Siphopteron leah, Klussmann-Kolb, Annette & Klussmann, Alexander, 2003

Klussmann-Kolb, Annette & Klussmann, Alexander, 2003, A new species of Gastropteridae (Gastropoda, Opisthobranchia, Cephalaspidea) from tropical Northeast Australia, Zootaxa 156, pp. 1-12 : 2-10

Siphopteron leah sp. nov.

Type material:

Four specimens, Australia: Queensland, Orpheus Island, Iris Point; 2mtrs depth, on the reef flat amongst algae

Type specimens deposited at the Museum of Tropical Queensland (MTQ), Townsville, Australia:

HOLOTYPE (accession number MTQ MO 40200), 4 mm. PARATYPES (accession number MTQ MO 40201, partly dissected; MTQ MO 40202), 3 and 4 mm.

OTHER MATERIAL EXAMINED, 4 mm (histological investigation).

Distribution

So far this species has only been found at Orpheus Island, Queensland, Australia. Considering the possibility, that the specimens shown by Masayoshi (2002) resemble S. leah sp. nov. due to similarity in colouration, the distribution would be extended to Japan (33N, 140E).

External morphology

Siphopteron leah sp. nov. is a small animal of a vivid yellow ground colour. Thin red lines are found in various positions: in three of the four investigated animals red lines were found along the outer edge of the head shield, at the apical tip of the dorsal ridge of the siphon, as well as along the sides of the siphonal crest and at the tip of the flagellum ( Fig. 1). Also in these three specimens one red line ran across the visceral hump just behind the head shield and two red lines were found further posteriorly. In the fourth specimen additional lines were found: one on each outer side of the parapodia and one around the edge of the visceral hump ( Fig. 2). In this animal the flagellum was completely red.

The head shield is almost triangular with small rounded corners ( Figs. 1, 2). The siphonal crest is elongate and bears a prominent median dorsal ridge which extends in front of the head shield when erect. The body is almost ovoid in shape and bears a conical shaped flagellum at the right side. The foot forms an elongate but broad tapering tail and bears large wing­like lateral parapodia. These are folded over the top of the visceral hump but do not meet each other, even when the animal is at rest. The parapodia are free moving thus enabling the animal to actively swim using rapid flapping motions. A small gill composed of five simple plicae is found on the right side of the body underneath the overlapping mantle. In the preserved animals glandular follicles are visible shimmering through the translucent epithelium of the parapodia. A large white foot gland is also visible at the posterior end of the foot.

Internal anatomy and microstructure

Epidermis: The notal epidermis consists of a thin pavement epithelium whereas the cells are more cylindrically shaped along the foot sole. The epithelial cells are ciliated on both ventral and dorsal surfaces. Few subepidermal glandular cells can be found on either surface but appear to be more numerous along the foot sole. The glandular cells protrude between the epidermal cells with elongate apical cell extensions, where they are discharging their heterogeneous acidophilic secretions.

Siphon: The outer epithelium of the siphon is composed of cylindrical non­glandular cells with central nuclei. However, large subepidermal glandular cells are found along the outer margin of the siphon. These cells are filled with acidophilic secretions which are discharged through small pores between the epidermal cells.

The epithelium of the lateral sides of the dorsal ridge of the siphon is folded ( Fig. 8) and appears to be glandular. The cuboidal cells contain dark basophilic secretory droplets and bear long cilia and a relatively thick apical fringe of unknown composition (see asterisks in Fig. 8). The siphonal crest as well as the median ridge are composed of dense connective tissue and are heavily innervated by numerous nerves (see arrows in Fig. 8).

Parapodial glands: A band of bottle shaped subepidermal glandular cells (see arrows in Fig. 9) runs along the inner margin the complete length of the parapodia. These cells are filled with dense masses of acidophilic secretions, sometimes forming smaller droplets in the apical parts of the cells. The apical part of the cells protrudes between the epidermal cells of the parapodia and secretions appear to discharge through small pores to the exterior surface.

Flagellum: The epidermis of the flagellum is thicker than in other parts of the notum. It is slightly folded and consists of cylindrical non­glandular cells covered by an apical fringe of unknown composition. No traces of nervous innervation of the flagellum could be found.

Gill: The gill plicae are composed of a simple folded, ciliated epithelium.

Foot gland: The foot gland consists of clusters of round to elliptically shaped glandular cells containing heterogeneous filamentous acidophilic secretions. The glandular cells discharge into a common ciliated duct which opens to the ventral side of the posterior foot.

Abbreviations: al, albumen gland; am, ampulla; bc, bursa copulatrix; ce, cerebral ganglion; cga, common genital aperture; dg, digestive gland; fl, flagellum; hd, hermaphroditic duct; hs, head shield; in, intestine; me, membrane gland; mu, mucous gland; oe, oesophagus; ot, oral tube; pa, parapodia; pb, penial bulb; pd, penial duct; pe, pedal ganglion; ph, pharynx; pl, pleural ganglion; pp, penial papilla; pr, prostate; ps, penial spines; sb, subinestinal ganglion; sc, siphonal crest; sgl, salivary gland; sgr, seminal groove; sp, supraintestinal ganglion; spo, spermoviduct; st, stomach; tl, tail; vap, vaginal pocket; vh, visceral hump; vi, visceral ganglion.

Scale bars: 1, 2: 1 mm; 5: 0.5 mm.

Digestive system: The mouth opening is located ventrally just anterior to the foot. The ciliated epithelium of the mouth opening is surrounded by large clusters of subepidermal glandular cells which contain filamentous acidophilic secretions (see asterisk in Fig. 7). A schematic outline of the digestive system is shown in Fig. 3. The oral tube is rather short and stout and leads into the muscular pharynx. No distinct jaw plates could be found, but the anterior pharynx is lined by cuticle with small projections at the ventral side (see arrow in Fig. 7). The radula sac is relatively large and positioned in a ninety degree angle towards the rest of the pharynx. The radula formula of one of the paratypes (MTQ MO 40201) is 18 x 4.1.0.1.4. The inner lateral tooth has a broad base, a sharp outer cusp and two stout denticles at the inner masticatory border whereas the marginal teeth are smooth ( Figs. 13, 14 ).

The oesophagus leaves the pharynx at its very posterior end. It is a non­ glandular tube composed of a simple folded, ciliated epithelium. At the transition of pharynx and oesophagus a pair of small short tubular salivary glands is found which enter the pharynx dorsally. The glandular epithelium of the salivary glands consists of cylindrical cells filled with acidophilic secretions of different structure and density. The glandular cells discharge into a common ciliated duct.

The stomach does not show any signs of a specialized epithelium. At its posterior end the oesophagus merely enters a widely spaced area lined by a simple ciliated pavement epithelium. Further posteriorly this epithelium also lines the central canal of the digestive gland. This canal communicates with the glandular tissue of the digestive gland throughout its whole length. The digestive gland forms one large glandular mass filling almost the entire body in the posterior half. The glandular tissue is packed into follicles which are composed of different types of glandular cells. The cells are always cylindrical and can contain either discrete acidophilic secretory granules, or they may contain smaller basophilic granules or large non­staining vacuoles.

Abbreviations: am, ampulla; bc, bursa copulatrix; go, gonad; ot, oral tube; ph, pharynx; pb, penial bulb; pr, prostate; vap, vaginal pocket.

Scale bars: 7: 200 µm; 8­12: 100 µm.

Abbreviations: fla, first lateral tooth; m, marginal tooth.

Scale bars: 13: 50 µm; 14: 25 µm.

The intestine leaves the stomach on its left side then curves over to the right side of the body where it discharges through the anus. The epithelium lining the intestine is simple and ciliated throughout its entire length. In its proximal region the intestine forms a tube with a small lumen, but the lumen widens further distally before it narrows again just prior to the anus. The anus is located just above the gill, posterior to the genital opening and the nephroporus. It is composed of a folded epithelium studded with long cilia. Large subepidermal glandular cells are present surrounding the anal opening. The glandular cells contain heterogeneous acidophilic secretions and small basal nuclei.

Nervous system: The central nervous system is situated around the anterior digestive system in form of a circum­oesophageal nerve ring ( Fig. 4). The large cerebral ganglia are almost oval in shape and closely appressed, a commissure is not found. All ganglia show the same pattern of peripheral nerve cells and central neuropil. The eyes are located medially at the base of the siphon.

Reproductive system: Siphopteron leah sp. nov. possesses the typical monaulic reproductive system found in most gastropterids. The gonad is located in the hind part of the body lying on the dorsal right side of the digestive gland. Male and female gametes are stored in separate follicles. The sperm follicles are surrounded by the oocyte follicles. The latter contain yolk granules as well as large nuclei. The sperm follicles contain both immature spermatogonia as well as mature sperm with their heads directed to the epithelial lining of the follicle. Some nursing cells can also be found.

A schematic outline of the distal reproductive system is shown in Fig. 6. The small internally ciliated hermaphroditic duct leaves the gonad well to the posterior and runs along the ventral side of the digestive gland anteriorly where it leads to the small tubular ampulla. The ampulla which is lined by a ciliated pavement epithelium contains masses of undirected sperm. It runs along the ventral side of the digestive gland for a short distance then curves to the anterior left side and leads dorsally where it enters the junction of the nidamental glands and the spermoviduct. The nidamental glands are composed of three different parts. The proximal albumen gland consists of highly cylindrical cells filled with large round to elliptically shaped basophilic secretory granules. Nuclei can be found basally in the glandular cells. Alternating with the glandular cells are ciliated supporting cells. The albumen gland shows a continuous transition into the following membrane gland. The latter is composed of a glandular epithelium consisting of cylindrical cells containing heterogenous masses of acidophilic mucus. The cilia of the supporting cells are longer in this glandular part than in the albumen gland. At the distal part of the nidamental glands a mucous gland is located. It forms the largest part of the glandular mass and consists of a highly cylindrical epithelium. The glandular cells contain dense masses of acidophilic mucus. Nuclei could not be distinguished in the glandular cells, neither could supporting cells. The nidamental glands discharge into the distal spermoviduct which leads to the common genital aperture.

A distinct receptaculum seminis could not be found in the specimen investigated histologically. A large globular bursa copulatrix can be found on the left side of the reproductive system. It is lined by a relatively thin pavement epithelium. The bursa is completely filled with baso­ and acidophilic secretions ( Fig. 11). The internally folded and ciliated vagina leads from the bursa distally. Just before it enters the genital aperture the vagina widens to a pocket­like structure which is surrounded by a thick muscle layer ( Fig. 12). This pocket contains masses of undirected sperm embedded in acidophilic secretions.

A seminal groove leads from the genital aperture anteriorly to the penial opening which is located on the right side of the mouth. The penis is a complex structure composed of various distinct parts ( Fig. 5). The penial bulb is muscular and two large spines are found at its base ( Figs. 5, 10). A few smaller spines are also present further anteriorly. The penial duct is followed by a penial papilla which does not bear chitinous spines.The large glandular prostate is composed of histologically distinct parts ( Fig. 11). At the proximal end close to the transition to the penial bulb and penial duct the prostatic epithelial lining is composed of cylindrical cells which apically contain large non­staining vacuoles; medially small basophilic secretory droplets are found and the nuclei are located basally. The glandular cells are ciliated. Further distally the glandular cells become more cylindrical, the apical vacuoles partly contain dense basophilic secretions and the medial small secretory droplets become much more prominent. In the most distal part of the prostate no large secretory vacuoles are found but the glandular cells are completely filled with smaller basophilic secretory droplets. The cilia in this part of the prostate are very short.

Excretory system: The kidney is located above the heart anteriorly and on top of the digestive gland further posteriorly. The epithelium lining the large lumina of the kidney is composed of cylindrical cells containing basal nuclei and large non­staining vacuoles. The nephropore is located just posterior to the genital opening on the right side of the body.

Circulatory system: The large heart can be found well to the anterior above the penis and the central ganglia. The pericard is lined by a very thin pavement epithelium. The ventricle and atrium are composed of loose connective tissue.

Figures

FIGURES 13 ­ 14: Radula morphology of Siphopteron leah sp. nov. 13: inner and outer laterals; 14: close up of inner lateral showing two denticles at inner margin.