Alexey V. Rybakov

The Chemoreceptive Lattice Organs in Cypris Larvae Develop from Naupliar Setae (Thecostraca: Cirripedia, Ascothoracida and Facetotecta)

Abstract Lattice organs are peculiar chemoreceptors found only in the Crustacea Thecostraca (Facetotecta, Ascothoracida, Cirripedia). In these taxa, five pairs occur in the head shield (carapace) of the terminal larval instar (y-cyprid, ascothoracid larva, cyprid), which is the settlement stage. Lattice organs represent an autapomorphy for the Thecostraca but their evolutionary origin and possible homologues in other Crustacea remain obscure. We have used scanning electron microscopy to describe the setation pattern of the head shield in late nauplii of one species of Ascothoracida, one species of Facetotecta and several species of the Cirripedia Thoracica, Acrothoracica, and Rhizocephala. The naupliar head shield always carries two pairs setae situated anteriorly near the midline. Each of these setae carry a single pore, and positional, structural and ontogenetic evidence show that these setae are homologous in all the examined species and that they represent precursors of the two anterior pairs of lattice organs of the succeeding larval stage, viz., the ascothoracid larva (Ascothoracida), y-cyprid (Facetotecta), and cyprid (Cirripedia). This leads us to infer that lattice organs are among the most highly modified sensilla in all Crustacea and they have in most cases lost all external resemblance to a seta. The nauplii of the Rhizocephala carry an additional three pairs of setae situated more posteriorly on the head shield and they could be precursors of the three posterior pairs of lattice organs. All other species examined lack these posterior setae, except the Facetotecta which have one posteriorly situated pair.

Revision of the Rhizocephala Akentrogonida (Cirripedia), with a List of All the Species and a Key to the Identification of Families

ABSTRACT A revision of the suborder Akentrogonida Hafele (Cirripedia: Rhizocephala) is proposed, based upon characters concerning ontogeny, larval morphology, and metamorphosis. The original diagnosis of the suborder is amended to include several genera, previously considered to be incertae sedis. The family Sylonidae Boschma is reduced to a synonym of Clistosaccidae Boschma. The diagnosis of the family Chthamalophilidae Bocquet-Vedrine is amended to include the genera Boschmaella Bocquet-Vedrine and Bocquetia Pawlik. The new families erected are: Duplorbidae, new family, for the genera Duplorbis Smith, Cryptogaster Bocquet-Vedrine and Bourdon, and Arcturosaccus Rybakov and Hoeg; Mycetomorphidae, new family, for the genus Mycetomorpha Potts, and Thompsoniidae, new family, for the genus Thompsonia Hafele. The genus Pirusaccus Lutzen is still considered as incertae sedis, though its possible affinity to Thompsoniidae is emphasized. A list of all species of the Rhizocephala Akentrogonida is given with notes on hosts and geographical distribution. A key allows identification of all the families of the Rhizocephala.

Larval Development in Peltogasterella Studied by Scanning Electron Microscopy (Crustacea: Cirripedia: Rhizocephala)

SEM and light microscopic studies on the larvae of the rhizocephalan barnacles Peltogasterella gracilis and P. sulcata reared in the laboratory revealed the presence of 5 naupliar instars, the newly found stage being instar 3. It resembles instar 2 but is larger and has a body slightly tapering towards the posterior end, a reduced subapical (preaxial) seta on the antennule, and both transverse grooves and a specific denticle ornamentation on the hind body of female larvae. In cultures, the larvae of both species develop into cyprids within 4-5 days after release. Peltogasterella nauplii have a size comparable to that seen in Sacculina and Peltogaster, but they are much smaller than the larvae of the cold-water Briarosaccus. The nauplii of Peltogasterella are lecitotrophic and have limb setation and other feeding structures highly reduced. Peltogasterella nauplii have a cuticular flotation collar encircling the body, but it is very small, delicate, and almost smooth. It therefore differs from the homolo- gous, but much larger, and heavily ornamented flotation collars seen in nauplii of Peltogaster and Briarosaccus. A characteristic feature in Peltogasterella nauplii is the single subterminal seta at the distal end of each frontolateral horn, whereas nauplii of Briarosaccus and Peltogaster have two such setae. The antennulary basipod carries a short additional seta, which represents the only remaining rudiment of gnathobases. Nauplii of Briarosaccus and Peltogaster have a homologous, but significantly longer seta, whereas Sacculina nauplii lack this seta altogether. The nauplii of P. gracilis and P. sulcata are very similar in size and morphology, but those of P. sulcata differ by somewhat longer frontolateral horns and furcal spines. Another distinctive feature of P. gracilis nauplii is the ven- tral ornamentation of the hind body that shows different pattern at different stages of development and reflects an internal development of segments. This ornamentation is only weakly pronounced in P. sulcata nauplii. Compari- son with larval development in Briarosaccus tenellusleads to the conclusion that the rhizocephalan ground pattern comprises six naupliar stages just as in other cirripedes (e.g., Thoracica). Comparison of lattice organ morphology indicates a sister group relationship between the families Peltogastridae and Lernaeodiscidae. Our study highlights the value of SEM revealed larval characters and of characters other than appendage setation as being important in rhizocephalan phylogeny.

The Ultrastructure of Retinacula in the Rhizocephala (Crustacea: Cirripedia) and their Systematic Significance

Abstract Scanning electron microscopy is used to examine the mantle surfaces of the reproductive body (externa) in a range of rhizocephalan barnacles. The cuticular structures called retinacula are described in detail and classified into various morphological types. The retinacula can be single or grouped in repeated patterns, or even united by a common base into a variety of complex structures. The surface of the individual retinaculum can be smooth or ornamented in various ways. The different types of retinacula intergrade to some extent and may all be derived from a single type, but they exhibit taxonomically informative differences. They are comparatively easy to study on traditional alcohol specimens and seem to offer the potential of an important suite of taxonomic characters which can be used also by non-specialists. It is recommended that all future species descriptions in the Rhizocephala use SEM to illustrate retinaculum morphology.

Infestation Level of the Crab Hemigrapsus sanguineus by a Parasitic Crustacean Polyascus polygenea (Crustacea: Cirripedia) in Vostok Bay, Sea of Japan

We studied the infestation level of the coastal crab Hemigrapsus sanguineus by a cirripede barnacle Polyascus polygenea in Peter the Great Bay (Vostok Bay, Sea of Japan) in 1998–1999. It is shown that the externae of the parasite usually appear on the surface of the host’s body at the time when the crab attains sexual maturity (at a carapace width of 14–15 mm). More often we encountered infested crabs of a medium size, with a carapace width of 25–29 mm. The level of crab invasion did not depend on the sex of the host. In the summer period the proportion of infested crabs in different habitats ranged from 7 to 84%. The greatest number of crabs bearing the externae of the parasite was registered in an area with a boulder-pebble surface and small surf waves.

Larval development of the rhizocephalanSacculina polygenea (Crustacea: Cirripedia)

Larval development of the rhizocephalanSacculina polygenea (Crustacea: Cirripedia: Rhizocephala) parasitizing the coastal crabHemigrapsus sanguineus was studied in Vostok Bay, the Sea of Japan. At 22–23°C, the entire cycle of larval development takes 2.5 days and includes five naupliar stages and one cypris stage. Like other rhizocephalans, the larvae ofS. polygenea are lecithotrophic and only grow slightly in size in the course of development, and like all sacculinids, they have no flotation collar. The naupliar stages IV and V have a tubercle between the furcal rami; this tubercle is absent in the larvae of the genusPeltogasterella, but it has been described inS. carcini. The first seta of the antennule only disappears completely at the fourth stage, although it is markedly reduced at the third stage. No morphological differences, except differences in size, are found between male and female nauplii.

Larval development of the parasitic barnacle Heterosaccus papillosus (Cirripedia: Rhizocephala: Sacculinidae) studied under laboratory conditions

Larval development of the parasitic barnacle Heterosaccus papillosus (Cirripedia: Rhizocephala: Sacculinidae) is described from larvae reared in the laboratory. Lecithotrophic nauplii of H. papillosus reached the cypris stage in 3.5 days, at 22 23°C. The development included five naupliar and the single cypris stage, thus following the typical pattern in the Rhizocephala Kentrogonida. Like the nauplii of all sacculinids, those of H. papillosus had a single pigmented nauplius eye, unsegmented frontolateral horns each with two long spines and a tubercle between the furcal rami, but they lack a flotation collar and have no seta on the antennal basipod. The male larvae are larger than female ones, the mean lengths in the two sexes never overlap. The most interesting feature of this species are the morphological differences between the male and female larvae. In the male nauplii III to V the anterior body margin is noticeably convex, that is not characteristic for female larvae. The male cyprid also has a more acuited anterior region, whereas that of the female is rounded.

Larval Development in the Rhizocephalan Barnacle Sacculina pilosella

We studied, under laboratory conditions, the larval development of a rhizocephalan barnacle Sacculina pilosellaVan Kampen et Boschma, 1925, which parasitizes the kelp crab Pugettia quadridens(de Haan) in Vostok Bay, Sea of Japan. It is shown that at 22–23°C, the whole cycle of larval development takes about 3 days. The larvae of S. pilosellaare lecithotrophic; their development, like in other rhizocephalans, comprises five naupliar instars. Like the larvae of all sacculinids, the nauplii of S. pilosellahave no flotation collar. In their structure, the larvae of S. pilosellaare similar to the nauplii of the “typical” sacculina, S. carcini(elongated body outline, long furcal branches, and weakly pronounced segmentation of the abdomen). On the other hand, the characteristic outgrowth inbetween the furcal branches that is characteristic of stages IV and V in S. carciniand S. polygeneais absent in the larvae of S. pilosella.The first seta on the antennula of S. pilosellacompletely disappears only at stage IV; however, at stage III, it is already significantly reduced. No morphological differences have been revealed between male and female larvae of S. pilosellaexcept certain size differences.

Anatomy of Arcturosaccus Kussakini, New Genus, New Species (Rhizocephala: Akentrogonida), from the Ultraabyssal Isopod Antarcturus Zenkewitchi Kussakin (Arcturidae)

ABSTRACT Arcturosaccuskussakini, new genus, new species, is a rhizocephalan parasite of the ultraabyssal isopod crustacean Antarcturus zenkewitchi Kussakin, collected off the east coast of Iturup Island, Kuril Islands, Russia. The morphology of the Rhizocephala Akentrogonida is reviewed and compared to this new species. Arcturosaccus has multiple layers of cuticle in the mesenteric canal, and a few spermatogenic islets floating free in the mantle cavity. By these characters, the new species seems most closely related to the genera Duplorbis and Cryptogaster, but differs from them by (1) the presence of the paired ovary, (2) the position of the mesentery, and (3) the mantle aperture, and by the shape of the externa. Naupliar instars are absent in the life cycle of Arcturosaccus kussakini and the larvae hatch as cyprids.

The taxonomic position of rhizocephalan crustaceans of the genus Parthenopea Kossmann, 1874 (Cirripedia: Rhizocephala) with validation of a new family, Parthenopeidae fam. nov

Data on the external morphology are reviewed and new information is provided about the structure of the first naupliar instar of Parthenopea subterranea Kossmann, a rare and poorly known species of rhizocephalan barnacles. The taxonomic position of P. subterranea, which has so far been uncertain, is discussed. A new family, Parthenopeidae fam. nov., is erected to include a single genus, Parthenopea, with a single species, P. subterranea. The distinctive characters of the new family are described.