New Species of Pseudovermis (Opisthobranchia: Aeolidacea) from New Zealand and the Solomon Islands

Transactions of the Royal Society of New Zealand : Biological Sciences, Volume 11, Issue 10, 13 October 1969, Page 153

New Species of Pseudovermis (Opisthobranchia: Aeolidacea) from New Zealand and the Solomon Islands

D. A. Challis

By

Department of Zoology, University of Auckland

[.Received by the Editor, 2 April, 1969.]

Abstract

Two new species of Pseudovermis are described; P. mortoni n.sp. from the Solomon Islands and P. hancocki n.sp. from New Zealand. The internal anatomy of P. mortoni is discussed. A brief resume of the feeding relationships of the genus is given together with information on the food of the New Zealand P. hancocki. A key to the species of Pseudovermis is included.

Introduction

Eight species of Pseudovermis Periaslavzeff, 1891, have been described from the littoral and sub-tidal sands of Europe and Brazil. In the course of the interstitial fauna survey carried out by the 1965 Royal Society Expedition to the Solomon Islands Protectorate a number of minute sand-dwelling opisthobranchs were collected. One of the most interesting of these was a species of Pseudovermis.

In January, 1968, a specimen of a further species of Pseudovermis was recovered from a sand sample dredged in the Bay of Islands, New Zealand. Despite intensive dredging, no further specimens were found at the original locality, but another single specimen was collected later from an adjacent area. This is the first interstitial opisthobranch collected from New Zealand sands.

Materials and Methods

The animals were separated from the sand by a modification of Kowalevsky’s (1901), method. The sand was placed in a bowl, covered with an inch or two of sea water, swirled around by hand until a vortex formed and the supernatant water containing the animals decanted off through a 100 micron mesh nylon gauze. The gauze was inverted in a petri-dish of sea water and the extracted animals sorted. Where possible microphotographs of the living animals were taken. The radula and jaws were prepared for examination by dissolving away the soft parts in 10 per cent potassium hydroxide and mounting the hard parts in Salmon’s (1949) poly-vinyl-lactophenol with Chlorazol Black. Serial sections of the Solomon Islands species were made in both transverse and longitudinal planes and the internal anatomy was reconstructed. Where the anatomy was complex, as in the reproductive system, the sections were each drawn with the aid of a camera lucida and a large scale stereographic diagram was made. Dissection was generally impractical

because of the very small size of the animals and the difficulty of holding specimens, but useful gross dissections of the buccal mass and of the central nervous system were made. Several whole animals were mounted in poly vinyl-lac tophenol permitting some superficial features such as the very small epidermal spicules to be seen under phase contrast.

The drawings were made from photographs or with the aid of a camera lucida.

Family PSEUDOVERMIDAE Pelseneer, 1906

Genus Pseudovermis Periaslavzeff, 1891

Pseudovermis mortoni n.sp

Description: Fully extended animal (Fig. la) approximately smm long; ratio of breadth to length approximately 1:17. Colour in life translucent white, digestive gland, creamy yellow stippled with black spots. Head completely ciliated, reasonably distinct from the body, having the greater diameter. Eyes absent. Body vermiform, tapering posteriorly, terminating in a short tail. Gerata non-retractile, simple, finger-like, shorter than head diameter, first pair opposite, remainder aproximately alternate, seven on left side of body, five or six on right in larger specimens, number variable in immature specimens. Epidermis of dorsal body wall behind first pair of cerata sparsely invested with small, colourless, calcareous spicules (Fig. If-g). Foot not set off in any way from body, distinguishable externally only by ciliated sole, bordered internally by two rows of cyanophil pedal glands from nerve ring backwards. Groups of similar gland cells scattered around internal body wall of anterior region (Fig. 4a-|C). Buccal bulb (Fig. 3a) supported by two lateral mandibles (Fig. Ib-c) having thickened anterior borders bearing approximately 24 teeth. Inner border of mandibles bent upwards and also thickened. Radular formula 1-1-1 X 34. Median plate (Fig. Id) having a large median tooth and three lateral denticles on either side increasing in both height and size from the innermost to the outermost denticle. Outermost denticle a little taller than median tooth. Lateral plates slender, tapering, curved, with a secondary denticle extending to about half the length of the plate.

Type Locality: The holotype and 17 paratypes were collected by the author in mid-September, 1965, from coarse, clean, shell-sand at Low Water Neap Tide 100 yards NNW of the southern end of Maraunibina Island, Marau Sound, East Guadalcanal.

Other Localities: Further material was collected from a similar tidal level at Komimbo Bay, West Guadalcanal, three specimens; Banika Beach, Russell Islands, two specimens; Pango Beach, Efate, New Hebrides, two specimens.

The beaches where this animal was collected were invariably sheltered to a large degree from direct wave action by an offshore fringing coral reef- They were generally short and separated from the reef by an area of silty, rubble-strewn, springtidal flat. The sand of the lower part of the beach to which the animal was restricted was coarse, clean, highly oxygenated and composed mainly of fine broken shell, small rock fragments and very large numbers of living micro-foraminifera.

A general account of the habitat and of the precise distribution of Pseudovermis and of the other interstitial opisthobranchs collected during the expedition is provided elsewhere (Challis, 1969).

Types: Of the total of 25 specimens of P. mortoni collected, 10 remain intact together with two other badly damaged specimens. The holotype with four paratypes and a slide of the jaws and radula of a further paratype have been deposited in the British Museum (Nat. Hist.), London. A series of four paratypes with a slide of the jaws and radula of another paratype has been deposited in the Dominion Museum, Wellington, New Zealand. The remaining paratypes and the sectioned material are retained by the author.

Remarks: While a number of species of Pseudovermis have individual characteristics in common with P. mortoni, for example, the general external body form, the length of the cerata, the double point of the lateral plates of the radula and the lack of eyes, only P. mortoni has these characters in combination. The number of

lateral denticles on the median plate of the radula, the presence of epidermal spicules and a reproductive system surprisingly different from that of the other members of the genus further distinguish this species.

Pseudovermis hancocki n.sp.

Description: Fully extended animal (Fig. 2a) approximately 4mm long, ratio of breadth to length approximately 1:15. Colour in life translucent white, digestive gland creamy yellow stippled with black spots. Head reasonably distinct from body and having the greater diameter. Two eyes present. Cerata simple, longer than the diameter of head, five on left side of body, four on the right, first pair opposite, remainder alternate. Body terminating in a short tail. Radular formula 1-1-1 X 36, median plate lacking usual large median tooth, having instead two or three very fine short denticles and four or five lateral denticles. Lateral plates slim, curved and pointed having a well-developed parallel secondary process (Fig. 2d). Mandibles (Fig. 2b and 2c), curved, slender with graduated series of about 16 teeth on their anterior border. Inner border of mandible bent upwards and thickened.

Type Locality: The holotype was collected by the author in January, 1968, from sand dredged in 20 feet of water on the southern side of Urapukapuka Island, Bay of Islands, New Zealand (G.R. 726585, N.Z.M.S.I, Sheet N. 12).

Other Localities: A second specimen was collected in sand dredged from a similar depth on the southern side of Motu Arohia Island, Bay of Islands, New Zealand (G.R. 672559, N.Z.M.S.I, Sheet N. 12).

Types: The two specimens of P. hancocki collected were both sacrificed for jaw and radular mounts. The holotype is a preparation of the radula and jaws on a microscope slide and is deposited together with microphotographs of the living animal in the Dominion Museum, Wellington, New Zealand. The paratype is a similar preparation and will be retained by the author in the meantime.

Remarks : P. hancocki is externally similar to several of the previously described species of Pseudovermis. Internally the central plate of the radula is remarkably different where it lacks the usual predominant median tooth. In view T of the uniqueness of this feature, the second specimen was sacrificed to confirm that it was typical of the species.

Biological Notes

The feeding of Pseudovermis has long been the subject of interest and speculation. That the animal feeds at least to some extent on hydrozoans like the larger aeolids is certain for it is known that where nematocysts are present in the cerata of aeolids they are invariably derived from the hydrozoans upon which these animals feed (see discussion in Graham (1938)). Sand-dwelling Gnidaria of a suitable size are, however, not common, nor have they invariably been found in the sands from which Pseudovermis has been collected.

Ev. Marcus (1953) found that nematocysts in the cerata of P. salamandrops corresponded closely in size and type with those carried by the free-living sand hydrozoan Psammohydra nanna Schultz, 1950. She failed to find Psammohydra in the sand but considered it possible that her extraction method was unsuitable for their recovery. Boaden (1961) collected P. hoadeni Salvini-Plawen, 1968, and Halammohydra vermiformis Swedmark and Teissier, 1957, from the littoral sands of the island of Anglesey in northern Wales. From a comparison of the nematocysts present in the two animals he considered it probable that P. hoadeni feeds upon H. vermiformis. Fize (1961) collected Pseudovermis setensis from the littoral sand of the Languedoc province of France, but despite the large numbers of Pseudovermis present he found no sand-dwelling hydrozoans. Pseudovermis of all stages of maturity were collected and he noted that the young Pseudovermis lacked nematocysts in their cerata. He observed P. setensis attack and eat specimens of a small Microhedyle present in the same sand and so corroborated the suggestion by Kowalevsky (1901) that Pseudovermis fed at least to some extent on Acochlidiacea. Fize concluded that P. setensis was probably a facultative rather than an obligate hydroid feeder like the larger aeolids.

When Pseudovermis mortoni was discovered in the Solomon Islands a thorough search was made for sand hydrozoans which had they been the prey species, could

reasonably have been expected to occur in greater numbers than their predator. No hydrozoans were found. However, when the Pseudovermis were placed in a petri-dish together with Paraganitus ellynnae Ghallis, 1968, a small acochlidiacean collected from the same sand, they consistently attacked the smaller animal, tearing off and ingesting pieces of its visceral sac.

Neither Acochlidiacea nor free-living sand hydrozoans have so far been found in the Bay of Islands sands, but relatively large numbers of a small, solitary sanddwelling pennarian hydroid Heterostephanus, of an apparently undescribed species (Fig. 2e), were dredged from a nearby locality. The hydrozoans were not collected in the same sample as the Pseudovermis, but a feeding relationship seemed possible and the nematocysts present in each were extracted and compared. Those of Heterostephanus are of three types: a moderately large stenotele, a slightly smaller holotrichous isorhiza and a very small nematocyst (Fig. 2f), that seems not to fit the general classification proposed by Weill (1930). The capsule and butt are each about 8 microns long and there is a slender flagellum of about 40 microns. I am unable to find spines of any sort either on the butt or on the flagellum. These, if present, would indicate that the nematocyst conformed to Weill’s category, “ microbasic mastigophore ”, a type most frequently encountered in the anemones, zoanthids and corals. In the absence of spines it seems reasonable to tentatively describe the nematocyst as an “ atrichous microbasic mastigophore ”. Nematocysts of precisely the same appearance and size were extracted from the Pseudovermis. Considering the unusual type of nematocyst concerned, this appears to be good circumstantial evidence of a feeding relationship of at least a facultative nature between P. hancocki and Heterostephanus. Thus the food of P. hancocki appears to be partially different from that previously considered for other species of Pseudovermis, and confirmation of this may help to elucidate the feeding behaviour of the genus generally.

Anatomy of Pseudovermis mortoni

Alimentary Canal: Mouth situated on the ventral side immediately anterior to the posterior margin of the head. Buccal cavity moderately though not excessively glandular, surrounded by a cuticular tube similar to the “ mug-shaped ” ring described for P. salamandrops Marcus Ev. du B-R (1953). Oral tube short and narrow, connecting the buccal cavity with the buccal mass. Musculature of buccal mass well developed, consisting of three substantial blocks of longitudinal muscle, two being enclosed by the rami of the mandibles, the third lying ventrally beneath the odontophore. Odontophore formed of a large central block of radial muscle fibres (Fig. 4b-c). Transverse sections of the reinforced inner border of the mandibles show different sectional shapes according to the region from which the section is taken and although the sectional configuration of this organ has been used in the past as a major taxonomic character it does not seem, in this species, constant enough to be useful. The mandibles and radula have been described. Salivary glands small, paired, tubular discharging into the oesophagus rather than the pharynx. Oesophagus, a short, thin-walled tube, connecting the posterior end of the buccal mass to a well differentiated stomach (Fig. 4d). Stomach a thick-walled tube lined internally with a single layer of ciliated columnar epithelium, sheathed externally with a thin coating of circular muscle, opening posteriorly and dorsally to the intestine and ventrally to the digestive gland (Fig. 4e). Intestine ciliated internally throughout its length, lying near the mid-dorsal line of the body cavity until it approaches the anus (Fig. sb), which opens on the right side some distance behind the genital openings. Digestive gland confined within the body cavity, connected to the cerata but not entering them. Anteriorly it lies on the ventral side but is displaced first towards the left side and then dorsally in the posterior region of the body where it overlies the gonad. It occupies rather less than half of the space in the animal’s body. Gland lined with tall, club-shaped digestive cells with narrow

bases and wider free tips. The cells in fixed material do not appear to be ciliated though Graham (1938) has shown in more normal aeolids that they usually bear cilia in life. Digestive cells highly vacuolated and in some the vacuoles are filled with ingested food particles. The nuclei are large, ovoid, and generally confined to the narrow proximal neck of the cell; they have nucleoli that are clearly visible in haemotoxylin-stained material. Scattered amongst the digestive cells are small numbers of cells packed with yellowish refractive granules, like those considered by Graham to have an excretory function.

The cerata (Fig. le) contain only cnidosacs with nematocysts. Proximally they communicate with the underlying alimentary canal by narrow winding ducts. The first pair are connected, the right ceras to the stomach and the left to the digestive gland as in P. salamandrops. Remaining cerata connected to the digestive gland. There appears to be no terminal pore distally, but the thinness of the distal epithelium and the ease with which it is ruptured by slight pressure suggests that the nematocysts may be liberated when the cerata are slightly damaged. The nematocysts are for the most part grouped in large vacuolated cells similar to those described from the anterior cerata of Eolidina alderi Graham (1938). These cells are arranged in a column in the lumen of the ceras. Some nematocysts were found lying free in the lumen of the cnidosac, but this may be the result of tissue damage caused by fixation.

Nervous System: Central nervous system (Fig. 3a-b) similar to that described for P. salamandrops lying immediately posterior to the buccal bulb and characteristic of the type generally found in members of the Aeolidacea. Cerebro-pleural complex fully coalesced, pedal ganglia situated in the dorsal half of the body. Pedal commissure running horizontally beneath the oesophagus behind the buccal bulb with the statocyst arranged on the outside of the cerebro-pedal connective. Only one statolith in the statocyst. Buccal ganglia lying between the buccal bulb and the oesophagus joining the cerebro-pedal connective immediately below the cerebropleural ganglia. Several large nerves run forward from the anterior region of the cerebro-pleural ganglia and from these a number of branches terminate in sensory cells scattered throughout the head (Fig. 4a-c). The sensory cells are not arranged in foliaceous groups as are those described for P. paradoxus Kowalevsky (1901). A number of nerves run ventrally from the pedal ganglia.

Renopericardial System: Kidney an elongated sac, extending on the right side from behind the genital openings almost to the gonad. Renal pore opening immediately beside the anus. Towards the posterior end of the kidney a short renopericardial duct runs dorsally to a small pericardium, lying between the kidney and the body wall. The pericardium does not appear to have the features described by Marcus (1953) for P. salamandrops. There is no “mug-shaped” cuticularised basement membrane in this species nor is the pericardium glandular in appearance. There are no further traces of a circulatory system.

Reproductive System (Fig. 3c-d) : Hermaphrodite gonad lying posteriorly beneath the digestive gland, consisting of three follicles, two containing only oocytes and a third containing only sperm. Gonad discharging anteriorly into a short common ampulla which immediately narrows to become the hermaphrodite duct, continuing anteriorly for a short distance to bifurcate into oviduct and vas deferens. Entrance to the oviduct guarded by a muscular sphincter.

Male System : Anterior to the bifurcation of the hermaphrodite duct the vas deferens extends forward almost to the level of the buccal bulb then turns abruptly upon itself and joins the anterior end of the prostate. This gland is a wide tube lined with strongly ciliated columnar cells, whose cytoplasm is densely packed with eosinophilous granules. Prostate extending for some distance posteriorly. From near its proximal end a narrow, ciliated “penial” duct runs forward again and

joins the anterior end of a penial gland. At this level the penial gland lies to the left, but as it extends backwards it passes across the dorsal surface of the body cavity, being restricted finally to the right side where it eventually opens into the atrium of the male genital aperture. Wall of the penial gland formed of a thin layer of very flattened epithelial cells. Medioventrally it is lined with large clusters of intensely cyanophilic gland cells (Fig. 4d-e). From the same surface a muscular process heavily coated with gland cells protrudes into the lumen of the gland. Whether this process is a “ penis ” in the sense of an extrusible copulatory organ is uncertain for although it is suitably muscular it terminates some distance from the external opening and appears to be firmly attached to the median wall of the penial duct. There is no “ tubular, winding, cuticular penis ” like that described by Marcus from P. salamandrops. Penial gland discharging into a short ciliated antrum quite free of glandular tissue, opening to the exterior on the right side at the same level and a little dorsally to the female aperture.

Key to abbreviations used in Figures: Al.gld., albumen gland; All.dct., allospermatic duct; An., anus; A.n., anterior nerves; Ant., antrum; A.sns., Anterior sensory cells; Buc.g., buccal ganglia; Bu.cop., bursa copulatrix; Gil.si., ciliated sole of foot; Gl.ep., ciliated epithelium; Cp.con., cerebro-pleural connective; Cpl.g., cerebro-pleural ganglia; D.gld., digestive gland; F.ch., fertilisation chamber; $ G.ap., male genital aperture; 2 G.ap., female genital aperture; Gld.c., cyanophil gland cells; H.dct., hermaphrodite duct; H.gnd., hermaphrodite gonad; Int., intestine; Jw., mandible; Jw.musc., musculature of mandibles; K., kidney; Mc.gld., mucous gland; Mth., mouth; Muse., muscle blocks; Od., oviduct; Oes., oesophagus; Ot., statocyst with otolith; Fed.con., pedal connective; Ped.g., pedal ganglia; Ped.gld., pedal glands; Pen.dct., Penial duct; Pen.gld., penial gland; Pr.gld., prostate gland; Rad., radula; Ref.gn., refractive granules; Sal.gld., salivary gland; Spnct., sphincter muscle; Stm., stomach; Vag., vagina; Vas.def., vas deferens.

Female System : The oviduct leaves the hermaphrodite duct and runs backwards for a short distance to enter a small but distinct ciliated sac, the fertilisation chamber. Here it is joined by the posterior end of the allospermatic duct carrying foreign sperm to the fertilisation site. From the fertilisation chamber the allospermatic duct extends forward for a short distance giving off a diverticula ending in a long, blind, sperm-filled sac, the bursa copulatrix. The allospermatic duct continues forward merging with the oviduct immediately before entering the vagina. After

leaving the fertilisation chamber the oviduct runs through the nidamental gland. This mass has a complex internal construction and although it is not possible on simple microscopic examination to assign precise functions to the histologically differentiated areas some indication can be given of the general topography of the mass and the sequence in which the different areas are traversed. The gland mass is deeply divided into two main lobes, one lying on the right and the other on the left of the haemocoele. The right lobe into which the oviduct enters after leaving the fertilisation chamber is the smaller. The lumen is very large and strongly ciliated. The lining is exceptionally granular and the cell outlines largely obscured. The whole lobe is clearly secretory and constitutes the “ albumen gland ”. Towards its anterior end it is linked by a transverse isthmus to the much larger left lobe, the mucous gland. This extends backwards to the level of the gonad where it occupies almost the whole ventral half of the body. Histologically the mucous gland is more complex than the albumen gland. Cells of three quite distinct types can be seen (Fig. sa-b), granular ones similar, though not identical to those found in the albumen gland (stippled) ; those staining almost black in haemotoxylin (black); and lightly staining cells with large but infrequent nuclei. The latter have a rather vacuolated appearance and invariably lie between the first two types (hatched). After leaving the mucous gland the oviduct unites with the allospermatic duct and enters a short ciliated vagina, opening to the female genital aperture immediately below and on the same level as the male aperture.

Discussion

While many aspects of the anatomy of P. mortoni are similar to those of the previously described species of the genus, the reproductive system differs remarkably. Whereas in P. salamandrops Marcus Ev. 1953, P. schulzi and P. axi Marcus Ev. and E. 1955 (species from which the reproductive system has been described), this system is essentially diaulic, that of P. mortoni is quite clearly triaulic. The possession, in this species, of a bursa copulatrix and a fully closed allospermatic duct fulfils the essential character of a triaulic system, despite the absence of a separate third genital aperture. The reproductive system of P. mortoni corresponds quite closely with that described by Chambers (1934) from Emhletonia fuscata. In his discussion of the classification of reproductive systems in opisthobranchs Chambers places systems conforming to this pattern in his category IV whose main characteristics lie in “ the relationship of the third channel (the allospermatic duct) to the seminal receptacle” . . . “ there is but a single receptacle and it is not entered by the third channel. The receptacle is a unit in itself connected to the vagina by a short duct. The third duct passes from the fertilisation chamber to the vagina at a point anterior to the receptacle ”. In P. mortoni there is no indication of the direct passage of sperm from the vas deferens to the exterior via the renal pore as there is in P. salamandrops.

A Key to the Species of the Genus Pseudovermis

The following key is an amended and amplified version of that presented by Salvini Plawen (1968).

1. Cerata prominent only on the preserved animal 2

Cerata prominent on the living animal 5

2. 11 to 13 pairs of cerata P. kowalevskyi Salvini Plawen Less than 10 pairs of cerata 3

3. Not more than 5 cerata in all P. schulzi Marcus and Marcus 7-8 pairs of cerata 4

4. Lateral plate of the radula with a single point, lateral denticles of median plate not increasing in size from interior to exterior P. paradoxus Kowalevsky

Lateral plate of the radula with two points, lateral denticles of median plate increase in size from the interior to the exterior P. boadeni Salvini Plawen

5. Cerata very short, button-like; foot thin 6

Cerata long, finger-like; foot broad 7

6. Not more than 7 pairs of cerata; eyes lacking; lateral plates of the radula having two points P. axi Marcus and Marcus

Not more than 5 pairs of cerata; eyes present; lateral plates of the radula having a single point P. setensis Fize

7. Lateral plates of the radula with a single point 8

Lateral plates of the radula with two points 9

8. Cilia of the head interrupted dorsally and laterally; eyes present; anterior border of mandibles not denticulate P. papillifera Kowalevsky

Cilia of the head not interrupted; eyes lacking; anterior border of mandibles denticulate P. salamandrops Ev. Marcus

9. Cerata longer than the diameter of the head; eyes present; median plate of the radula lacking large median tooth, having instead 2 or 3 very fine short denticles P. hancocki n.sp.

Cerata shorter than the diameter of the head; eyes lacking; median plate of the radula with large median tooth and three lateral denticles on each side, epidermis bearing spicules P. mortoni n.sp.

Acknowledgments

I am grateful to the Royal Society of London for their invitation to participate in the 8.5.1. P. expedition, the South Pacific Research Programme Committee of the University of Auckland for granting funds which enabled me to do so. I thank Professor J. E. Morton of the University of Auckland, after whom P. mortoni is named, for his encouragement and advice during the expedition and subsequently, and Dr M. C. Miller for making his knowledge of the Aeolidacea available to me during this study.

I am grateful to Mr M. Plancock of the Whangarei Conchology Club after whom P. hancocki is named. He collected the original sand samples from which this animal was extracted and subsequently placed himself, and his yacht at my disposal during the search for further specimens. Mrs M. Borowska of the University of Auckland Library translated Marcus Ev. and E. (1955).

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