Stylochus pygmaeus, Merory & Newman, 2005

Merory, Marsha & Newman, Leslie J., 2005, A new stylochid flatworm (Platyhelminthes, Polycladida) from Victoria, Australia and observations on its biology, Journal of Natural History 39 (28), pp. 2581-2589 : 2582-2586

Stylochus pygmaeus sp. nov.

? Stylochus z anzibaricus Laidlaw 1903; Skerman 1960a, p 612, Figure 2 .

Material

Holotype: MV F93543 , WM, 29 January 2002, Breakwater Pier , Williamstown, Victoria, Australia . Paratypes: MV F94086 , WM, same data ; MV F94087 , LS, same data , MV F94088 , same data , MV F94089 , four specimens, S, same data .

Description

Animals are relatively small, body oval to elongate oval, thick and fleshy, without marginal ruffles. Smaller animals tended to be more rounded oval. Sizes ranged from 4.9× 3 mm to 8.2 mm × 3.5 mm.

The dorsal colour pattern is cream-beige with densely scattered dark brown and lighter brown mottling, margin without markings ( Figure 2 ). The colour pattern is variable; larger worms are darker with denser markings. Paired, elongate retractile nuchal tentacles are located posterior to the first quarter of body length from the anterior margin ( Figure 3 ). The ventral surface is grey-white, without markings.

There are three to four rows of scattered marginal eyes along the anterior margin, the eyes are more densely packed anteriorly, then reducing to two to three rows extending for a short distance along the margin to the level of tentacles ( Figure 3 ). Tentacular eyes (about 10–15 each side) are present within the tentacles and there are about 20 scattered cerebral eyes between the nuchal tentacles, however, the dark markings of the worm tend to obscure the cerebral eyes.

The pharynx is central, about one-quarter the body length, highly ruffled and mid-body with complex ruffled folds ( Figure 4 ). The mouth is in the middle of the pharynx. Gonopores are found close together but separate, posterior to the pharynx, proximal to the posterior margin. The vas deferens extend anteriorly from the male pore.

Testes are scattered throughout the body. The seminal vesicle has a single muscular lobe (0.72 mm long) and lies ventrally ( Figure 5 ). The seminal vesicle leads into a long and narrow ejaculatory duct which joins the prostatic duct before it leads into the penis papilla. A short prostatic duct leads to a large prostate (0.80 mm long) with numerous ducts leading into the lumen which is lined with a folded epithelium. Both the seminal vesicle and prostate are of similar size. Penis papilla simple, extremely short within a shallow male antrum. Ovaries scattered dorsally throughout the body. Ova collect into the uteri on each side of the pharynx, running posteriorly to the female pore, curve dorsally and join at the distal end of the vagina. Details of the vagina were not evident from serial sections.

Diagnosis

Small, cream-beige Stylochus sp. with scattered brown mottling, with three to four rows of anterior marginal eyes, 10–15 tentacular eyes each side and about 20 cerebral eyes.

Etymology

Named from the Latin, pygmaeus 5dwarf, for its relatively small size.

Remarks

Skerman (1960a) reported relatively small stylochid flatworms from the Port of Auckland, New Zealand and his specimens were identified by L. Hyman as Stylochus zanzibaricus Laidlaw, 1903 . A black and white photograph ( Skerman 1960a, Figure 2 ) indicates that these stylochids are similar to Stylochus pygmaeus sp. nov. They were also thought to prey on the same barnacle species.

Although the present species bears some resemblance to the worms described briefly by Laidlaw (1903) as Stylochus zanzibaricus from Zanzibar ( Tanzania), his original description, from a single fixed specimen, is without colour notes or any details of the reproductive anatomy. Both Laidlaw (1903) and Meixner (1907) believed that the copulatory structures of S. zanzibaricus were identical to S. neapolitanus Lang, 1884 from the Mediterranean and that these species are ‘‘probably identical’’. Furthermore, the colour figure ( Lang 1884, Plate 1, Figure 7) of S. neapolitanus Lang, 1884 is clearly different to our specimens since this stylochid is large and elongate (not small and rounded oval), with long striped nuchal tentacles and distinct transverse marginal banding. These features are clearly not found in S. pygmaeus sp. nov.

Moreover, there are distinct morphological differences between Skerman’s (1960a) specimens and S. pygmaeus sp. nov. Laidlaw (1903) reported that the marginal eyes are in a band of equal width that does not continue around the anterior margin as far as the tentacles, whereas in the present specimens the band of eyes continues posterior to the tentacles. In S. pygmaeus sp. nov. there are three to four rows of eyes at the anterior margin and two to three rows laterally. Nor does Skerman document the presence of cerebral eyes that are embedded in the epidermis between the tentacles. Furthermore, the tentacular eyes are clearly clustered within the tentacles, not on them, as stated by Laidlaw (1903). In our specimens, the nuchal tentacles are more elongate that those shown by Laidlaw, although this may be due to the fact that he examined a fixed specimen and we are describing live animals. The diminutive size at sexual maturity of this new species sets this species aside from any congenerics.

Distribution

Common during summer months within barnacles attached to pier pilings in the greater Melbourne ports area, Victoria, Australia.

Figures

Figures 1, 2. Stylochus pygmaeus sp. nov. (1) Inside the barnacle Balanus amphitrite Darwin, 1854. (2) With eggmass deposited on a perspex plate in the absence of barnacles of suitable size.

Figures 3–5. Stylochus pygmaeus sp. nov. (3) Details of the eyes arrangement. (4) Morphology of the holotype from ventral view. (5) Reconstruction of the reproductive anatomy of a paratype.