Introduction

Saccocirridae Bobretzky, 1872 is a relatively small family of interstitial annelids common in shallow water marine sediment (Westheide 2008). The 22 species known from the family have been traditionally divided in two groups, “papillocercus” and “krusadensis,” mainly based on the presence or absence of a ventral pharynx, as well as different ciliary patterns and chaetal types (Brown 1981). These groups have been largely recognized in the literature (Brown 1981; Jouin and Gambi 2007), but only recently, phylogenetic analyses revealed that they actually constitute monophyletic clades (see Di Domenico et al. 2014b) leading to the formal definition of two respective genera, Saccocirrus Bobretzky, 1872 and Pharyngocirrus Di Domenico, Martínez, Lana & Worsaae, 2014. The position of Saccocirridae within Annelida has been controversial. The group was placed amongst, or closely related to, the archiannelids by early workers (see review by Hermans 1969). This controversial phylogenetic position motivated detailed morphological investigations on the family (Eakin et al. 1977; Martin 1978; Sasaki and Brown 1983; Purschke and Jouin 1988; Purschke 1990; Purschke 1992), which in overall suggested a close relationship between Saccocirridae and the families Protodrilidae Purschke & Jouin, 1988 and Protodriloididae Purschke & Jouin, 1988. This relationship was recovered by an extensive cladistic study (Rouse and Pleijel 2001) and more recently by phylogenomic analyses (Andrade et al. 2015; Struck et al. 2015; Helm et al. 2018).

Saccocirridae is known from all over the world. However, most of the European records have been attributed to Saccocirrus papillocercus Bobretzky, 1872, and until recently all the species of the “krusadensis” group were known solely from Brazil or the Indopacific (Purschke and Jouin 1988; Bailey-Brock et al. 2003; Jouin and Gambi 2007). Soon after the description of the first Pharyngocirrus species in the Mediterranean (Jouin and Gambi 2007), more extensive sampling confirmed that both clades occur in all temperate and tropical oceans, with the highest diversity around the equator (Di Domenico et al. 2014b). Despite their geographic overlap, the two genera rarely occur sympatrically, with species of Pharyngocirrus generally inhabiting sheltered sandy habitats, and members of Saccocirrus being more common in reflective sandy beaches (Di Domenico et al. 2014a, b, c). Yet, these apparent differences in habitat preference across each genus remain to be tested explicitly.

Until now, four species of saccocirrids are known from the Eastern Atlantic and the Mediterranean Sea (Westheide 2008). Saccocirrus papillocercus is recorded from the Black Sea (Bobretzky 1868; Bobretzky 1870; Repiachoff 1881; Salensky 1907; Gusjewa 1929; Mastepanova 2004; Surugiu 2006), the Mediterranean Sea (Pierantoni 1907; Magagnini 1980; Villora-Moreno 1997; Jouin and Gambi 2007), and the Atlantic Ocean (Langerhans 1880; Cabioch et al. 1968; Dauvin 1978; San Martin 1987; Núñez et al. 2005), whereas Saccocirrus major Pierantoni, 1907; S. parvus Gerlach, 1953; and Pharyngocirrus goodrichi (Jouin & Gambi, 2007) are exclusively known from the Mediterranean Sea (Pierantoni 1907; Jouin and Gambi 2007). Despite the family being commonly found in the Canary Islands (Gusmão et al. 2016; Riera et al. 2017), its diversity in the archipelago remains uninvestigated and so far only S. papillocercus has been reported from the area (Núñez et al. 2005).

The goal of this study is to formally describe one new species recovered from the Canary Islands during recent phylogenetic studies on the family (Di Domenico et al. 2014b) and include new information about Saccocirrus parvus in an emended description. The study is based on material collected during the First International Workshop to Marine and Anchialine Ecosystems, Lanzarote in November 2011. Saccocirrus slateri sp. nov. is described, according to the available material, combining light and scanning electron microscopy observations, as well as DNA sequences. Furthermore, we include new information about Saccocirrus parvus from Canary Island, and we discussed the functional morphology of Saccocirrus and adaptations to occur in instable substratum coupled with wave action, highlighting their mobility and ability to deal with surf and swash environment.

Material and methods

Sample collecting and processing

A total of nine sandy beach localities were sampled along the Canary Islands (Fig. 1), including Los Abades, Barranquilla, Punta del Hidalgo, and Punta Sama in Tenerife; Charco Verde in La Palma; Las Canteras in Gran Canaria; and Charco del Palo, Mala in Lanzarote. La Barranquilla, Punta Sama, Punta del Hidalgo, Charco Verde, Los Cancajos, and El Golfo are exposed sandy beaches, with volcanic sand. Los Abades is a small semi-exposed beach serving as a harbor for fisherman. Las Canteras beach is an organogenic sandy beach, semi-exposed, and protected by a fossil reef. Charco del Palo is an exposed rocky shore coast with sandy patches.

Fig. 1
figure 1

Map of study area showing sampled localities

Full size image

Sediment samples were collected manually at the intertidal zone or by SCUBA diving from subtidal environments. Animals were extracted using the MgCl2 decantation technique through a 63-μm mesh (Higgins and Thiel 1988). Live animals were sorted and photographed in the field using a Nikon D300 (NIKON, Tokyo, Japan), mounted on an Olympus SZX16 stereomicroscope. After a second relaxation of the animals with isotonic MgCl2, fixations were done in 2% glutaraldehyde in cacodylate buffer (24 h, subsequently transferred to 0.1 M cacodylate buffer) or 2% paraformaldehyde in PBS with 0.1 M sucrose (24 h at 4 °C, subsequently rinsed in PBS and stored with 0.2 M sucrose and 0.01 NaN3).

Morphological examinations

Morphological examinations were done using light microscopy (LM) and scanning electron microscopy (SEM).

Light microscopy observations were performed on whole mounted specimens prepared in 100% glycerol following a dilution series. Measurements and photographs were taken using an OLYMPUS DP73 camera mounted on an Olympus IX70 inverted compound microscope (OLYMPUS, Tokyo, Japan), equipped with CellSens Entry v.1.9 software.

Examinations were preferably done on glutaraldehyde-fixed specimens. After 60 min post-fixation in 1% osmium tetroxide (in 0.1 M cacodylate buffer), specimens were rinsed in demineralized water, dehydrated through a graded ethanol series, transferred to 100% acetone, and critical-point dried. Dried specimens were mounted on aluminum stubs, sputter-coated with platinum, and examined with a JEOL JSM-6335F field emission scanning electron microscope at the Natural History Museum of Denmark, University of Copenhagen.

DNA extraction and amplification

Specimens used for DNA extraction were preserved in 100% ethanol. DNA extractions were performed on single individuals using a Qiagen DNeasy Tissue and Blood Kit following protocols provided by manufacturer. Approximately 600 base pairs of the 16S ribosomal RNA (16S rRNA), and 1750 of the 18S ribosomal RNA (18S rRNA) were amplified using universal primers (see Di Domenico et al. 2014b). These markers were selected since 16S rRNA are widely recognized as useful markers for species delimitation (Silva et al. 2017), whereas 18S rRNA is the most widely use marker in high throughput sequencing studies, and therefore, it is useful for OTUs identification (Fonseca et al. 2014). Polymerase chain reactions (PCR) were performed following the manufacturer’s protocol for the Illustra PuReTaq Ready-To-Go PCR Beads, including 2 μl of template DNA and 1 μM of each primer. PCR reactions were carried out using a Bio-Rad S1000 Thermal Cycler. PCR protocol involved a 2-min initial denaturalization at 96 °C, followed by 40–45 cycles consisting of a denaturation step (94 °C, 30 s), annealing (38–55 °C, 30 s), and extension (72 °C, 1 min), ending with a final extension at 72 °C for 7 min. PCR products were resolved on a E-Gel 2% SYBR Safe agarose gels (Invitrogen) and purified with E.Z.N.A. Cycle Pure kit. Purified products were sent to Macrogen Europe Lab (the Netherlands) for sequencing. Sequences were assembled with Sequencer 4.10.1 (GeneCodes Corporation, Ann Arbor, MI, USA) and submitted to GenBank (GenBank accession numbers included in each species description).

Alignment and phylogenetic analyses

We based our phylogeny on the dataset on Di Domenico et al. (2014b), which we updated in the current study by including Saccocirrus minor Aiyar & Alikuhni, 1944 and Saccocirrus major. Therefore, our phylogeny included 10 species of Saccocirrus with all Atlantic-Mediterranean species available for molecular comparison. The species Pharyngocirrus sonomacus (Martin, 1977), Pharyngocirrus gabriellae (du Bois-Reymond Marcus, 1946), and Pharyngocirrus jouinae (Brown, 1981) were selected as outgroup (see Di Domenico et al. 2014b).

Sequences for each gene were visualized using Genious® 7.1.9. and aligned using the MAFFT online platform (Katoh and Standley 2013). Protein coding H3 was translated into amino acids and checked for indels and stop codons. We selected interactive refinement algorithm Q-INS-I (Katoh and Toh 2008) for alignments of 16S rRNA, 18S rRNA and 28S rRNA. The option ‘nwildcard’ was selected for in both cases as it does not designate missing data as gaps. Gene fragment H3 was constant in length and therefore trivial as sequences showed no variation in length, but we checked for directionality using the quick interactive refinement algorithm L-INS-I (Katoh et al. 2005). Individual gene datasets were concatenated using Genious ® 7.1.9.

Concatenated molecular datasets were analyzed using maximum likelihood (ML) and Bayesian inference (BI) methods. Maximum likelihood (ML) partitioned analyses were conducted using with RAxML version 7.2.8 (Stamatakis 2006). A default general time reversible, with corrections for a discrete gamma distribution (GTR + G), was specified for each partition. Nodal support was estimated via non-parametric bootstrap (Felsenstein 1985), with 1000 replicates. Bayesian analyses (BA) were performed using MrBayes version 3.2.5 (Ronquist and Huelsenbeck 2003). Prior to the analyses, jModelTest (Posada 2008) was used to infer the optimal evolutionary model for each gene, which was selected using the corrected Akaike information criterion (AICc) (Posada and Buckley 2004). GTR + G for 28S rRNA, 16S rRNA, and a proportion of invariable sites (GTR + I + G) were selected for 18S rRNA and H3.

Additional material for comparison

Type material from the following species were examined as comparative material: Saccocirrus alanhongi Bailey-Brock, Dreyer & Brock, 2003 (USNM POLY 1012494–1012497); Saccocirrus oahuensis Bailey-Brock, Dreyer & Brock, 2003 (USNM POLY 1012490–1012491); Saccocirrus waianaensis Bailey-Brock, Dreyer & Brock, 2003 (USNM POLY 1012492–1012493); Saccocirrus pussicus du Bois-Reymond Marcus, 1948 (ZUEC-Pol 14069–14099; ZMUC-Pol 2299); Pharyngocirrus eroticus Gray, 1969 (USNM POLY 36064–36066); Pharyngocirrus tridentiger Brown, 1981 (USNM POLY 62034–62040); Pharyngocirrus jouinae (Brown, 1981) (USNM POLY 62027–62033); Pharyngocirrus sonomacus Martin, 1977 (USNM POLY 53050–53052); and Pharyngocirrus gabriellae (du Bois-Reymond Marcus, 1946) (ZUEC-Pol 14053–14063).

Additionally, we examined newly collected specimens of Saccocirrus pussicus from Uruguay (Rodríguez et al. 2013); Pharyngocirrus gabriellae from Colombia (Lagos et al. 2018), Pharyngocirrus sonomacus from California, USA; Pharyngocirrus tridentiger and Pharyngocirrus jouinae from New South Wales, Australia; Pharyngocirrus krusadensis Alikunhi, 1942, from New South Wales, Australia and Phuket, Thailand; and Saccocirrus papillocercus from Black Sea and Gulf of Napoli, as well as Saccocirrus major, Saccocirrus parvus, and Pharyngocirrus goodrichi from Italy.

Results

Phylogenetic analyses

Bayesian consensus and maximum likelihood trees were congruent, except for the position of S. pussicus (Fig. 2). All species of Saccocirrus were recovered as a maximally supported monophylum (maximum likelihood bootstrap, MLB = 100; Bayesian posterior probability, BPP = 1) branching off as two sister clades. Clade A (MLB = 100, BPP = 0.99) included Saccocirrus major Pierantoni, 1907 and the undescribed Saccocirrus sp. 9 (Belize), Saccocirrus sp. (France), as well as Saccocirrus sp. 8 (Bermuda), while clade B (MLB = 100, BPP = 1) consisted of undescribed Saccocirrus sp. 10 from Mono Island (Solomon Island) sister to a supported clade (MLB = 100, BPP = 1) with S. slateri sp. nov., S. papillocercus, S. parvus, and S. pussicus. S. slateri sp. nov. was consistently recovered as sister taxon to S. papillocercus (MLB = 97, BPP = 1), while all the sequences included for S. parvus from the Canary Island and Italy were identical.

Fig. 2
figure 2

Phylogenetic relationships of Saccocirrus based on four molecular markers. Tree topology is based on Bayesian analyses, whereas nodal values correspond to maximum likelihood bootstrap (MLB) followed by Bayesian posterior probabilities (BPP). (a) and (b) indicate the clades (see results). Asterisk indicates nodes with full support

Full size image

Systematic account

Genus Saccocirrus Bobretzky, 1872

Saccocirrus slateri sp. nov.

http://zoobank.org/95ACD363-37AD-4582-90D0-464AB3AA8B81

Saccocirrus sp. 1 (Di Domenico et al. 2014b; Gusmão et al. 2016)

Type material. Holotype (ZMUC-Pol 2309). 21 mm long female. Los Abades, Tenerife. Intertidal gravel at the ramp of the small fishermen harbor, 28° 9′ 50.48″ N, 16° 25′ 54.14″ W. Col: M. Di Domenico and A. Martínez. December 22, 2010. Paratypes: seven whole mounted specimens (MZUSP 3124–3130, same locality and date as the holotype; DNA information: 16S rRNA (GenBank Acc. KF954445), 18S rRNA (GenBank Acc. KF954467).

Additional material. Two whole mounted females (MZUSP 3131–3132), from Charco Verde, La Palma. Exposed sandy beach, with volcanic sand, 0–1 m depth, 28° 34′ 19.39″ N, 17° 53′ 58.63″ W. October 14, 2011. Four specimens on SEM stubs (Worsaae’s lab scientific collection), same locality and date as the holotype.

Etymology. Species named in honor of surfer Kelly Slater, who has been crowned World Surf League Champion 11 times. The genus Saccocirrus is often found at exposed sandy beaches, where they are able to cope with the turbulence produced by the waves by moving horizontally along the beach slope.

Diagnosis. Body grayish-brown with dark pigmentation along the gut. Trunk robust, with 100–155 segments, last 7 smaller and achaetous. Prostomium rounded with two pigmented eyes and long filiform palps. Palp ampullae extended posteriorly to segment 3. Ventral muscular pharynx absent. Parapodia with three types of chaetae: (1) 1–2 long chaetae, robust and forked with equal prongs; (2) 2–3 medium spatulated chaetae, with 5–6 denticles; and (3) 2–3 short simple chaetae, with notched apex. Paired pygidial lobes with 11–22 transverse adhesive ridges. Females with bilateral ovaries in segments 24–93 (along 65 segments), each with 10–14 large oocytes. Pyriform spermathecae present. Males with bilateral seminal vesicles in segments 22–72 (along 50 segments), with paired hooked penises in segments 46–52.

Description. (Measurements provided from holotype; ranges from adult paratypes in parentheses). Body long, very robust and dark grayish-brown (Fig. 3a, e), 21 mm long (18–25 mm, n = 8) and 695 μm wide (460–730 μm, n = 80), up to 117 segments (105–155, n = 8). Last 7 segments (5–7 segments in adults, n = 8) smaller and achaetous.

Fig. 3
figure 3

Saccocirrus slateri sp. nov. light micrographs of male holotype a whole animal dorsal view; b anterior end in dorsal view; c details of the mouth; and paratypes showing d bilateral oocytes and spermatheca from a female; e glandular esophagus; f transition between esophagus and gut; g bilateral oocytes; h female ventral gonopore; i parapodium showing a bundle of chaeta; j male circular seminal vesicle and hooked penis; k female oocytes and spermatheca; l male hooked penis shape; m pygidium in dorsal view. ag adhesive glands, bp black pigmentation, oe esophagus, go glandular esophagus, gu gut, hp hooked penis, lc longest chaeta, mc medium chaeta, mo mouth, no nuchal organ, oc oceli, oo oocytes, pa palp, pp parapodium, pr prostomium, py pygidium, pyl pygidium lobe, s setae, sc short chaeta, se spermatheca, sv seminal vesicle, tr transition

Full size image

Prostomium rounded, 150 μm long (100–200 μm, n = 8) and 200 μm wide (150–200 μm, n = 3), with two dorsal pigmented eyes (Fig. 3b), 30 μm in diameter (20–35 μm, n = 2). Paired annulated palps, 2740 μm long (1850–2980 μm, n = 8), reaching to segment 12. Palp with internal channels connected to oval ampullae, 210 μm long (200–240 μm, n = 6), extending posteriorly along peristomium until segment 3. Palp surface without motile cilia but with scattered non-motile ciliary tufts (Fig. 4a, d). Paired oval ciliated nuchal organs, extending dorsally between the prostomium and peristomium (no; Figs. 3b and 4b), 30 μm long (25–35 μm, n = 3).

Fig. 4
figure 4

Saccocirrus slateri sp. nov. scanning electron micrographs of a anterior end in lateral view; b prostomium and peristomium in dorsal view; c mid-body segments in lateroventral view; d mouth and prostomium in ventral view; e gonoducts in ventral view; f ciliary tufts on the epidermal ciliary; g parapodium showing a bundle of chaeta; h medium chaeta; i short chaeta; j long chaeta; k pygidium in dorsal view. ag adhesive glands, dt denticle, go gonad opening, lc longest chaeta, mc medium chaeta, mo mouth, no nuchal organ, pa palp, pg prongs, pp parapodium, pr prostomium, py pygidium, pyl pygidium lobe, sc short chaeta, sp spatulated chaeta

Full size image

Peristomium 200 μm long (200–300, n = 3) and 250 μm wide (250–400, n = 3), with a ventral slit-shaped mouth (mo; Figs. 3c and 4d). Mouth cavity continues into glandular esophagus, extending along three trunk segments (Fig. 3f). Ventral muscular pharynx absent (Figs. 3b and 4d).

Paired cylindrical retractile parapodia projecting laterally on each trunk segment, except the last seven. Each parapodium up to 10 μm long (10–15 μm, n = 2) (pp; Figs. 3i and 4c, g), uniramous, and with three types of chaetae: (1) 1–2 long forked chaetae (lc; Figs. 3i and 4g, j), terminally bifurcated, 2 μm wide at the tip (n = 3) and twice as long than the medium chaetae, with two short, equal, terminal prongs (pg; Fig. 4j), with a denticulated area between them (dt; Fig. 4j); (2) 2–3 spatulated chaetae (mc; Figs. 3i and 4g, h), 5 μm wide terminally (n = 2) with two thick lateral teeth (te; Fig. 4h) separated by a thinner area provided with 4–6 smaller denticles (dt; Fig. 4h); and (3) 2–4 simple short chaetae (ss; Fig. 4g, i), thin (1 μm wide, n = 2) notched terminally (arrow head; Fig. 4i). Special penis structure called uncini sensu Jouin (1975), 40 μm long (n = 1), 10 μm wide (n = 1) (hp; Fig. 4j, l).

Epidermal ciliation absent except for few scattered ciliary tufts on peristomium (Fig. 4f), more abundant on the peristomium (Fig. 4b, d). A conspicuous ciliary band with abundant cuticular pores, presumably representing adhesive glands, present in each trunk segment (sg; Fig. 4k). Although these pores presumably connect to subepidermal glands, these glands could not be observed with light microscopy.

Pygidium with two robust lobes (pyl; Figs. 3m and 4k), 250 μm long (200–300 μm, n = 6) and 200 μm wide (180–250 μm, n = 3), each with 11–22 ventral adhesive ridges (ag; Figs. 3m and 4k). Anus opening terminally between pygidial lobes, without distinct ciliation (an; Fig. 3m).

Females with ca. 70 fertile segments (max. 72, n = 4) from segment 25 (24–26, n = 4) to 97 (79–99, n = 4). Each fertile segment with 10–14 mature oocytes (oo; Fig. 3d, g, k), 140–180 μm in diameter (110–195 μm, n = 4), at both sides of the gut. Fertile segments with a pair of ventral gonoducts (gc; Fig. 4c, e). Spermathecae present from segment 60–67 to segment 100–104 (se; Fig. 3d, k), all piriform, ca. 200 μm long (150–200 μm, n = 2). Males with 50 fertile segments (segments 22 to 72 (n = 1)). Paired circular seminal vesicles (sv; Fig. 4j,), 300 μm in diameter, at each side of gut; with terminal hooked shape penis (hp; Fig. 4j, l).

Distribution and ecology. The new species was collected in volcanic coarse sand at exposed reflective beaches and gravelly cinders at the slope of a little fishermen harbor. Accompanying fauna included the protodrilid annelids Lindrilus sp. and Claudrilus cf. hypoleucus (in Martínez et al. 2015), as well as tricladid flatworm Procerodes sp., several species of proseriate flatworms, nemertean Ototyphlonemertes sp., and the amphipod Ingolfiella cf. canariensis Vonk & Sánchez, 1991. Saccocirrus slateri sp. nov. is an omnivorous non-selective feeder, since unicellular algae, pieces of animals, organic debris, as well as microfibers (probably microplastic) have been found in the stomach of several individuals (Gusmão et al. 2016).

Taxonomical remarks. Saccocirrus slateri sp. nov. and S. heterochaetus are the only species of Saccocirrus described with a hooked shape penis. However, S. slateri sp. nov. differ in the presence of longer pygidial lobe with up to 22 pygidial papillae, absent in S. heterochaetus, and the longest chaeta with equal prong lengths, unequal in S. heterochaetus. Saccocirrus slateri sp. nov. morphologically resembles S. papillocercus, S. minor, S. major, and S. orientalis in the presence of forked chaetae with equal prongs. It differs from S. papillocercus in the presence of fewer fertile segments, more pygidial papillae, and the presence of a hooked shape penis; Saccocirrus slateri sp. nov. differs from S. major by its smaller size, the presence of fewer fertile segments, less oocytes per segment, and the presence of teeth in the median chaetae, which are absent in S. major. It differs from S. minor and S. orientalis in the larger size and shape of the pygidium. The differences to the remaining species of the family are summarized in Table 1.

Table 1 Meristic and morphometric characters of the species of Saccocirrus. Abbreviations: L, length; W, width; Max, maximum; N, number; Fem, Female; unk, unknown; ca. circa (about), segm, segment
Full size table

Saccocirrus parvus Gerlach, 1953

Delamare-Deboutteville et al. (1954), Fize (1963), Gerlach (1953), Villora-Moreno et al. (1991), Villora-Moreno (1997), and Westheide (1972).

Material examined from Canary Islands. One whole mounted, female, 13 mm long (MZUSP 3133). Punta Sama, Tenerife. Patches of coarse pumitic sand amongst boulders, 2–3 m depth, 28° 7′ 22.21″ N, 16° 27′ 29.64″ W, December 12, 2007. Col: A. Martínez and B. González. Two whole mounted specimens, immature (MZUSP 3134–3135), same locality, date and collectors as the female. DNA information: 18S rRNA (GenBank Acc. MK604223). One whole mount (MZUSP 3136) Playa de las Canteras, Las Palmas de Gran Canaria. Coarse sand in a sheltered bay, 3–4 m depth, 28° 8′ 38.56″ N, 15° 26′ 13.61″ W. May 8, 2008. Col: A. Martínez and K. Worsaae. One whole mount (MZUSP 3137). One specimen mounted on SEM stubs (Worsaae’s lab scientific collection). Charco del Palo, Mala, Lanzarote. 4–6 m depth. Coarse sand amongst rocks. 29° 4′ 59.75″ N, 13° 26′ 58.32″ W. October 13, 2011. Col: M. Curini-Galetti.

Emended diagnosis. Body brownish, 54–70 segments; last 5–10 smaller and achaetous. Prostomium rounded with two pigmented eyes, transverse ciliary band extends between palps and nuchal organs, long filiform palps. Ventral muscular pharynx absent. Parapodia with three types of chaetae: (1) 1–2 long chaetae, robust and forked with unequal prongs; (2) 2–3 medium long spatulated chaetae with 8–10 denticles; and (3) 2–3 short simple chaetae with notched apex. Paired pygidial lobes with 3–5 transverse adhesive ridges. Females with bilateral ovaries from segments 31 to 53 (along 22 segments), each with 5 to 6 large oocytes. Spermathecae absent.

Emended description. Body brownish (Fig. 5a), 12 mm long (5–13 mm, n = 5) and 280 μm wide (210–270 μm, n = 5), up to 70 segments (54–70, n = 5). Last 7 segments (5–7 segments, n = 5) smaller and achaetous.

Fig. 5
figure 5

Saccocirrus parvus light micrographs of fixed paratype a anterior end animal view; b chaetae; c pygidium in lateral view. lc longest chaeta, pa palp, oc oceli, py pygidium

Full size image

Prostomium rounded, 100 μm long (90–110, n = 5) and 150 μm wide (120–170, n = 5) with two dorsal pigmented eyes (oc; Fig. 5a), 8 μm in diameter (4–10 μm, n = 3). Short ciliary band extending transversally between palp insertion and nuchal organ (plc; Fig. 6h). Paired palps, annulated and hollow, 1300 μm long (720–1300 μm, n = 5), extending to segment 7 (6–9, n = 5) (pa; Fig. 5a). Palp ampullae oval, 20 μm long (n = 1). Palp without motile cilia, but with scattered non-motile ciliary tufts on the palp surface (Fig. 6b, g). Paired nuchal organ dorsal, oval and ciliated (no; Fig. 6a), 20 μm long (n = 1).

Fig. 6
figure 6

Saccocirrus parvus scanning electron micrographs a anterior end in dorsal view; b mouth and prostomium, ventral view; c mid-body segments in ventral view showing the gonoducts; d parapodium showing a bundle of chaeta in lateral view; e longest chaeta; f medium chaeta; g pygidium in ventral view; h palp ciliation; i patch of lateral ciliation on the prostomium. ag adhesive glands, an anus, dt denticle, fo forked chaeta, go gonad opening, lc longest chaeta, mc medium chaeta, mo mouth, no nuchal organ, pa palp, pg prongs, plc palp lateral ciliation, pp parapodium, pr prostomium, py pygidium, pyl pygidium lobe, s setae, sc short chaeta, sp spatulated chaeta

Full size image

Peristomium 350 μm long (300–350 μm, n = 2) and 150 μm wide (100–150 μm, n = 2), with a ventral slit-shaped mouth. Ventral muscular pharynx absent (Fig. 5a).

Parapodia cylindrical and retractile, up to 30 μm long (n = 1), projecting transversally from each trunk segment, absent on last 7 segments (5–7, n = 5) (pp; Fig. 6c, e). All parapodia uniramous with three types of chaetae: (1) 1–2 long forked chaetae (lc; Fig. 6c, d), thin and bifurcated, 2 μm wide at the tip (n = 2) and ca. twice longer than the medium chaetae; bearing two short, unequal, terminal prongs (pg; Fig. 6d) with a denticulated area between them (dt; Fig. 6d); (2) 2–3 spatulated medium chaetae (mc; Fig. 6c, e), 4 μm wide terminally (n = 2) with 8–10 denticles (dt; Fig. 6c, e); and (3) 2–3 simple short chaetae (ss; Fig. 6c, e), thin (2 μm wide, n = 2) and terminally notched (arrowhead; Fig. 6e).

Epidermal ciliation absent, except for prostomial ciliary bands and few, small ciliary tufts, more abundant on the peristomium (Fig. 6b, h). A transverse band with abundant cuticular pores, presumably representing adhesive glands, present in each trunk segment (sg; Fig. 6b).

Pygidium with two short lobes (pyl; Fig. 6f), 100 μm long (60–100 μm, n = 2) and 100 μm wide (100–120 μm, n = 3), each with five (3–5, n = 5) ventral adhesive ridges (ag; Fig. 6f). Anus opening terminally between pygidial lobes, without distinct ciliation (an; Figs. 5c and 6f).

Females with ca. 22 fertile segments (n = 1), from segment 31 (n = 1) to 53 (n = 1). Each fertile segment with 5–6 mature oocytes, 140–190 μm in diameter (n = 1), at both sides of the gut. Spermathecae absent, pairs of gonoducts present in each fertile segment (gc; Fig. 6c). Males were not found.

Distribution and ecology. The type locality of S. parvus is Marina di Pisa, Golf of Naples, Italy (Gerlach 1953), but it has been recorded from several beaches in the Mediterranean, in France, Italy, and Tunisia (Gerlach 1953; Delamare-Deboutteville et al. 1954; Fize 1963; Westheide 1972; Villora-Moreno et al. 1991; Villora-Moreno 1997). The Saccocirrus parvus from Canary Island has been collected in shallow water (depth 2–5 m) in semi-exposed well-sorted coarse sandy sediments. Fauna accompanying the new species included several species of annelids such as the nerillids Trochonerilla sp., Mesonerilla cf. luederitzi and Nerillidium sp., the psammodrilid Psammodrilus sp. (in Worsaae et al. 2018), and the protodrilids Meiodrilus sp. nov. 3 and Claudrilus cf. hypoleucus (in Martínez et al. 2015). Further details of the accompanying fauna in Lanzarote are provided in Martínez et al. (2019).

Taxonomical remarks. Saccocirrus parvus from Canary Island is genetically identical in gene fragments of 28S rRNA and H3 to S. parvus from Italy to S. parvus from Italy. Our specimens also fit well the original description of the species, although showing some differences, e.g., in the presence of more pygidial ridges (one in S. parvus from Italy, versus 3–5 in S. parvus from the Canary Islands) and unequal prongs in the longest chaeta (described as equal in S. parvus). Compared to other species of Saccocirrus, S. parvus from the Canary Island resembles S. pussicus, S. heterochaetus, and S. minor in size, number of segments, and pygidial length size. Saccocirrus parvus differs from S. slateri sp. nov. by the absence of spermathecae.

Discussion

Geographic distribution of Saccocirridae

Saccocirridae is a relatively species-poor family with 23 described species (Di Domenico et al. 2014b, c). The family was traditionally divided in two groups, so-called krusadensis and papillocercus, which were believed to be restricted to the Indo-Pacific and Atlantic oceans respectively. However, recent phylogenetic analyses erected these groups as two separated genera and showed that their presumed geographical restrictions reflected previous sampling bias rather than real biogeographical patterns (Di Domenico et al. 2014b). Today, both Saccocirrus and Pharyngocirrus are considered widespread in tropical and temperate seas (Di Domenico et al. 2014b).

In contrast to other interstitial annelids, species of Saccocirridae exhibit comparatively large distribution areas, probably facilitated by the presence of larvae (Curini-Galletti et al. 2012). Therefore, the discovery of S. parvus in our samples from the Canary Island is not surprising, despite it is the first record of the species outside the Mediterranean. Large distribution areas have also been found in ongoing molecular studies on the genus Pharyngocirrus. Sequences of 16S rRNA gene fragments indicate that P. gabriellae, originally described from Brazil (MDD, unpublished data), might occur along the entire Atlantic and Pacific coasts of South America, as well as the Canary Islands (see Martínez et al. 2019). However, whether these high genetic similarities reflect the presence of gene flow amongst distant populations of saccocirrids or are an artifact derived from the election of too conservative markers demands more exhaustive studies.

In contrast to these high molecular similarities, morphological traits seem to vary much more than previously reported (Brown 1981; Jouin and Gambi 2007). Species of saccocirrids are mainly diagnosed using the chaetal morphology, the arrangement of the gonads, and the shape and number of pygidial adhesive ridges (Brown 1981; Jouin and Gambi 2007). However, our results here, as well as the examination of type and newly collected material, suggested that these traits might vary more than previously expected. These variations could be linked to different developmental stages, which might affect the arrangement of the gonads or the size of the pygidium. If this is the case, this variation could potentially be understood by sampling the same localities at different times of the year to get complete developmental series (Sasaki and Brown 1983; Westheide 2008). However, variation in chaetal morphology is more difficult to explain in this way, since the shape of chaetae is determined by the temporal and spatial modification of the microvilli pattern of the chaetoblast during the chaetogenesis (Hausen 2005) and just seems more plastic than previously reported.

While these observations are preliminary, they collectively highlight the importance of an integrative approach in the future studies of the family, preferably including large number of specimens collected at different periods of the year and combining morphological observations with several independent molecular markers. This approach has been already followed in several studies done in other groups of interstitial annelids, such as Protodrilidae (Di Domenico et al. 2013; Martínez et al. 2013), Nerillidae (Worsaae et al. 2019), Psammodrilidae (Worsaae et al. 2018), and Parergodrilidae (Struck et al. 2017), which collectively suggests that the evolutionary histories of each lineage of these ancient lineages is unique, and more complex than any simple single general model can predict (Martínez et al. 2015; Gonzalez et al. 2017; Ramey-Balcı et al. 2018; Worsaae et al. 2019). This is not surprising, as this has also been shown to be the case in many other interstitial lineages of metazoans (Fontaneto 2011; Curini-Galletti et al. 2012; Jörger et al. 2014; Sánchez et al. 2016).

Habitat preferences of Saccocirridae

While both cosmopolitan, it has been proposed that each genus of Saccocirridae exhibit distinct habitat preferences with members of the Pharyngocirrus preferring sheltered sandy habitats, in contrast to the specialization of exposed sandy beaches showed by members of Saccocirrus. Although this hypothesis remains to be explicitly tested, it was congruent with the results obtained in the last phylogenetic analysis of the family (Di Domenico et al. 2014b).

These differences in habitat preferences seem to be related to the presence of different trophic guilds and morphological traits in each clade. Species of Pharyngocirrus are specialized to selectively graze on biofilms of bacteria-diatom using their muscular ventral pharyngeal bulbous and ventral ciliation around the mouth. In contrast, members of Saccocirrus are non-selectively microphagous, as indicated by the presence of sand grains and even microfibers in the gut content of many specimens (du Bois-Reymond 1948; Di Domenico et al. 2009; Jouin and Gambi 2007; Gusmão et al. 2016). Since most of the species of Saccocirrus have been found in exposed beaches, where the high turbulence prevents the deposition of organic matter, it has been proposed that palps, rather than pharynx, play a more decisive role in feeding in these species (Di Domenico et al. 2014a, b). Since food is more difficult to locate and catch under turbulent conditions, palps might be essential for locating and reaching food particles in flow. High rates of ingestion, efficient digestion and assimilation, and subsequent energy conservation are common adaptation found in other animals dwelling in sandy beaches (McLachlan and Defeo, 2017).

However, our findings provide new information on habitat preferences of Saccocirridae species, suggesting that even within Saccocirrus different species might be adapted to specific grain sizes and turbulence. These adaptations might involve changes in the number and position of adhesive glands, as well as body and palps size (Martin 1978; Di Domenico et al. 2014b). Saccocirrus slateri sp. nov. has been exclusively found in very exposed, coarse sediments, following the general pattern of the genus. Two features, a large species with well-developed palps, are found in other annelids adapted to this energetic environment. For example, the protodrilid Protodrilus albicans Jouin, 1970 (Jouin 1970; Martínez et al. 2018) is one of the largest species in the genus and undulates its long body to swim back into the sediments after being suspended in the water column by turbulence; once in the bottom, it uses its ciliated palps to gather suspended particles of organic matter. Saccocirrus slateri sp. nov. does not swim but instead curls around heavy sand grains, adhering to them using both pygidial and segmentally arranged adhesive glands (Di Domenico et al. 2014a). The large body of the species might therefore provide a better grip to the large particles integrating the coarse sediments that the species inhabits. In contrast, S. parvus has been found in subtidal semi-exposed environment, a habitat resembling those where species of Pharyngocirrus are typically found. S. parvus is also one of the smallest species in the genus, possessing comparatively short palps. Its small body more likely represents an evolutionary secondary reduction in size, given the position of S. parvus in our phylogenies, consistently recovered nested amongst larger species of Saccocirrus. The available information suggests that this reduction might facilitate the exploration of smaller interstitial spaces found in the semi-exposed environments where the species was recorded, favoring the access to interstitially deposited organic matter (Westheide 1987).

Extant sandy beach environments may be considered recent sedimentary deposits, dating only from the Holocene. Meanwhile, fossil records of organisms of the Annelida group date back to the beginning of the Cambrian Period (~ 535 m) (Parry et al. 2014). Considering the ancient evolutionary history of the interstitial annelids (Andrade et al. 2015; Struck et al. 2015), we hypothesize that sediments in the beach environments may have been conquered by Saccocirrus for a relatively short time (Di Domenico et al. 2014a, b). Many of the morphological adaptations and behaviors that characterize these sandy-beach animals may be driven by the instability of the substratum coupled with wave action (Di Domenico et al. 2013, 2014c; McLachlan and Defeo 2017). Thus, burrowing behavior or adhesive glands to attach to the sand grains of high-energy sandy beaches may be essential to not be swept away by incoming waves and swash (Martin 1978; Di Domenico et al. 2014c).