Sacculina nectocarcini, Rybakov & Høeg & Kuris, 2006

Rybakov, Robert H. Gurney Alexey V., Høeg, Jens T. & Kuris, Armand M., 2006, Sacculina nectocarcini, a new species of rhizocephalan (Cirripedia: Rhizocephala) parasitising the red rock crab Nectocarcinus integrifrons (Decapoda: Brachyura: Portunidae), Zootaxa 1332 (1332), pp. 37-50 : 41-48

Sacculina nectocarcini sp. nov.

Materials

We examined three specimens from the red rock crab, Nectocarcinus integrifrons (Latreille, 1825) ; Tasmanian Museum and Art Gallery ; ref. nos. G5467, G5468 and G4029. Holotype: G5467 (from Stoney Point , Western Port , Victoria , May 1997: externa attached to the modified abdominal flap of a male crab) is deposited in the Tasmanian Museum and Art Gallery ( Hobart , Tasmania ). Paratype: G5468 (from Stoney Point , Western Port , Victoria , April 1997: detached externae) deposited in the Zoological Museum , University of Copenhagen ( Copenhagen , Denmark). G4029 was dissected and prepared for SEM. The holotype is 11 × 19 × 8 mm in L x H x W; the stalk is very short, about 1.5 mm in diameter. The paratype is 13 × 16 × 12 mm in L x H x W; the stalk is 2 mm in diameter and 4.5 mm long. The third examined specimen is 7.5 × 16 × 5.5 mm in L x H x W .

Other material: Virgin externae were rectangular with a smooth cuticle. A mature externa (15 x 27 x 11 mm in L x H x W) was photographed ( Fig. 1) and subsequently sectioned and stained with H & E ( Fig. 7).

Colouration

Preserved externae were light yellow-brown. In live specimens colour ranged from light yellow, in smaller, presumably younger, specimens, to light to medium brown in larger specimens. Externae with larvae became increasingly deep purple as the larvae developed eyespots. Virgin externae were light yellow.

Position on host Externae emerged from the ventral cuticle of the second or third abdominal segment.

Diagnosis and description

The mature externa is brown and almost symmetrical, with the dorsal side slightly larger than the ventral one. The shape varies from more or less rounded to trapezoidal, with the anterior region significantly larger than the posterior one. The anterior margin is almost straight and continues into a pair of conspicuous dorsal and ventral protuberances (the ‘shoulders’). The latter are pointed or rounded; the ventral shoulder is usually larger than the dorsal one. The posterior margin is concave and may continue into a second pair of ‘shoulders’, which, when present, are much smaller than those of the anterior margin and directed backwards. The stalk is rather thin and, in some specimens, also rather long: it arises from the centre of the posterior margin. The stalk is slightly flared at both ends where it attaches to the host at one end and to the externa at the other. No annuli could be found. The mantle opening is located at the center of the anterior margin. It is somewhat shifted to the left side and slightly elevated above the surface of the externa.

The cuticle surface becomes increasingly wrinkled in mature specimens, with wrinkles running parallel along the anterior and dorsal and ventral margins. Older specimens may have worn or abraded cuticular surfaces ( Fig. 2). The cuticle is very thick and rigid and consists of numerous layers. Below it extends a thick muscle layer with fibres running in different directions ( Fig. 3). The external surface is densely covered with simple spiniform papillae. They are 6–9 µm long, about 1 µm wide at the base and arranged in groups of 3–12 so that their basal parts are fused to form a common base ( Fig. 4). The internal surface of the mantle is smooth or, in some areas, covered with minute setae and it bears a few scattered, but large retinaculae. The retinaculae consist of a cylindrical basal part with dimensions 20–35 µm in diameter (sometimes swollen) and 30–65 µm in height. 11–25 barbed spindles, 14–22 µm long, project from the distal tip of the basal cylinder ( Fig 5). The subjacent layer of cuticle has a very peculiar structure. It consists of a loose fibrous material that forms irregular elevations separated by narrow grooves. In some areas retinaculae can be seen as groups of smooth spindles located at the bottom of oval depressions in the cuticle ( Fig. 6).

The mesentery is complete, thin and extends from the stalk to the mantle opening. The visceral sac is asymmetrical, with the dorsal side significantly larger than the ventral one. There is a deep groove on the right side of the sac, running from the base of the stalk to the mantle opening and a corresponding ridge extends on the left side. The colleteric glands are located in the centre of the left and right surfaces of the visceral sac. They consist of numerous (ca. 100–120) canals, visible in longitudinal sections through the central part of the gland. The canals are arranged in 6–7 rows ( Fig.7) and villiform projections (5–7 µm in length) line their inner surface. The male receptacles are embedded in the visceral mass close to the stalk; they are directed dorso-ventrally and closely applied to each other, but not fused. The left receptacle is better developed, straight and cylindrical in shape; it has very thick walls with smooth inner surface and a wide lumen. The right receptacle is much smaller, also straight, strongly compressed laterally and crescent-shaped in transverse section, so that it has almost no lumen. Whether this difference is caused by one receptacle being invaded by a male trichogon and the other being sterile is unknown as only one specimen has been sectioned. The receptacles gradually pass into short, somewhat tortuous ducts, furnished with numerous distinct ridges along the inner surface. The right duct is shorter than the left. The receptacle ducts are almost as wide as the receptacles, and they exit into the mantle cavity at the level of the stalk.

Remarks

Ideally, any new species should be described in terms of supposed automorphic traits compared to other closely related species. In the Rhizocephala this is not yet possible using morphological characters only. The combination of characters seen in the new species is not known from any described rhizocephalan, and for the present this, and an analysis of the mitochondrial DNA (CO1 region) ( Gurney et al. 2006) and ribosomal RNA ( ITS 1 region) ( Murphy and Goggin, 2000) which distinguished this parasite from Sacculina granifera , Sacculina oblonga and S. carcini , must suffice to separate it from other species.

Etymology

The name is derived from its known host Nectocarcinus integrifrons and is consistent with the naming convention used for the rhizocephalan Sacculina carcini which parasitises Carcinus maena s, a portunid crab from the same family (Carcininae) as N. integrifron s.

Effects on Nectocarcinus integrifrons

The parasite modified the morphology of the male abdomen by broadening it so that it more closely resembled the female abdomen. However, modification was not complete and parasitised males remained distinguishable from females because the male pleopods were not modified or fused as a consequence of infection. This finding contrasts with the observations of Haswell (1888) who described extensive modification to the abdominal appendages of sacculinised male and female N. integrifrons . The parasitised male abdomen lost its concave lateral outline, becoming flattened and slightly convex as described for Portunus pelagicus parasitised by S. granifera ( Weng, 1987) . We did not find any evidence for hyperfeminisation (an abnormally wide abdomen) of parasitised female N. integrifrons .

No ovaries nor testes were present in crabs with externae. Degenerating ovaries were observed in female crabs infected with an interna.

Prevalence and distribution

The first survey of Western Port (May 1997), covered 7 sampling locations within the Western Channel and North Arm ( Fig. 1). Sacculina nectocarcini was common at three of the seven locations with prevalences ranging from 13.3 to 56.5% ( Table 1). Of the 33 parasitised specimens from the May collection, 60% had an externa with the remaining 40% having only interna. The highest prevalence of infected N. integrfrons (56.5%) came from waters 100 to 200 m off the south-east coast of Sandstone Island. Two subsequent collections from Sandstone Island. (October 1997, April 1998) revealed prevalences of 33.3% (internae only) and 56.0% (externae only) respectively.

The prevalence of Sacculina nectocarcini did not differ between the sexes for crabs from Sandstone Island and Tortoise Head, but female crabs from Stony Point had nearly double the percentage prevalence of males from the same location ( Table 2).

Nectocarcinus integrifrons is of marginal commercial value and is not heavily fished. This may, in part, explain the paucity of reports of rhizocephalan infections for this crab. Variable spatial and temporal prevalences are common features for many rhizocephalans ( Heath, 1971; Hochberg et al. 1992; Hines et al. 1997). Despite its common presence in Western Port, this body of water is little studied compared with its neighbouring bay to the west, Port Phillip Bay, which may also have contributed to its lack of detection.

Biology The seasonal life-cycle of this rhizocephalan is unknown but it may well follow the life-cycle elucidated for Sacculina carcini in the northern hemisphere ( Høeg and Lützen, 1995). That is, small crabs are infected in the summer or autumn with female cyprids produced from female nauplii released from mature externae. The interna develops within the infected crab over 9 months. The virgin externae emerge in late spring and are fertilised in summer to mature and produce maximal broods in autumn. We presume a similar life-cycle for this rhizocephalan based on our three collections which revealed large numbers of mature externae in autumn (May 1997 and April 1998) and the absence of externae but presence of internae in spring (October 1997). This seasonal pattern of infection may be the reverse of that described for Sacculina granifera infesting Portunus pelagicus in Moreton Bay, Queensland (27°– 28° S and 153°– 153.25° E) ( Sumpton et al., 1994) and Mornington Island in the Gulf of Carpentaria (11°– 17° S and Long. 136°– 142° E) where maximum infection, as evidenced by externae, occurred during summer. However, because we did not sample N. integrifrons during the summer, we are unable to confidently support this seasonal pattern.

Three separate attempts were made to hatch and rear nauplii larvae from infected N. integrifrons in recirculating marine aquaria with water temperature maintained at 17ºC. In each attempt either eggs or eyed nauplii were aborted by the externa with the host crab usually dying shortly after. Development, from egg to eyed larvae, took 3–4 weeks, at which point larvae were aborted and the externa either withered and dropped off, leaving a black scar on the cuticle of the host, or began brooding another batch of eggs. N. integrifrons were difficult to keep alive in aquarium conditions and no fully developed cypris larvae could develop for morphological description because of naupliar putrifaction in the dying externa.

Two double infections were recorded; each consisted of a large and small externa. The differences in externa size suggested that the larger externa was the first to be implanted with a trichogon initiating immediate growth ( Høeg and Ritchie, 1985). Embryonic development was asynchronous between the externae of double infections.