Introduction

Platyctenes (order: Platyctenida) are dorso-ventrally compressed ctenophores. Most ctenophores from this order have secondarily lost the characteristic ctene rows of the order in the adult stage or may not have them at all (Komai 1922). There is only one genus of benthic ctenophores that has ctene rows as an adult – Ctenoplana Korotneff, 1886. The order Platyctenida is composed of five different families (Coeloplanidae, Ctenoplanidae, Tjalfiellidae, Lyroctenidae, and Savangiidae), with the Coeloplanidae being the richest in species (Mills 2014). Although Platyctenes resemble nudibranchs and flatworms, they can be distinguished by their thread-like, branched, retractile tentacles. Most species from the family Coeloplanidae are commensal epibionts and live in close association with diverse groups of invertebrates (e.g., octocorals, echinoderms, ascidians) and seaweeds (except C. meteoris Thiel, 1968, which dwells on the sediment, C. mesnili Dawydoff, 1938 and C. tattersalli Devanesen and Varadarajan, 1942, which were collected from the plankton). Yet, the nature of the symbiosis has never been studied and thus is unclear. The degree of specificity between benthic ctenophore species and their hosts is used as a useful taxonomic character. Unlike planktonic ctenophores that reproduce sexually, benthic ctenophores are also known to reproduce asexually by excising fragments from their body that can regenerate and form an entire animal. While benthic ctenophores are highly modified compared to their planktonic ancestors of the order Cydippida (Podar et al. 2001), they retain many basic features of those ancestors, especially in their characteristic cydippid larval form.

Traditionally, the taxonomy of benthic ctenophores relies on several morphological characters, including the identity of their hosts. These characters include coloration and pattern, number and arrangement of aboral papillae, maximal length along the tentacular axis, location and shape of tentacular sheath, presence of oral lappets and an oral groove (Gershwin et al. 2010; Matsumoto 1999; Matsumoto and Gowlett-Holmes 1996; Rankin 1956). Some of these characters have been shown to be controversial as they tend to change pattern, shape, and number between live versus fixed specimens, even in the same individual, and also depend on the individual state (e.g., relaxed versus contracted) and environmental conditions (e.g., oxygen level in the surrounding water). For example, Gershwin et al. (2010) reported that live specimens of Coeloplana mellosa have 20 aboral simple papillae, while fixed specimens have more than 100. In addition, although color pattern has been useful for distinguishing between species of benthic ctenophores, these pigments are lost post fixation, making it challenging to describe fixed museum samples.

To date, there are 29 described species (including the two spp. varieties of C. agniae and the three spp. varieties of C. gonoctena) in the family Coeloplanidae. These species belong to two different genera, Coeloplana Kowalevsky, 1880 and Vallicula Rankin, 1956 (Mills 2014). Coeloplana is more speciose compared to the other genus, Valiculla, which contains only one species. Vallicula differs from Coeloplana mainly by the presence of an additional cross-piece in the tentacle sheath, making this structure resemble an anchor (Rankin 1956).

Prior to this study, there were only two known species of benthic ctenophores from the Red Sea: (1) Coeloplana bannwarthi Krumbach, 1933 which was described from the Gulf of Suez and was found on the spines of the sea urchin Diadema setosum (Krumbach 1933) and also reported from the Jordanian coast of the Gulf of Aqaba (Hulings 1989); and (2) Coeloplana metschnikowii Kowalevsky, 1880, described from the Suez canal (near Al-Tur), living on the seagrass Zostera.

The present study offers a comprehensive description of coeloplanid taxonomic identification, the first survey of the benthic ctenophore fauna in the Red Sea and a first compilation of the species list in the area.

Materials and methods

Collection and observation

Benthic ctenophore species were collected during 2012–2013 from various invertebrates and algae by scuba diving in three sampling sites along the Israeli shore of the Gulf of Aqaba (29°30′ N, 34°56′ E): (1) North Oil Jetty – NOJ, (2) Underwater Observatory – UO, (3) Interuniversity Institute for Marine Sciences - IUI (Fig. 1). Some specimens were collected together with their hosts, and later dislodged from them in the lab using a pipette with a gentle stream of water. In other cases, the ctenophores were dislodged from their host in situ using a small pipette. Sampling was done mainly at night, as most benthic ctenophore species were easier to locate due to their extended tentacles and better contrast with the dark background water. Each ctenophore encountered was photographed in situ using a Nikon D-90 camera enclosed in a Hugyfot (TM) housing. For each specimen collected, the date, site, depth, and host were registered. Due to the expected difficulties inherent in morphological examination of fixed material, each collected live specimen was inspected in the lab and photographed using a high-resolution camera mounted on a stereoscope. The identification of species was conducted based on all existing Coeloplanidae literature.

Fig. 1
figure 1

Map of collection sites: North Oil Jetty (NOJ), Underwater Observatory (UO), Interuniversity Institute for Marine Sciences (IUI)

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Fixation

Following identification and documentation, animals were given museum catalog numbers and were preserved in 4 % (v/v) formalin in filtered seawater and later transferred to 70 % EtOH. Prior to fixation, ctenophores were relaxed, allowing them to fully extend their bodies, using 2.5 % (m/v) MgCl2 in filtered sea-water. Specimens were in good state even after 2 years post fixation. In addition, some whole specimens were preserved in 95 % EtOH for future molecular research, as well as in 2.5 % Glutaraldehyde for electron microscopy.

Scanning electron microscopy (SEM)

Ctenophore samples preserved in 2.5 % Glutaraldehyde underwent dehydration through a graded series of ethanol (30 to 99 %), critically point-dried with liquid CO2 and then gold-coated. Ctenophores were viewed under a JEOL JSM 840a SEM (Tokyo, Japan) at 25 kV.

Results

Systematic account

Phylum Ctenophora Eschscholtz, 1829

Class Tentaculata Eschscholtz, 1825

Order: Platyctenida Bourne, 1900

Family: Coeloplanidae Willey, 1896

Diagnosis: Creeping or sessile ctenophores, compressed in oral-aboral axis. Adults lack comb rows, which are present only in the cyddipid larval stage. Two pinnate tentacles with numerous tentilla branching on their ventral side, with colloblasts on their surface. Each tentacle extends from and retracts into a tentacular sheath. Meridional canals are branched and anastomosed. Pharynx is permanently everted. Body length < 6 cm. When resting, the outline is roughly oval, longer in the tentacular axis. However, the shape is extremely fluid, changing by local advances and retractions of the margins. When creeping, the animal becomes elongated and flatworm-like. Statocyst located on the aboral side near the midpoint of the body, appearing as a tiny white granule. Aboral papillae, which are out-growths of the gastro-vascular canals, are usually present. An oral groove along the tentacular axis may be present on the oral side and may be used to form temporary “chimneys”. The family comprises two genera: Coeloplana and Vallicula.

Genus: Coeloplana Kowalevsky, 1880

Diagnosis: Possess the features of the family. Coeloplana species have high specificity to their hosts. Tentacle sheath is flask-shaped in aboral view.

Coeloplana loyai Alamaru and Brokovich, n. sp.

Material examined: Eilat, Israel, Gulf of Aqaba, Red Sea (Fig. 1). Holotype: TAU-CO35558 (sampled on September 5th 2012 at 6 m depth from the aboral side of the coral Herpolitha limax Esper, 1797 at the reef in front of the underwater observatory (UO), preserved in 4 % formalin and later transferred to 70 % EtOH), Paratype 1: TAU-CO35559 (sampled on September 5th 2012 at 6 m depth from the aboral side of H. limax coral at the reef in front of the underwater observatory, preserved in 4 % formalin and later transferred to 70 % EtOH). Paratype 2: TAU-CO35560 (sampled on September 5th 2012 at 6 m depth, several specimens on their coral host, H. limax, fixed in 4 % formalin). All types were deposited at The National Collections of Natural History at Tel-Aviv University, Israel. In addition, ten specimens collected by A.A. and E.B. were inspected for size measurements. These samples were fixed in 70 % EtOH and later sacrificed for molecular analysis.

Type locality: Eilat, Israel, Gulf of Aqaba, Red Sea (Fig. 1): 29°30′13.27″N, 34°55′07.50″E.

Etymology: Named in honor of Prof. Yossi Loya, a world-renowned researcher of coral reefs.

Diagnosis: Found on mushroom corals of the genera Herpolitha and Ctenactis. Transparent cream-colored with light-brown spots, making it look mottled. Unique ruff-like structure surrounding the statocyst on the aboral side may be seen using a stereoscope. Oral groove and oral lappets not apparent.

Description: This is a relatively large Coeloplana species, with individuals collected ranging in maximal length between 30–40 mm along the tentacular axis (n = 10). Characteristic shape, with the features of the family. Transparent cream-colored with light-brown spots composed of tiny granular pigments, making it appear mottled (Figs. 2 and 3). When the tentacles are retracted into their tentacle sheaths, they form a white line along the tentacular axis, seen as a black line when backlit (Fig. 2b, c). This species was not observed to extend its tentacles in situ either during the day or at night. No apparent papillae observed on the aboral side in situ. However, in the laboratory, after prolonged duration in still water, papillae were observed along the meridional canals (Fig. 3b). Coeloplana loyai n. sp. has a unique structure resembling a ruff that surrounds the sensory organ (Figs. 2c and 3). The length of the ruff ranges from 4.5–8.2 mm along the tentacular axis (n = 4 measured ctenophores). This structure was present in all specimens examined and is not known from other benthic ctenophore species. The statocyst resembles a granular mass, with a diameter of ∼55 μm, located at the center of the aboral sensory organ (Fig. 4a). Due to the transparency of this species, the meridional canals and the reticulations of the gastro-vascular system are clearly visible (Figs. 2c, 3b and 4). As in ordinary planktonic ctenophores, there are eight meridional canals: two tentacular canals, which later split into two canals that surround the tentacle sheath (sub-tentacular canals); and four sub-pharyngeal canals, which are located exteriorly to the sub-tentacular canals (Fig. 3). Oral groove and lappets not present. Extremely thin and delicate species with a tendency to tear when handled.

Fig. 2
figure 2

Coeloplana loyai n. sp. from the Gulf of Aqaba, Red Sea. a C. loyai n. sp. in situ on its host, the stony coral Herpolitha limax. The tentacular axis of each individual ctenophore has been highlighted in opaque white line in order to visualize the density of ctenophores on their coral host. b Close-up of two specimens showing the color pattern as seen in situ. The outline of one individual is marked in grey. c Aboral view of a live specimen ∼40 mm in length (scale bar: 10 mm) photographed using rear light transmitted through the specimen. The ctenophore was sedated prior to photographing using cold MgCl2 solution in a small petri dish

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Fig. 3
figure 3

Aboral view of a live specimen of C. loyai n. sp. a Coeloplana loyai n. sp. showing the unique ruff-like structure surrounding the aboral organ (marked by a black arrow) and the arrangement of female gonads (FG) full of eggs along the meridional canals (scale bar: 1 cm). b Close-up showing the canals of the gastro-vascular system, including the tentacular canal (TC), sub-pharyngeal canal (S.PH.C), sub-tentacular meridional canal (S.T.C) ), papillae (P), and tentacles enclosed inside tentacle sheaths (TS) (scale bar: 25 mm). Sample preparation was the same as in photograph 2c. Papillae were not observed in normal conditions and protruded only after prolonged duration in a low-oxygenated environment

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Fig. 4
figure 4

Coeloplana loyai n. sp. a Aboral view of the sensory organ showing the granular statocyst in the middle, surrounded by the pole plates (scale bar: 400 μm). b Aboral view of the periphery of the animal showing the reticulations of the gastrovascular system, which later converge towards the central area into the main gastrovascular canals. (scale bar: 3 mm). c Aboral view showing developed testes (TE) (scale bar: 3 mm). d Embryos or fertilized eggs discovered in a specimen in September 2012 surrounded by a membrane (scale bar: 200 μm)

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Reproduction/developmental biology: In samples collected in September 2012, both female and male gonads were observed (Figs. 3a, b, and 4c). The ovaries develop along the meridional canals and the sperm masses develop on both sides of the tentacular sheath and are brownish in color (similar structures were described by Komai 1922). In some of these September specimens, brooded eggs were found on the edges of the oral side, still surrounded by the egg membrane (Fig. 4d).

Distribution and habitat: Coeloplana loyai was observed in all three sampling sites between 1–20 m (Fig. 1). The ctenophores were found on the aboral side of the stony corals H. limax and C. echinata, but were more abundant on H. limax (Fig. 2). In most cases, a few individual ctenophores were observed on each coral host (Fig. 2). On rare occasions, a density of over 20 ctenophores was seen on one fungid host. Although the holotype and paratypes were not collected from Ctenactis, several other specimens were collected from C. echinata corals and inspected for their detailed morphology. During this research, we examined over 200 fungid polyps belonging to the most abundant mushroom coral species of the Gulf of Aqaba. However, C. loyai was found only on two species of mushroom corals and was relatively rare.

Ecology and behavior: This benthic ctenophore was never observed to occur on the oral side (upper side) of the mushroom coral polyps. Moreover, this species was not observed to extend its tentacles for feeding either during the day or at night (even though its tentacles were clearly visible when inspected in the lab). Brownish pigmentation raised an hypothesis that this species may host algal symbionts, as does its coral host. However, inspection of specimens under UV light clarified that this was not the case.

Remarks: Although the general structure of C. loyai n. sp. resembles that of other Coeloplana species, this new species is distinguished from other Coeloplana species by several unique characters: (1) coloration pattern: transparent cream-colored with light-brown spots composed of tiny granular pigments, making it appear mottled; (2) a unique ruff-like structure surrounding the aboral sensory organ, which is not known in other Coeloplana species; (3) lack of aboral papillae when observed in situ or in well aerated conditions; (4) its host, C. loyai n .sp., is the first ctenophore species to be described from the stony corals H. limax and C. echinata belonging to the family Fungiidae. The association of benthic ctenophores with stony corals was first reported in Hoeksema et al. (2013). In addition to C. loyai described herein, Hoeksema et al. reported the association of an undescribed ctenophore species found on Pleuractis moluccensis and Fungia fungites from Payar Island (Malaysia), Cycloseris costulata from Sabah (Malaysia) and Lithophyllon repanda from South Sulawesi (Indonesia). Unfortunately, the fixation state of the samples from Malaysia and Indonesia made it impossible to describe them morphologically. Therefore, at this stage we cannot assign a valid name for them and it is not clear if these individuals are in fact C. loyai.

Coeloplana yulianicorum Alamaru, Brokovich and Loya, n. sp.

Material examined: Eilat, Israel, Gulf of Aqaba, Red Sea (Fig. 1). Holotype: TAU-CO3571 (sampled on September 3rd 2012 at 8 m depth from a colony of the soft coral Sarcophyton glaucum at the reef in front of the underwater observatory (UO). Preserved in 4 % formalin and later transferred to 70 % EtOH). Paratypes: TAU-CO35572, TAU-CO35573, TAU-CO35574 (sampled on September 3rd 2012 at 8 m depth from a colony of the soft coral Sarcophyton glaucum at the reef in front of the underwater observatory. Preserved in 4 % formalin and later transferred to 70 % EtOH). Two additional samples (TAU-CO35575, TAU-CO35576), each containing five ctenophores, were also deposited. All types were deposited at The National Collections of Natural History at Tel-Aviv University, Israel.

Type locality: Eilat, Israel, Gulf of Aqaba, Red Sea (Fig. 1): 29°30′13.27″N, 34°55′07.50″E.

Etymology: Named after the parents of A.A., Yulian and Anica Alamaru.

Diagnosis: Found on the soft coral Sarcophyton glaucum. Color creamy-white with whitish spots and scattered pinkish-reddish dots. Twenty- four dorsal papillae in an X shape (5-7-7-5) surrounding the aboral sense organ. Another 12 papillae, six flanking each tentacular bulb, three on each side. Oral lappets and oral groove present.

Description: Color creamy-white with whitish spots (∼500 μm in maximal diameter) and scattered pinkish-reddish dots (∼85 μm in diameter), across the entire body on the aboral side (Fig. 5a, b, c). Individuals collected ranged in maximal length between 10.5 and 32 mm (n = 23) along tentacular axis. Twenty-four dorsal papillae forming an X shape (asymmetric with 14 on one side and 10 on the other side) were identified in live specimens surrounding the sense organ (Fig. 5c, d). Another 12 papillae were identified, six flanking each tentacular bulb, three on each side (Fig. 5c). On the aboral side, pole plates are not visible, but the sense organ and the statocyst can be clearly seen under a light microscope (Fig. 5e). Reticulations of the gastrovascular system are clearly seen, with radial canals anastomosed, forming a network along the periphery of the animal. Statocyst diameter ∼55 μm. Gonads were not observed. On the oral side, lappets and oral groove present and well defined (Fig. 5f). Pharyngeal folds are seen inside the oral opening (Fig. 5f). After fixation in Formalin, the specimens become opaque-cream color (Fig. 5f), making the internal structure (e.g., gastrovascular system) and papillae challenging to observe, although the fixation was generally good (not like planktonic ctenophores that usually dissolve).

Fig. 5
figure 5

Coeloplana yulianicorum n. sp. from the Gulf of Aqaba, Red Sea. a Ten specimens of C. yulianicorum n. sp. in situ on their host, the soft coral S. galucum, b in situ close-up of a single specimen of C. yulianicorum n. sp. showing the milky white spots and pink dot markings. c Aboral view of live specimen showing the x-shaped arrangement of papillae (scale bar: 1 cm). d Close-up of the aboral side showing the simple papillae protruding (scale bar: 500 μm). e Aboral view of a live C. yulianicorum n. sp. showing the aboral sense organ with the statocyst in the middle and the meridional canals (scale bar: 500 μm). f Oral view of a formalin-fixed specimen showing the oral lappets (OL) and oral groove (OG) parallel to the tentacles axis (scale bar: 0.5 cm)

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Distribution and habitat: This species is an epibiont found on the inner margins (underside) of the umbrella on colonies of the soft coral Sarcophyton glaucum down to a depth of at least 30 m. (Fig. 5a, b). It is one of the most common species of Coeloplana found in the Gulf of Aqaba, Red Sea.

Ecology and behavior: Usually, we found several individuals aggregating on each colony (feasibly a result of asexual reproduction, although not observed) (Fig. 5a). Coeloplana yulianicorum has two pinnate tentacles, extended both at night and during the day, to capture zooplankton (Fig. 5a). Tentacles seem to be extended farther at night, and may reach a length of at least 20 times that of the maximal animal length.

Remarks: This species differs from all other Coeloplana species by its coloration pattern and the identity of its host. To date, four other Coeloplana species are known to live on Sarcophyton: (1) C. lineolata Fricke, 1970, which has a unique patterns of lines parallel to the tentacular axis (2) C. punctata Fricke 1970, which is characterized by small green-yellowish dots, (3) C. wuennebergi Fricke, 1970, which has large red-violet spots, and (4) C. mellosa Gershwin, 2010, which in fact may be the same species as C. punctata (see discussion). Out of the four, it appears to be closest to C. punctata, which is commensal on Sarcophyton colonies and was described by Fricke (1970) from Madagascar. However, C. yulianicorum differs from C. puncatata Fricke, 1970 by the color of the dots (pink-red in C. yulianicorum versus green-yellowish in C. punctata), the presence of relatively large (∼500 μm) whitish spots in C. yulianicorum and by the arrangement and number of papillae (Fig. 5a, b, c, Table 1).

Table 1 All known species of the family Coeloplanidae, including their morphological characters (size, color, aboral papillae, tentacle sheath location), based on the original descriptions of the species. In cases where additional descriptions, different from the originals, were reported in the literature, we added a citation next to the addition. Geographical distribution and identity of hosts is compiled based on all available literature. *As stated in the text, the validity of this species should be re-assessed
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Coeloplana huchonae Alamaru, Brokovich and Loya, n.sp.

Material examined: Eilat, Israel, Gulf of Aqaba, Red Sea (Fig 1). Holotype: TAU-CO35566 (sampled on September 6th 2012 at 6 m depth from a colony of the soft coral Dendronephthya hemprichi at the reef in front of the underwater observatory (UO). Preserved in 4 % formalin and later transferred to 70 % EtOH. Paratypes: TAU-CO35567, TAU-CO35568, TAU-CO35569 (sampled on September 6th 2012 at 6 m depth from a colony of the soft coral Dendronephthya hemprichi at the reef in front of the underwater observatory. Preserved in 4 % formalin and later transferred to 70 % EtOH). A sample of several ctenophores with their coral host (TAU-CO35570) was also deposited. All types are deposited at The National Collections of Natural History at Tel-Aviv University, Israel.

Type locality: Eilat, Israel, Gulf of Aqaba, Red Sea (Fig. 1): 29°30′13.27″N, 34°55′07.50″E.

Etymology: Named in honor of Dr. Dorothee Huchon, phylogeny and molecular evolution expert, for her endless support, encouragement, and friendship to A.A.

Diagnosis: Found on the soft coral Dendronephthya hemprichi (Fig. 6a). Color opaque white with deep scarlet elongated (though small) markings (Fig. 6b, c). No apparent aboral papillae, no oral groove or oral lappets present.

Fig. 6
figure 6

Coeloplana huchonae n. sp. (on the left column) vs. Coeloplana anthostella, (on the right column, adopted from Song et al. (2011). a Coeloplana huchonae n. sp. from the Gulf of Aqaba, Red Sea in situ on its host, the soft coral Dendronephthya hemprichi. White arrows indicate ctenophores on the stem of the coral colony. b Aboral view of a live specimen of C. huchonae n. sp. showing the tentacles (T) and the tentacle sheath (TS) (scale bar: 1 mm). c Close-up of the aboral sense organ of C. huchonae n. sp. showing the statocyst in the middle (white granular mass indicated by the white arrow) (scale bar: 1 mm). Note the tiny pink elongated markings. d Coeloplana anthostella on a colony of Dendronephthya spinulosa collected in Japan (scale bar: 2 mm). White arrows indicate ctenophores on the stem of the coral colony. e Live specimen of C. anthostella, arrows show the tentacle sheaths (scale bar: 1 mm). f Close-up of the aboral sense organ showing the statocyst of C. anthostella (scale bar: 0.5 mm). Note the difference in size and shape of the pink markings of C. huchonae n. sp. from the Red Sea versus C. anthostella from Japan.

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Description: Color of live specimens is opaque white with deep scarlet elongated (though small) markings (Fig. 6b, c). Post fixation the ctenophores become opaque-yellowish with brown tiny markings. No apparent papillae on aboral side (Fig. 6b). Individuals collected ranged in size between 4.6-4.8 mm (n = 4) maximal length along tentacular axis. Oral groove not present. Oral lappets visible on oral side, easily seen in fixed animals. When tentacles are retracted inside the tentacular sheath, they resemble white, flask-shaped pouches that are easily recognized on the coral host (Fig. 6a, b). When tentacles are extended, the tentacular sheaths form chimney-like structures (Fig. 6b). The statocyst (∼45 microns in diameter) is composed of small granular structures and is located in the middle of the sense organ, between two fleshy plate-like structures (the pole plates) (Fig. 6c). Radial canals easily seen along the periphery of the animal. Gonads not seen.

Distribution and habitat: Found on the stems and branches of the soft coral Dendronephthya hemprichi (Fig. 6a). We found this species only on one large colony of D. hemprichi. On this colony, the ctenophores reached a density of ∼100 specimens on a 20 cm stem.

Ecology and behavior: At night, the extended tentacles form a mesh between the coral branches. In the lab, the ctenophores extended their tentacles and rolled into a ball, making it challenging to measure and photograph them.

Remarks: This species differs from all other Coeloplana species by its coloration pattern and the identity of its host. Currently, there are only two other species of Coeloplana that were reported to live on Dendronephthya colonies: Coeloplana bocki Komai, 1920 and Coeloplana anthostella Song and Hwang, 2010. Coeloplana bocki differs remarkably from the new species found in the Gulf of Aqaba by its color pattern (Table 2). However, Coeloplana huchonae n. sp. resembles C. anthostella described by Song and Hwang (2010) from Korea. Nevertheless, the new species’ scarlet markings are less than half the size of C. anthostella’s markings and are elongated in shape, versus a star-like shape in the Korean species (Table 2, Fig. 6c, e, f). Moreover, the two species are found on different hosts (D. spinulosa and others in Korea versus D. hemprichi in the Red Sea) (Table 2, Fig. 6a, d).

Table 2 Comparison between species of Coeloplana that live on Dendronephthya corals: Coeloplana huchonae n. sp., Coeloplana anthostella Song and Hwang, 2010 and Coeloplana bocki Komai, 1920
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Coeloplana fishelsoni Alamaru, Brokovich and Loya, n. sp.

Material examined: Eilat, Israel, Gulf of Aqaba, Red Sea (Fig. 1). Holotype: TAU-CO35561 (sampled on September 6th 2012 at 6 m depth from a colony of the soft coral Xenia umbellata at the reef in front of the underwater observatory (UO). Preserved in 4 % formalin and later transferred to 70 % EtOH). Paratypes: TAU-CO35562, TAU-CO35563, TAU-CO35564 (sampled on September 6th 2012 at 6 m depth from a colony of the soft coral Xenia umbellata at the reef in front of the underwater observatory. Preserved in 4 % formalin and later transferred to 70 % EtOH). Additional sample of two ctenophores on their coral host, X. umbellata (TAU-CO35565) were also deposited. All types are deposited at The National Collections of Natural History at Tel-Aviv University, Israel.

Type locality: Eilat, Israel, Gulf of Aqaba, Red Sea (Fig. 1): 29°30′13.27″N, 34°55′07.50″E.

Etymology: Named in the honor of the late Prof. Lev Fishelson, a world-renowned marine biologist who passed away in 2013.

Diagnosis: Found on the soft corals Xenia umbellata (Fig. 7a) and Paralemnalia sp. (Fig. 7b). Color opaque white with white dot markings (Fig. 7c, d). No apparent papillae on aboral side.

Fig. 7
figure 7

Coeloplana fishelsoni n. sp. from the Gulf of Aqaba, Red Sea. a One Coeloplana fishelsoni in situ on its host, the soft coral Xenia umbellata b One Coeloplana fishelsoni n. sp. in situ with its tentacles extended on its host, the soft coral Paralemnalia sp. c Aboral view of a live specimen on the stem of the Xenia coral. d Aboral view of a live specimen on the stem of Paralemnalia. White arrows indicate the ctenophore

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Description: Color opaque white with small white dot markings creating a uniform cover (Figs. 7c, d and 8a). No apparent papillae on aboral side (Fig. 8a). Individuals collected ranged in maximal length between 4 and 9 mm along tentacular axis (n = 10). Oral groove and lappets not present. Although gonads were not found, samples collected in September 2012 were observed to brood embryos or fertilized eggs (∼200 microns) under the margins of the oral side (Fig. 8b). Statocyst size is ∼30 μm (Fig. 8c).

Fig. 8
figure 8

Coeloplana fishelsoni n. sp. a Live, fully extended specimen showing the tiny white markings covering the entire body, oral view (scale bar: 3 mm). b Oral view of a specimen sampled in September 2012 showing brooded embryos or fertilized eggs on the margins of the animal (scale bar: 1000 μm). Blowout shows the embryos/fertilized eggs surrounded by a membrane. c Aboral view of the sense organ showing the granular statocyst in the middle surrounded by two pole plates (scale bar: 100 μm)

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Distribution and habitat: Found on the stems of the soft corals Xenia umbellata and Paralemnalia sp. at shallow depths of ∼6 m (Fig. 7).

Ecology and behavior: During the night, the ctenophores crawl on top of the polyps towards the exterior part of the colony and extend their tentacles into the water column in order to capture food (Fig. 7a, b).

Remarks: This species differs from all other Coeloplana species by its coloration pattern and the identity of its host. To date, there are no Coeloplanidae species that are known to live in association with Xenia and/or Paralemnalia soft corals. However, it is known that C. agniae Dawydoff, 1930 lives on Sinularia colonies, which resemble Paralemnalia. Nevertheless, C. agniae differs from C. fishelsoni by its opalescent colors and variety of pigments, whereas C. fishelsoni is opaque white with white dots (Table 1). Moreover, C. agniae has four distinct rows of papillae forming an X-pattern through the statocyst and four additional rows of papillae flanking the tentacle sheaths—C. fishelsoni n. sp. does not have aboral papillae.

Coeloplana punctata Fricke, 1970 – NEW RECORD

Material examined: Eilat, Israel, Gulf of Aqaba, Red Sea (Fig. 1). Sample: TAU-CO35585, (sampled on September 5th 2012 at 4 m depth from a colony of the soft coral Sarcophyton glaucum at the reef in front of the underwater observatory (UO). Preserved in 4 % formalin and later transferred to 70 % EtOH). Sample is deposited at The National Collections of Natural History at Tel-Aviv University, Israel.

Diagnosis: Epibiont on colonies of the soft coral Sarcophyton (Fig. 9a). Greenish-yellowish dots on aboral side (Fig. 9b). Extremely high number of aboral papillae (70–100) arranged in 10-12-12-10 pattern (Fig. 9c).

Fig. 9
figure 9

Coeloplana punctata from the Gulf of Aqaba, Red Sea. a In situ C. punctata on a colony of the soft coral Sarcophyton glaucum. Note extended tentacle and tentilla to the right of the ctenophore. Ctenophore indicated by a white arrow. b Live specimen of C. punctata, with the left tentacle extended, showing the round brown marking pattern (scale bar: 0.5 cm). c Live, fully-extended specimen showing the x-shaped arrangement of aboral papillae crossing the aboral organ (scale bar: 2.5 mm). d Close- up of the aboral sense organ showing the statocyst (scale bar: 300 μm)

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Description: Color milky white with brown-yellowish round spots across the entire body and green-yellowish dots (much smaller, but more numerous than the spots) scattered across the entire body surface (Fig. 9a, b). Only three individuals were collected, therefore few data are available on their size range. Length along tentacular axis ranged between 10.4 and 21.3 mm, and width ranged between 5.46 and 10.8 mm (n = 3). Papillae arranged in four rows creating an X-shape around the statocyst (Fig. 9c). Eleven papillae in each row (parastomial papillae). Four rows of paratentacular papillae, 14 papillae in each row. Total of 100 papillae (14-11-11-14) (Fig. 9c). This extremely high number of papillae in live organisms is in agreement with the original description of Fricke (1970). Reticulations of the gastrovascular system easily seen. Meridional canals branch repeatedly and form a reticulated network towards the periphery of the body. Individuals collected in Madagascar by Fricke (1970) were ∼ 30 mm in length. Statocyst seen as a granular mass located at the middle of the aboral sense organ ∼ 45 μm (Fig. 9d). Gonads were not observed.

Distribution and habitat: Found on colonies of the soft coral Sarcophyton glaucum (Fig. 9a).

Ecology and behavior: Coeloplana punctata could be found together with C. lineolata on the same coral colony.

Remarks: In the original description of the species, Fricke 1970 described two color variants which differ by the size of the aboral dots and the body background color (whitish or greenish). Unfortunately, the accompanying black and white photos do not seem to fit the description of both variants. As the samples collected in the Red Sea and described herein fit all the characters described in Fricke’s paper (but not the photo), we assign them the name C. punctata. It should be noted that during our literature review, we noticed that the Red Sea specimens resemble the pictures of Coeloplana mellosa Gershwin, Zeidler, and Davie, 2010 from Australia. However, they differ greatly in the number of aboral papillae present in live specimens (20 versus 100). Therefore, at this point it is unclear whether C. mellosa is in fact a synonym of C. punctata. Support for this hypothesis is that Gershwin et al. reports observing 100 papillae in preserved specimens of C. mellosa, which is in agreement with the original description of Fricke. Obtaining samples of C. mellosa from Australia and comparing them to our samples may resolve the issue.

Coeloplana lineolata Fricke, 1970 – NEW RECORD

Material examined: Eilat, Israel, Gulf of Aqaba, Red Sea (Fig. 1). Sample: TAU-CO35584 contains two ctenophore specimens, (sampled on May 27th 2012 at 10 m depth from a colony of the soft coral Sarcophyton glaucum at the reef in front of the Interuniversity Institute of Marine Sciences of Eilat (IUI). Preserved in 4 % formalin and later transferred to 70 % EtOH). Sample is deposited at The National Collections of Natural History at Tel-Aviv University, Israel.

Diagnosis: Found on the soft coral Sarcophyton glaucum (Fig. 10a, b). Characteristic pattern of brownish parallel lines parallel to the tentacular axis on aboral side (Fig. 10b, c). There are 60–70 aboral papillae arranged in 9-7-7-9 pattern (Fig. 10d).

Fig. 10
figure 10

Coeloplana lineolata from the Gulf of Aqaba, Red Sea. a Two C. lineolata specimens in situ on their soft coral host (Sarcophyton glaucum), extending their tentacles for feeding at night. b C. lineolata in situ on S. glaucum, showing the color pattern of parallel brown lines. c Aboral view of a live specimen of C. lineolata (scale bar: 1 cm). d Aboral view of a MgCl2 treated backlit specimen showing the papillae arrangement (scale bar: 1 cm). e Close-up of the aboral sense organ showing the granular statocyst in the middle (scale bar: 300 μm). f Oral view of a fixed specimen showing the oral opening surrounded by oral lappets (OL). White arrow indicates the oral grove (OG) (scale bar: 1 mm)

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Description: Transparent yellowish-brown with unique pattern of brownish lines parallel to the tentacular sheaths (Fig. 10b, c). Lines (∼185 μm in width) are composed of multiple tiny brown pigment granules (Fig. 10b, c). The width of gaps between adjacent lines is ∼300 μm. Individuals collected ranged in maximal length from 9 to 35 mm (n = 7) along tentacular axis and from 6 to 13.6 mm maximum width. To date, this is the largest species of all three Coeloplana species found on the soft coral Sarcophyton glaucum in the Red Sea. Thirty-six dorsal papillae in an X-shape were identified in live specimens surrounding the aboral sense organ (Fig. 10d). An additional 20 papillae were identified, ten flanking each of the two tentacular bulbs, arranged in two rows of five (one row on each side, 9-5-5-9). After examining several specimens, it seems that the total number of aboral papillae ranges between 38 and 56. Statocyst diameter is ∼50 μm (Fig. 10e). Oral lappets present surrounding the mouth, oral groove is present but narrow (Fig. 10f). Reticulations of the gastrovascular systems are seen; however, the structure of the meridional canals was not observed. Gonads were not observed.

Distribution and habitat: Found on the inner margins (underside) of the umbrella of S. glaucum colonies. The unique pattern of parallel brownish lines seen on the aboral surface of the animal match the underside of the Sarcophyton colonies (Fig. 10a).

Ecology and behavior: This species was observed to be on the underside during the day, when it needs to be camouflaged, while during the night it crawls to the upper parts of the colony to feed (Fig. 10a, b). Coeloplana lineolata may be found on the same coral colony with individuals of C. punctata.

Remarks: The individuals of C. lineolata found in the Gulf of Aqaba match the original description by Fricke (1970) with regards to color and pattern, and are identical to the photographs included in the original manuscript. There are small differences in the number of papillae (64 in the original description by Fricke vs. 38–56 in this research), which may be attributed to natural variation. It is important to note that another striped Coeloplanid, Coeloplana agniae var striata Dawydoff, 1938, was identified as a variant of C. agniae, as individuals were found on the same host and differed only in color pattern. Fricke did not compare his species to that of Dawydoff and it may well be that they are the same species.

Coeloplana bannwarthi Krumbach, 1933

Material examined: Eilat, Israel, Gulf of Aqaba, Red Sea (Fig. 1). Samples: TAU-CO35581 contains five ctenophore specimens, TAU-CO35582 contains seven specimens, TAU-CO35583 several specimens on the spines of the host (sampled on May 27th 2012 at 3 m depth from the spines of the sea urchin Diadema setosum (Leske, 1778) at the reef in front of the Interuniversity Institute of Marine Sciences of Eilat (IUI). Preserved in 4 % formalin and later transferred to 70 % EtOH). Sample is deposited at The National Collections of Natural History at Tel-Aviv University, Israel.

Diagnosis: Found on the spines of the sea urchin D. setosum (Fig. 11). Characteristic dark purple / brown color. No aboral papillae present.

Fig. 11
figure 11

Coeloplana bannwarthi (a) In situ on the spines of the sea urchin Diadema setosum, (b) close-up of two C. bannwarthi on white spines of D. setosum with their tentacles extended

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Description: Aboral side dark brown in color (Fig. 12a). When the ctenophores are seen in situ, attached to the spines of the urchin, they may seem black in color (Fig. 11a, b). Oral side is milky white in live specimens. Ramifications of gastrovascular system are not clearly visible in either live or fixed animals. In some MgCl2 treated specimens, the meridional canals are visible (Fig. 12a). When the animal is contracted, the area surrounding the sense organ resembles a six-lobed inflated cushion (Fig. 12b). Pole plates are visible in live specimens only after treating the animal with MgCl2 for relaxation. Statocyst is granular and ranges in diameter between 32 and 72 μm (Fig. 12c), and its diameter increases with the animals maximal length. In normal state, no apparent papillae are visible. However, in specimens that were kept for several hours in petri dishes, ephemeral papillae started to emerge on the aboral side (Fig. 12d). Individuals collected ranged in size from 1.5 to 16 mm (n = 23) maximal length along tentacular axis, and 0.98 to 6.96 mm in width. In fixed specimens, oral lappets are clearly seen surrounding the oral opening (Fig. 12e). Pharynx with many pharyngeal folds (Fig. 12e).

Fig. 12
figure 12

Coeloplana bannwarthi morphology. a MgCl2 treated fully extended backlit C. bannwarthi specimen showing the meridional canals arrangement. TC – tentacular canal, S.PH.C – Sub pharyngeal meridional canal, S.T.C – subtentacular canal (scale bar: 5 mm). b Aboral view of a contracted specimen demonstrating the six-lobed cushion shape of the polar area (scale bar: 2 mm). c Close-up of the aboral sense organ showing the granular statocyst in the middle (scale bar: 200 μm). d Side view of a specimen showing three ephemeral papillae that emerged after keeping the specimen for several hours in a petri dish. e Oral view of a formalin fixed C. bannwarthi. Note the well-developed oral lappets surrounding the mouth. Oral groove not present (scale bar: 2.5 mm)

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Distribution and habitat: Found on the spines of the sea urchin D. setosum in shallow water reefs in the Gulf of Aqaba (Fig. 11). According to Dafni (2008), this benthic ctenophore species was not seen in the Gulf of Aqaba for at least a decade. However, during our survey, we found it to be very common across several sites.

Ecology and behavior: During the day, the ctenophores are concentrated near the sea urchin’s test and are difficult to see. During the night they crawl to the upper parts of the spines and extend their tentacles for feeding (Fig. 11). Infestation rate of the sea urchin may exceed 100 individuals per urchin (Fig. 11a).

Reproduction /developmental biology: Several early stage embryos were observed in the petri dish with an individual ctenophore collected on September 6th 2012. The average diameter of the embryos was 285 μm. In addition, asexual reproduction was inferred, as some of the specimens collected were small and lacked some of the structures.

Vallicula Rankin, 1956

Diagnosis: Possesses the characters of the family. Body flattened in oral-aboral axis with pharynx extended for creeping on the oral surface, or body folded in two along tentacular axis and floating or similarly folded and sessile on brown algae, hydroids, or ascidians. Comb plates absent. Oral grooves extending from mouth along tentacular axis to opening of tentacular sheaths on aboral surface with oral edges in central region, either produced as a pair of lappets, or further extended to form an accessory creeping sole. Anchor-shaped tentacle sheaths with extensible tentacles up to 20 or 30 times the length of the body. Branching and anastomosed gastrovascular system with aboral papillae capable of altering their position and shape and with well-marked spherical chambers near each tentacle sheath.

Vallicula multiformis Rankin, 1956 – NEW RECORD

Material examined: Eilat, Israel, Gulf of Aqaba, Red Sea (Fig. 1). Samples: TAU-CO35577 three specimens, TAU-CO35578 six specimens, TAU-CO35579 few specimens, TAU-CO35580 three ctenophores on their Sargassum sp. host (all specimens were sampled on May 27th 2012 at 8 m depth from a colony of the algae Sargassum at a sandy-rubble patch located between the Interuniversity Institute of Marine Sciences of Eilat (IUI) and the Underwater observatory (UO). Preserved in 4 % formalin and later transferred to 70 % EtOH). Sample is deposited at The National Collections of Natural History at Tel-Aviv University, Israel.

Diagnosis: Unique anchor shape of tentacles sheath with a distinct “cross piece” (this character discriminates between the genera Coeloplana and Vallicula) (Fig. 13a, b). Transparent, either colorless with whitish markings or lightly colored on aboral surface, with brown, pink, or green pigments (Fig. 13a). Color is more pronounced in area immediately surrounding the apical organ, along tentacular axis and around periphery.

Fig. 13
figure 13

Vallicula multiformis from the Gulf of Aqaba. a Live specimen of V. multiformis, showing coloration pattern, unique tentacle sheaths and the ramifications of the gastrovascular system. b Close-up of the tentacle sheath area showing the cross-piece structure, which is unique to the genus (scale bar: 2 mm) P – Pappilae, T.S.CP – Tentacle sheath cross-piece. TO – Tentacle opening with white tentacle tip protruding out. c Oral view of a preserved specimen of V. multiformis showing oral lappets (OL) and oral groove (OG) (scale bar: 1 mm). d Scanning electron micrograph of V. multiformis (scale bar 500 μm) OG – Oral Groove; OL – Oral lappets

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Description: Color ranges from colorless to transparent green to transparent yellow, depending on its host. Small white granular pigments concentrated along the animal’s perimeter and surrounding the aboral sense organ (Fig. 13a). In some specimens, light orange pigmentation is concentrated around the sense organ (Fig. 13a). All specimens have brown pigments located in cells that resemble chromatophores. Tentacle sheath differs from that of Coeloplana by having an additional cross-piece, resembling an anchor (Fig. 13a, b). Individuals’ (n = 28) maximal length ranges between 3 and 32 mm along tentacular axis, and their width ranges between 1 and 19 mm. Individuals have eight large club-shaped papillae, four flanking each tentacle bulb, two from each side (Fig. 13b). When tentacles are contracted and packed into the tentacle sheath, the tentacle sheaths look greenish in color. Aboral sense organ resembles a four-leaf clover, having four large papillae, one on each lobe. Statocyst diameter ∼60 μm. Oral lappets and a well-defined oral groove present (Fig. 13c, d). Reticulations of the gastrovascular system are clearly visible (Fig. 13a). Gonads were not observed

Distribution and habitat: Found on unidentified brown algae (Fig. 14a), on the vesicles of Sargassum seaweed (Fig. 14b) and on the spines of the holothurian Pearsonothuria graeffei (Fig. 14c). This is a generalist species with a variety of hosts, ranging from algae to invertebrates, in many global locations.

Fig. 14
figure 14

Vallicula multiformis from the Gulf of Aqaba, Red Sea on different hosts. a V. multiformis on an unidentified brown alga. Note extended tentacles in the flow, b V. multiformis on the seaweed Sargassum. Note the aboral papillae from a lateral view (marked by white arrow) (scale bar: 2 mm). c Two V. multiformis (marked by white arrows) on the sea cucumber Pearsonothuria graeffei

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Ecology and behavior: This species was found to be prevalent in the Gulf of Aqaba mostly during the winter, when the water column is mixed and there is a massive algal bloom. During this period Vallicula reached densities of hundreds of individuals per Sargassum algae. This is in agreement with previous observation in Hawaii of over 3000 V. multiformis per square meter (Carlton and Eldredge 2009).

Discussion

This study presents the first survey of the benthic ctenophore fauna in the Gulf of Aqaba. We have found four new species of benthic ctenophores (C. loyai n. sp., C. yulianicorum n. sp.; C. huchonae n. sp. and C. fishelsoni n. sp.) and three new records for the Gulf of Aqaba (C. lineolata Fricke, 1970, C. punctata Fricke, 1970 and Vallicula multiformis Rankin, 1956). The fact that half of the species documented in this study are new species demonstrates that this group holds great potential for uncovering new, unknown species. Moreover, because benthic ctenophores are epibionts on various algae and invertebrates that are common in coral-reef areas throughout the world, it may indicate that the biodiversity of this group is expected to be high in coral-reef ecosystems. This hypothesis is corroborated by the frequent documentation of these animals in coral-reef areas by amateur underwater photographers, as revealed when searching the keywords “benthic ctenophores” on Google.

Geographic range of Vallicula

Our new record of V. multiformis in the Red Sea represents an extension of the species’ geographic distribution, which was known until recently only from the Atlantic ocean and Hawaii. The first record from the Indian Ocean was recently reported by Prasade et al. (2015), who collected specimens in the Gulf of Kutch, West coast of India. Vallicula multiformis was originally described from Jamaica (Rankin 1956) and later recorded from Brazil (Marcus 1957; Oliveira et al. 2007), Bermuda (Freeman 1967), Hawaii (Eldredge and Miller 1995), Madeira Island in Portugal (Wirtz 1998), California (Mills and Haddock 2007), Canarias Islands, Cuba (Moro et al. 2010) and Florida (Glynn et al. 2014). At this point, we are unable to determine whether Vallicula is a circumglobal species or an easily transported invading species, dispersing through shipping routes (either by ballast water or as part of fouling organisms). The ability of Vallicula to live on a variety of hosts gives it an advantage and probably enables its large geographic distribution, whether invasive or not.

Morphological identification of benthic ctenophores

The seven main morphological characters traditionally used for benthic ctenophore identification are: color, pattern, size, papillae number, shape of tentacle sheath and the presence or absence of an oral groove and oral lappets. Oral groove presence and the shape of the tentacular sheath originally distinguished between the two genera in the family (Coeloplana vs. Vallicula), but these characters were not used to distinguish between different species from the genus Coeloplana. The use of these morphological characters in benthic ctenophore species identification is complicated and may not be unequivocal. The reason is that these characters are altered post chemical fixation, rendering taxonomic revisions based on existing museum records challenging.

Discrimination between Vallicula and Coeloplana

Although Rankin (1956) suggested that Vallicula should be considered as a new genus due to the presence of an oral groove, oral lappets and anchor-shape tentacle sheaths that are not found in Coeloplana, we argue that an oral groove and oral lappets do in fact occur in some species of Coeloplana, but not in all. For example, C. lineolata and C. yulianicorum n. sp., the new species found on S. glaucum, both have an oral groove, although in C. lineolata it is very narrow. Moreover, an oral groove is known to be present in two other species described from Australia: C. scaberiae and C. thomsoni, both designated as new Coeloplana species by Matsumoto (Matsumoto 1999; Matsumoto and Gowlett-Holmes 1996). Also, oral lappets were observed in the framework of this study in C. bannwarthi collected from the spines of the urchin Diadema setosum (Fig. 12e). We conclude that the only morphological character that is valid for identifying the genus Vallicula is the unique anchor-shape of the tentacle sheaths (having an additional cross-piece, Fig. 15). The fact that only the anchor shape distinguishes between Vallicula and Coeloplana raises a doubt regarding the standing of Vallicula as a separate genus. However, recent molecular evidence based on 18S and ITS sequences supports the validity of Vallicula as a separate genus (Simion et al., 2015).

Fig. 15
figure 15

A drawing of Valiculla multiformis showing the unique anchor-shape of tentacles sheath with a distinct additional cross-piece (CP)

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Reviewing recent literature on Coeloplanidae, we noticed in the original description (Gershwin et al. 2010) that C. reichelti appears to have an anchor-shape tentacle sheath with a distinct cross-piece (see Fig. 2F in Gershwin et al. 2010). Moreover, comparing Fig. 2e from Gershwin et al. (2010) to Fig. 13a in the current manuscript (and other photos in our possession) shows that there are similarities in color, pigmentation and general morphology between the specimens. Therefore, it is possible, even likely, that C. reichelti may be in fact Vallicula. We suggest that the issue may be easily resolved applying molecular methods as shown in Simion et al. (2015).

Discrimination between Coeloplana species

Color and pattern

Based on our observations, the color and pattern of Coeloplanidae species does not change in live specimens, even after detaching them from their host and keeping them for a few weeks in the laboratory. Transparency may change as a result of the animal’s state (i.e., relaxed versus contracted). However, based on existing literature, there seem to be several color morphs for certain species (e.g., C. gonoctena var. rosea, C. gonoctena var. natalensis and C. agniae var. striata). We cannot rule out that these varieties are in fact different species. At least for the species described here from the Gulf of Aqaba, color and pattern are useful and descriptive and allow species differentiation.

Size

Size measurements are challenging, as these animals are very flexible and tend to change shape constantly. In addition, size measurements are also affected by the animal’s state (e.g., relaxed vs. contracted) and also change greatly post fixation, as both formalin and ethanol dramatically alters the body shape. However, based on a comparison of length between all known species of Coeloplanidae (Table 1), it is clear that there are species that are generally smaller (e.g., C. anthostella, C. duboscqui, C. perrieri), while others may reach almost 60–70 mm when mature (e.g., C. willeyi, C. loyai, C. lineolata).

Aboral papillae

Historically, the number of aboral papillae was used to distinguish between coeloplanid species. However, we argue that the aboral papillae are not informative for distinguishing between species as their number can vary greatly, even in the same individual as reported here for C. loyai and also noted by Gershwin et al. (2010). Several authors have hypothesized that these protruding extensions of the gastrovascular system are involved in gas exchange, and therefore, papillae number will increase when placing an individual in a low oxygenated environment (Abbott 1907a, b; Rankin 1956). This hypothesis was corroborated by our observations on various species found within the framework of this study. After placing the animals in small petri dishes for observation, the number of papillae increased. This even happened in species with no observed papillae when initially examined, whereas after several hours these structures appeared (e.g., as seen in C. bannwarthi). In addition, the number of these papillae changes post fixation. For example, Gershwin et al. (2010) reported that live specimens of C. mellosa have 20 simple papillae, whereas post fixation in formalin solution they seem to have > 100 papillae.

Oral groove and lappets

Although the presence of oral groove and lappets cannot be used to discriminate between Vallicula and Coeloplana, they may be used as supporting characters when identifying Coeloplana species. Some Coeloplana species exhibit both structures (e.g., C. yulianicorum, Fig. 5f and C. lineolata, Fig. 8e), some have only oral lappets (e.g., C. bannwarthi, C. huchonae) or only the oral groove (e.g., C. sophiae) and other do not show any of these characters at all (e.g., C. loyai). These characters are better seen in fixed animals than in live ones.

Host specificity as an identifying character

The association between ctenophores and their hosts is an important character for species identification and may be used to discriminate between species from the family Coeloplanidae. The degree of specificity and host identity varies between different species of Coeloplanidae. Some ctenophore species are host specific and restricted to only one host (e.g., C. bannwarthi on the spines of Diadema setosum and C. huchonae on the stems of Dendronephthya hemprichi). Some are restricted to a group of morphologically similar hosts as in C. loyai on the mushroom corals H. limax and C. echinata, or Coeloplana waltoni that lives on various species of shallow water gorgonians. Others like V. multiformis are generalist, living on various hosts and habitats, and can be found on hard substrate like beach rocks as well as on various hosts ranging from algae to invertebrates. C. willeyi is found on both algae and on the spines of sea urchins.

Conclusion and future directions

Available literature and our own data suggest that: (1) differentiation between Coeloplana and Vallicula is possible based on only one character—the shape of the tentacle sheath; and (2) differentiation between Coeloplana species is possible with a combination of five characters (i.e., color, pattern, oral groove, oral lappets and host identity). As we have shown, color and pattern are very important characters in the taxonomy of benthic ctenophores. Because these two characters will change post fixation, we recommend basing ctenophore species identification on live specimens, using color photos and videos as part of the identification.

Finally, there is a need to develop molecular markers for species identification that will allow a thorough revision of the known species. We are currently in the process of doing this analysis.