edge of the ctenidium.

Nervous system and sensory organs

The central nervous system consists of four paired ganglia (cerebral, buccal, pleural and pedal ganglia) and four unpaired ganglia (osphradial, sub- and supraoesophageal and visceral ganglion). Due to contraction and the poor preservation, not all nerves could be detected (e.g. those of the epipodial sense organs or the neck lobe).

Each cerebral ganglion is situated below the base of a cephalic tentacle flanking the transition zone of pharynx and oesophagus latero-distally. Both ganglia are interconnected by the cerebral commissure (Fig. 2f–h). Each cephalic tentacle nerve forms a small cephalic tentacle ganglion at its base in front of the eyes. The short and thin buccal-cerebral connectives emerge at the ventral side of the cerebral ganglia. The buccal ganglia are located proximally to the cerebral ganglia and ventral to the pharynx (Fig. 2f). The buccal ganglia are interconnected via the buccal commissure, which forms a ventral loop and marks the beginning of the anterior oesophagus.

The pedal ganglia (Figs. 2f and 3e) are the largest ganglia and quite elongated. They are close to each other, so that the pedal commissure is formed by the proximal attachment zone of both ganglia. Further, posterior pedal commissures were not detected. The flat (squashed?) statocysts (see also sense organs) were attached posterior to the pedal ganglia (Figs. 2g and 3e). Neural cords with many nerve cells emerge from the pedal ganglia and innervate the foot and epipodial tentacles. A nerve emerges from the right pedal ganglion leading into the right parts of the body and mantle roof, whereas a left counterpart of this nerve could not be found. The cerebro-pedal connectives are located in front of the cerebro-pleural connectives. The long, conical pleural ganglia are located close to the pedal ganglia (hypoathroid condition; Fig. 2h). While the left pleuro-pedal connective is thin and short, but distinct, the right pleural ganglion is closely attached to the right pedal ganglion.

The visceral loop is streptoneurous. The pleuro-supraoesophageal connective emerges from the posterior right pleural ganglion, crosses the anterior oesophagus dorsally and leads to the supraoesophageal ganglion at the left side (interactive Fig. 2). This ganglion is located dorso-laterally above the posterior oesophagus. From there, the quite short left visceral connective emerges backwards and reaches the visceral ganglion. The osphradial ganglion is found above the supraoesophageal ganglion. Both are interconnected by a short but broad connective. The suboesophageal ganglion supplies the left mantle roof and the ctenidium. The left pleural ganglion is continued by the short pleuro-suboesophageal connective, which reaches the suboesophageal ganglion (Fig. 2g) by crossing the posterior oesophagus at its ventral side. The suboesophageal ganglion is quite large, round and depressed dorso-ventrally. A thin nerve emerges latero-frontally and leads in a loop backwards to reach the visceral ganglion, thus forming the right part of the visceral loop. The small visceral ganglion lies quite medially, at the level of the posterior end of the mantle cavity.

Cephalic and epipodial tentacles have been described above. The eyes lie somewhat embedded in the body surface (Figs. 2f–h and 3e), lack a lens, but the vesicle is filled with a vitreous body. In the section series available, the eyes vesicles are always devoid of pigment, a known bleaching artefact of the alcohol storage (pers. comm. A. Warén). A closed and pigment-less eye vesicle is also present in D. lignicola. The depressed (squashed?) statocysts are located adjacent to the pedal ganglia (Figs. 2g and 3e). Each statocyst contains several to many (D. lignicola) statoconia (Fig. 3f). Bursicles of the ctenidial leaflets have been described above. The single and densely ciliated osphradium (diameter 30 μm) is located on the left side of the mantle roof and is directly underlain by the large osphradial-ganglion.

Anatomy of L. minima (Tenison-Woods, 1887)

The anatomy of this species resembles those of the true skeneids; thus, only a short description with a focus on differences is provided.

As in the skeneid species, the ESOs are attached to the ventral basis of the first papillate epipodial tentacle. There are two shell muscles. The mantle cavity is provided with a monopectinate ctenidium, and the leaflets of which lack respiratory zones, but are equipped with skeletal rods and bursicles. Right of the rectum, there is a large hypobranchial glands consisting of several types of large mucous cells. Two kidneys are present; the right one also releases the gametes via a urogenital opening.

However, the genital system shows significant differences to the skeneid species: there is a true hermaphroditic gland and the large and yolky eggs are provided with a thin vitelline layer. A copulatory organ is absent and a receptaculum is lacking.

The gut resembles that of skeneids. The jaws with rod-like elements are delicate, the long radular sheath shows a large loop and the two pair of radular cartilages are slender. The anterior oesophagus lacks significant oesophageal pouches, shows torsion and enters ventrally into the large stomach, which is equipped with toothed gastric shield and tooth and two separated digestive glands. The intestine again shows a longitudinal rim, and the rectum runs through the heart and makes a large loop along the mantle roof.

Conditions of the nervous system do not differ from those of the skeneid species. However, the eyes show closed vesicles and pigment. The osphradium is large, and the statocysts contain several tiny statoconia.

Discussion

General remarks

In the following discussion, only genera with a propodial penis (see Table 3 for data) are referred as Skeneidae. Descriptions of internal conditions are so far restricted to S. serpuloides and concern either an unpublished thesis (Brückner 2003) or preliminary data (Brückner et al. 2004; Kunze et al. 2008). In addition, Warén and Bouchet (1993: 22ff) provided sketchy anatomical information on Protolira valvatoides.

The considerable degree of homoplasy of many phenotypic characters in basal ‘archaeo-’ gastropods (reviewed in detail e.g. by Haszprunar 1988b; 1993; Ponder and Lindberg 1997; Sasaki 1998) limits the application of these data to infer relationships. Although the molecular data are far from being entirely non-homoplasious, they are usually more suitable concerning phylogenetics. Nevertheless, since we also want to infer also evolutionary trends and environmental adaptations, an evaluation of morphological characters concerning the systematic position of Skeneidae is desirable. Assis et al. (2011), Richter and Wirkner (2014), Lee and Palci (2015) and Giribet (2015) have provided general points of view on the subject.

Character analysis

The small teleoconch lacking nacre cannot be used to define Skeneidae. However, protoconchs of skeneid species studied by SEM are generally smooth or finely granular; most of them also show few longitudinal fine cordlets, which might be a useful character (Hoffman et al. 2008). Accordingly, we regard all species with a sculptured pattern of the protoconch as doubtful members of Skeneidae.

The papillate condition of the cephalic and epipodial tentacles and the mantle edge reflect the vetigastropod nature of Skeneidae (von Salvini-Plawen and Haszprunar 1987; Haszprunar 1988b; Ponder and Lindberg 1997). TEM-details of these papillae have been provided by Crisp (1981) and Künz and Haszprunar (2001).

The two ciliated necklobes (Fig. 1a) of S. serpuloides were also reported by Fretter and Graham (1977), Rubio-Salazar (1991), Warén and Bouchet (1993) and Rubio and Rolán (2013b). As the right eyestalk is much larger than the small and inconspicuous left one (Fig. 1a, b), it was interpreted originally as a penis by Fretter and Graham (1977), but SEM and our histological data identify it as an enlarged eyestalk.

In living animals, the epipodial tentacles are quite long (Clark 1851b; Rubio-Salazar 1991; Rubio and Rolán 2013b), but because the specimens under investigation were heavily retracted in the shell, these tentacles appear short and stumpy in the reconstructions (Fig. 1a, b). Fretter and Graham (1977) did not mention the presence of epipodial sense organs (ESOs) in S. serpuloides, whereas Rubio-Salazar (1991: Fig. 1) described three ESOs: one ventral to the first right epipodial tentacle and two ESOs associated with the first one left. In contrast, we found only a single, equally sized pair of ESOs attached to the most anterior pair of epipodial tentacles (Figs. 1a, b, d and 3d). Our data corroborate the SEM data by Rubio and Rolán (2013b), who described nominally ‘four epipodial tentacles’, the first, third and fourth pair is papillate (true epipodial tentacles), but the second one much shorter and non-papillate (ESO). The available data on further skeneids (Table 3 and own observations) show considerable variation in number of epipodial tentacles and ESOs. ESOs of all skeneid species examined are located at the ventral base of an epipodial tentacle, a condition generally found in Haliotidae, Trochoidea and Phasianelloidea (Crisp 1981; TK pers. obs.). In contrast, species of Seguenzioidea (in the concept of Kano 2008 and Kano et al. 2009 including several skeneimorph taxa) have separated ESO tentacles (Kunze et al. 2016).

The retention of both shell muscles is a plesiomorphic character of Gastropoda (Haszprunar 1985b, 1988b). Whereas this condition in found in many Vetigastropoda, it is rare among Trochoidea, where usually only the left adult shell muscle is retained (e.g. Bandel 1982).

The single left ctenidium is monopectinate in all Skeneidae, which might be a matter of small size or paedomorphosis, since this condition is also known in early juveniles of taxa with bipectinate conditions as adults. As typical for most Vetigastropoda and several Neomphalida each ctenidial leaflet is equipped with a bursicle and skeletal rods (Szal 1971; Haszprunar 1987, 1988b; Heß et al. 2008; Kunze et al. 2008).

Retention of two asymmetric kidneys with different structure and functions is diagnostic for the Vetigastropoda and Patellogastropoda (Andrews 1985; von Salvini-Plawen and Haszprunar 1987; Haszprunar 1988b, 1993; Ponder and Lindberg 1997; Sasaki 1998). The structure of the left kidney as a papillary tube is typical for Vetigastropoda as is the release of the gametes via visceral gonoducts proper and a urogenital opening with the right kidney.

Simultaneous hermaphroditism with separated ovary/oviduct and testis/vas deferens has been observed in all Skeneidae studied so far. Similar conditions occur in lepetelloidean Vetigastropoda (Haszprunar 1988a, 1998) and also in the heterobranch Architectonicidae (Haszprunar 1985a) and Omalogyridae (Bäumler et al. 2008), although the latter have a common distal and glandular gonoduct.

The most striking feature of the species investigated is the propodial penis (Figs. 1a and 3c, g), which was until now the only diagnostic morphological synapomorphy for the Skeneidae (Warén 1991, 1992; Warén and Bouchet 1993). There is little doubt that copulatory organs have repeatedly evolved among gastropods in general and also among Vetigastropoda, in particular, in species which are small or inhabit deep-sea or chemosynthetic habitats, but a propodial penis remains unique. However, it should be noted that such a propodial penis may be quite small and inconspicuous as in the Dillwynella species studied. Indeed, the penis was not mentioned in the descriptions of the head-foot of Dillwynella species by Marshall (1988), Hasegawa (1997) or Kunze (2011) and can only be detected in serial sections.

The yolky eggs of all species under investigation have a vitelline layer, a diagnostic character for Vetigastropoda (Ponder and Lindberg 1997). In addition, all three skeneid species investigated have a separated seminal receptacle in the left and central mantle roof; a bursa copulatrix is present only in S. serpuloides (Fig. 3k). Similar conditions occur in a number of other vetigastropod taxa, namely within the Lepetelloidea (Haszprunar 1988a; 1998) and the Seguenzioidea (Kano 2008; Kano et al. 2009; Kunze et al. 2016).

Paired jaws with rod-like elements are typical for Neomphalida and Vetigastropoda, but occur also in hyalogyrinid and many other Heterobranchia (Haszprunar et al. 2011). Many skeneimorph species show a quite similar radular type. However, Warén (1990) showed that juvenile turbinids or trochids have a very similar rhipidoglossate radula, even if the radula type substantially differs in adults. This similarity of the radulae might be due to similar food being targeted in juveniles, namely grazing on biofilms on various substrates (Warén 1990), and thus might be a matter of convergence. Alternatively, it might be a homologous structure, if so probably a vetigastropod plesiomorphy or the radular type of the vetigastropod (plus neomphalidan) stem species, respectively. Finally (and most likely), the radular structures of these small species are due to paedomorphosis and thus again a matter of parallelism. In any case, the skeneid radula type cannot be used to infer systematic relationships.

Two pairs of radula cartilages and a papillate oesophagus were found in many vetigastropod clades (e.g. Sasaki 1998; Katsuno and Sasaki 2008), whereas the conditions of the stomach with its gastric shield and two digestive glands reflect basic gastropod conditions. Often, it is difficult to determine whether the rectum is encircled by the heart completely, because this region is often damaged by hardened haemolymph during fixation and the pericardial wall is very thin and fragile. A complexly coiled rectum has been also reported in Turbo stenogyrum (Sasaki 1998), in certain Seguenzioidea (e.g. Carenzia carinata; GH pers. obs.) and in ectobranch (valvatoidean) Heterobranchia (Haszprunar et al. 2011).

Conditions of the skeneid hypoathroid and streptoneuran central nervous system reflect plesiomorphic gastropod conditions.

S. serpuloides has pigmented eyes, which are black in living animals (Jeffreys 1865; Fretter and Graham 1977; Rubio-Salazar 1991). The eyes are closed vesicles with a vitreous body (sensu Sasaki 1998) like those found in the Scissurellidae (Bourne 1910; Strasoldo 1991), Fissurellidae (Boutan 1885; Illingworth 1902) or Phasianellidae (Marcus and Marcus 1960).

Marshall (1988: 953) constituted ‘eyeless optic tentacles’ in the diagnosis of the genus Dillwynella. However, Hasegawa (1997: 89) reported black eyes in Dillwynella vitrea and Dillwynella planorbis, pigment-less eyes in Dillwynella fallax and total lack of eye and optic tentacle in Dillwynella seishinmaruae. Kunze (2011) did not mention an eye in D. voightae; thus, it is likely that they are devoid of pigment also in the living animal. The same is certainly true for the deep-water D. lignicola and very likely for S. profunda. Accordingly, eye conditions are quite variable in Dillwynella and generally in Skeneidae.

Conditions of the statocysts with several statoconia reflect again vetigastropod relationships.

Ecology of Skeneidae

Skeneid species are found in various marine habitats from intertidal gravels or shallow coastal waters down to the bathyal plane, but for many locations detailed data about the habitat or bottom structure are not available (e.g. Høisæter 1968; Bouchet and Warén 1979). S. serpuloides is known from infra- and circalitoral amphioxus sand and maërl in depths between 15 to 145 m (Fretter and Graham 1977; Rubio-Salazar 1991; Rubio and Rolán 2013b; herein), whereas S. profunda lives on sunken wood in deep waters beyond 2000 m. Other Skenea species live in depths between 50 and 3500 m on algae, rock, sunken wood, sand and silty bottoms (Rubio-Salazar 1991; Warén 1991, 1993; pers. comm. C. Schander; pers. obs. TK). Dikoleps and Skeneoides species live in intertidal gravel, in shallow water from 0 to up to 160 m on different bottoms like sand, maërl and also on stones, algae and corals (Rubio-Salazar 1991; Warén 1992). Both known species of Protolira, Protolira thorvaldssoni Warén, 1996 and Protolira valvatoides live in depths between 850 and 3700 m in hydrothermal vent habitats, among mussels in sediments and also on whale bone (Warén and Bouchet 1993, 2001). All species of the genus Dillwynella inhabit deep-water sunken wood and algal holdfasts (Marshall 1988; Hasegawa 1997; Kunze 2011). Thus, skeneids occur in most benthic habitats.

According to WMSDB, most Skenea, Dikoleps and Skeneoides species have been described and recorded from the European Atlantic coast, from Spain to Svalbard and around Iceland, while many Dillwynella species were exclusively found in the Pacific Ocean. However, there is little doubt that many species still remain to be discovered, rendering biogeographic data highly preliminary.

Constraints of small size

Skeneidae are small. Based on various similarities with the related (see below for systematics) trochid or turbinid juveniles, we assume that the small size probably is a secondary condition probably been reached by progenesis, i.e. acceleration of sexual maturation into a juvenile stage (e.g. Raff 1996). Accordingly, skeneid gastropods (and other skeneimorph species as well) show several special conditions and circumstances:

A nacreous layer of the teleoconch is typical for trochoid and seguenzioid Vetigastropoda, but is often missing in small species (Hickman 1983).

Most of the larger (>5 mm) trochoidean species have an ESO at the ventral bases of each epipodial tentacle (e.g. Crisp 1981). In contrast, skeneids often have only a single ESO attached to the first or second epipodial tentacle pair; occasionally, there is a scattered distribution of ESOs at the epipodial tentacles.

All vetigastropods have yolky eggs and show lecithotrophic development. Thus, to provide enough yolk for the developing embryo and larva, the size of mature eggs cannot be reduced. With a decreasing body size, the number of ripe eggs decrease dramatically (assuming the same overall shape 50 % body length equals only 12.5 % of volume, 20 % body length equals 0.8 % of volume and thus egg numbers!); thus, fertilization success by internal or entaquatic (pallial) contact of sperm and eggs becomes a must. Hence, most microgastropods have copulatory structures or seminal receptacles. However, there are exceptions: so, for example, the small (1.7 mm) seguenzioid species Putilla porcellana (Tate & May, 1900) and the even smaller L. minima (<2 mm) neither have any copulatory organ nor receptacula (pers. obs. TK). The same is true for certain small Scissurellidae such as Incisura lytteltonensis (Smith, 1894) (about 1 mm; cf. Bourne 1910) or Scissurella jucunda (about 2 mm; cf. Strasoldo 1991; Baborka 2007). In these cases, fertilization success of the few eggs might be enhanced by special reproductive behaviour or spermatophores.

A monopectinate gill is also found in other minute vetigastropod species (Kano 2008; Geiger et al. 2008), and this condition might reflect paedomorphosis, since a monopectinate condition occur during early ontogeny of bipectinate species (e.g. Crofts 1937; Strasoldo 1991). However, there are small species with bipectinate ctenidia like Leucorhynchia caledonica (2 mm) and several scissurellids, and relatively large species like D. voightae (5.8 mm) with a monopectinate ctenidium; so this is not a strict rule. In addition, the ctenidia of many small gastropods do not show distinct respiratory areas but probably use their lateral cilia as ventilators of the mantle cavity: In these cases, respiration is mainly provided by the thin mantle roof and (in limpets) the subpallial epithelia.

The nervous system of skeneids appears more concentrated and more voluminous than those of larger vetigastropods. Again, this reflect both size constraints (the number of neurons cannot be reduced below a certain point and the size of neurons remains constant) and paedomorphic conditions.

Systematics

The vetigastropod nature of Skeneidae is beyond doubt and well supported by all molecular studies and morphological data including: papillate conditions of tentacles and mantle rim, ctenidium with skeletal rods and bursicles, two different kidneys, the left one a papillary tube, the right one ramifying between the viscera and forming an urogenital opening, eggs with a vitelline layer, paired and rod-like jaws, a rhipidoglossate radula and papillate anterior oesophagus, a hypoathroid and streptoneuran nervous system and statocysts with several statoconia.

xThe recent molecular studies support inclusion of the Skeneidae among Trochoidea close to the Turbinidae. Morphologically, this is supported by the epipodial conditions with attached ESOs, which separate Trochoidea (and Phasianellidae and Haliotidae) from Seguenzioidea (sensu Kano 2008; Kano et al. 2009) with separated ESOs (Kunze et al. 2016), the latter conditions also occurs in Scissurellidae, Clypeosectidae and Lepetodrilidae (TK pers. obs.).

It is more difficult to define monophyletic Skeneidae by phenotypic characteristics. At our current stage of knowledge, the genital apparatus appears to be the most promising character set. All skeneid species investigated are true hermaphrodites with separated ovary/oviduct and testis/vas deferens and show a unique propodial penis. The current concept based on the presence of a propodial penis includes the genera (alphabetic order):

  • Dikoleps Høisæter, 1968 with type species Margarita pusilla Jeffreys, 1847

  • Dillwynella Dall, 1889 with type species Teinostoma (Dillwynella) modesta Dall, 1889

  • Lissospira Bush, 1897 with type species Cyclostrema proxima Tryon, 1888

  • Protolira Warén and Bouchet, 1993 with type species P. valvatoides Warén & Bouchet, 1993

  • Pseudorbis Monterosato, 1884 with type species Fossarus granulum Brugnone, 1873

  • Skenea Fleming, 1824 (type genus) with type species H. serpuloides Montagu, 1808

  • Skeneoides Warén, 1992 with type species Delphinula exilissima Philippi, 1844

However, most of these genera need to be studied also anatomically to confirm inclusion. Based on consistent molecular data (Kano 2008; Williams 2012), we tentatively add here Cirsonella Angas, 1877 (syn. Tharsiella Bush, 1897) with type species Cirsonella australis Angas, 1877 (currently considered as a synonym of Cyclostrema weldii Tenison-Woods, 1877; see Rosenberg 2015), although this should be confirmed by checking the type species for a propodial penis.

Soft part characters of Iheyaspira lequios (Okutani et al. 2000); the type species of its genus, in particular, a tentacle-like right neck lobe; the separated ESO-tentacle and the lack of a propodial penis (Okutani et al. 2000; Nye et al. 2013) suggest seguenzioid affinities of Iheyaspira rather than the stated classifications among Trochidae or Skeneidae. The molecular analysis of Nye et al. (2013) inferred Iheyaspira bathycodon in a clade together with Skeneidae, but is not sufficient to prove inclusion, because in particular seguenzioid taxa have not been considered in that study. Most recently, however, Chen et al. (2015) showed that I. lequios Okutani et al. 2000 is composed of four lineages belonging to both, Vetigastropoda—Skeneidae and Neomphalina.

The record of a propodial penis in Lodderena catenoides by Warén (1992) and thus for Lodderena in general is misleading, because L. catenoides is now accepted as Skenea catenoides (Monterosato, 1877) (WoRMS). The current data on the type species of Lodderena, L. minima (Tenison-Woods, 1878), revealed significant differences in the genital system, namely a true hermaphroditic gland, lack of propodial penis and lack of a receptaculum. In addition, molecular data (H3 + COI) group Lodderena with Trochidae/Turbinidae rather than with Skeneidae (Kano 2008; Williams 2012). Therefore, Lodderena Iredale, 1924 should be excluded from the Skeneidae.

The type species of Leucorhynchia Crosse, 1867, L. caledonica Crosse, 1867, also shows papillate conditions, but a bipectinate ctenidium, a true hermaphroditic gland with a common gonoduct and a propodial penis on the left side (pers. obs. TK). Accordingly, we also exclude Leucorhynchia Crosse, 1867 from Skeneidae.

For the numerous other genera, which have been assigned to Skeneidae or Skeneinae, molecular or soft part studies (ideally both) are required for in- or exclusion, hard part characters alone clearly are not sufficient, although a protoconch with few spiral cordlets (e.g. Haplocochlias Carpenter, 1864; cf. e.g. Rubio & Rolan 2015) raises the probability of inclusion. Representatives of only two genuine skeneid genera with confirmed propodial penis, Dillwynella (Warén 1992: 152) and Protolira, and the tentatively assigned Cirsonella, were included in molecular analyses (Table 2).

We cannot exclude the possibility that Skeneidae in the present diagnosis is only a subclade within a broader clade of trochoid vetigastropods which may also include taxa without a propodial penis (or with a left one as in L. caledonica Crossé, 1867; Anders Warén pers. comm.). On the other hand, we prefer a clear diagnosis of Skeneidae instead of continuous usage of a family name as a lumping pot for small vetigastropods, which are better assigned as ‘Vetigastropoda incertae sedis’. Such a diagnosis now is available, and there is little doubt that many skeneid-like species need to be excluded from Skeneidae characterized in this way. This is also a plea to study microgastropods more intensively than up to now. Aside from shell, protoconch and radula, the soft body (by SEM and/or 3D-morphology) also needs consideration, and ideally molecular data also (see e.g. Chen et al. 2015 as a tale of caution) should be added in order to proceed in our understanding of ‘what [else] is Skeneidae?’ To conclude, ‘Before we came here we were confused about this subject. Having done our work we are still confused—but on a higher level’ [modified from Enrico Fermi, nuclear physicist] (Mackey 1991: 90).