研究報告　Research reports

An undescribed lobate ctenophore, Lobatolampea tetragona gen. nov. & spec. nov., representing a new family, from Japan

TOBA AQUARIUM Horita T.
鳥羽水族館　堀田拓史

Horita,T.An undescribed lobate catenophore,Lobatolampea tetragona gen.nov.& spec.nov.,representing a new family,from Japan.
Zool.Med.Leiden 73(30),6.iii.2000:457-464,figs 1-3.-ISSN 0024-0672.
Takushi Horita,Marine Biological Laboratory,Toba Aquarium,Toba,Mie 517-8517,Japan.

Key words:Ctenophora;Lobatolampeidae fam.nov.;Lobatolampea tetragona gen.nov. & spec.nov.;lobate;medusoid;Japan.
Lobatolampea tetragona gen. nov. & spec. nov. was collected from coastal waters of Ise Bay, near Toba, Pacific coast of middle Japan. It assumes a medusoid bell-shape when floating, but is nearly discshaped when resting on the bottom. It is characterized by four C-shaped gonads present on the distal part of the subpharyngeal meridional canals, beyond the oral end of the comb rows. "Primitive" oral lobes are also formed and ctenes are found at the site of the presumptive auricles. The morphology of this species differs from any previously described larval or adult form, and is intermediate between the orders Cydippida and Lobata. The larvae are of the cydippid type.

Key words: Ctenophora; Lobatolampeidae fam. nov.; Lobatolampea tetragona gen. nov. & spec. nov.; lobate; medusoid; Japan.

fig.1

fig.2

　　Description (mainly based on lost holotype).− The specimen resembled a thalassocalycid medusa when floating (fig. 3), but was roughly disc-shaped when resting on the bottom (fig. 3). Body flattened along the oral-aboral axis, with the aboral part somewhat domed. Prominences, keels and papillae lacking in the aboral region around the sensory organ. Polar plates of moderate length. Tentacular plane slightly narrower than pharyngeal plane, measuring 21.3 x 23.1 mm in width. Comb-rows short, extending to level of tentacle bulbs; subtentacular rows slightly shorter than subpharyngeal rows, running to very near tentacular plane, whereas subpharyngeal comb-rows each extending roughly to centre of its respective quadrant. Number of comb plates 16 per row. Oral margin markedly incurved to form an Omega-shaped cleft at each end of the tentacular plane, up to halfway up the bell. Oral periphery fringed wholly with a broad, thin, and transparent velar membrane, hanging down when floating.

　　Very short perradial canals and blind paragastric canals present as in Cydippida.Interradial canals and bifid tentacular canals distinct. Adradial canals join respective meridional canals at the level of aboral one-fifth of the comb-rows. Subpharyngeal meridional canals end blind orally, and bear gonads in the distal portion beyond the oral end of each comb-row, Situated near interradial axis. Subtentacular meridional canals in each quadrant aligned almost along tentacular plane and extending along the margin of the simple oral lobe. After meeting at the middle of the lobe margin in the pharyngeal plane, both canals run together along the pharyngeal axis on the vestibular wall over the pharynx and extend to the central entrance region of the folded portion of the pharynx; connected respectively to partner extension of neighbouring subtentacular meridional canal on opposite side of tentacular plane. Thus, the circumoral canal is formed. It is accompanied by the ciliated groove along its whole length and with the secondary tentacles along the periphery of the oral lobes.
　　Four remarkable, well developed C-shaped gonads are present, one each, in the distal part of each of the four subpharyngeal meridional canals, beyond the oral edge of the comb rows. Subtentacular meridional canals sterile.
　　Primary tentacles reduced, represented only by conspicuously pigmented bulbous bases, approximately at level of distal end of subtentacular row of comb-plates (about the level of aboral one-fourth of bell height); tentacle sheath vestigial. A pair of stout tentacular stems of the secondary tentacular system issuing from each tentacle base; each stem running towards pharyngeal plane and subsequently turning along this plane towards a peripheral notch at the middle of each oral lobe. Stems extend along the periphery of the oral lobes, on the interior surface, to entrance of cleft in tentacular axis. Many side-branches issued from this stem in each quadrant. In life, side-branches actively extended or contracted and entangled near the proximal portion; however, they were gradually torn off and later only a small number of short ones remained.
　　Colour.− The whole body is transparent, but the testicular side of the gonads and the tentacle bulbs are golden in colour. The ovarian side of the gonads, the base of the ctenes at the site of presumed auricles and the upper part of the infundibulum are conspicuously red.
　　Remarks.− Food particles caught by the tentacles were carried along the ciliated groove. Even food obtained near the tentacular plane was sent along the lobe periphery onto the pharyngeal plane, and subsequently to the pharynx.
　　Embryos in various stages of cleavage were observed in the gonads of the holotype. The specimen continued to release cydippid larvae intermittently for a few days after capture. In the other specimen, gonads were emptied by release of germ cells. Larvae were pear-shaped when just released, but changed to nearly spherical within 24 hours, and gradually elongated. After about two weeks the mouth of the larvae began to spread and the tentacular sheath opened on the bell at the level of the infundibulum. Tentacles, furnished with side-branches, measured more than three times body length in early stages, but gradually shortened to nearly body length after two weeks. Further rearing of larvae failed at this point.
　　Discussion.− Another remarkable feature of Lobatolampea tetragona spec. nov., in addition to the four C-shaped gonads, is its medusa-like bell shape when floating, changing to nearly disc-shape when resting on the substrate. The animal is provided with "primitive" oral lobes (figs 1,3B), and has no primary tentacles. The existence of ctenes at the site of the presumptive auricles as in Lobata is noteworthy. Internally, the gastrovascular canal system resembles closely that of Cydippida, except for the subtentacular meridional canals extending to form the circumoral canal. These features indicate a position of L. tetragona intermediate between Cydippida and Lobata.
　　Lobatolampea tetragona superficially resembles Ganeshida, in which, however, the body is compressed along the tentacular axis, the tentacle sheaths open on the oral margin, the perradial canals are missing, the interradial canals arise directly from the infundibulum and lead to the aboral end of the meridional canals, and the subpharyngeal canals are connected to the circumoral canal. These features are clearly different from those in the present species.
　　In Thalassocalycida and Lobata, four medusoid forms are depicted by the following authors: Chun, 1880; Moser, 1910; Dawydoff, 1948; and Madin A Harbison, 1978. Ttlalassocalyce inconstans, described by Madin & Harbison (1978) as an adult medusoid ctenophore, is about twice as large as L. tetragona spec. nov., and bears the tentacles located near the mouth as in Lobata. Neither oral lobes, nor ctenes are present at the site of tile auricles. In addition the gastrovascular canals show a complicated configuration. These features are quite different from those seen in L. tetragona. The medusoid stage of Leucothea multicornis (Chum, 1880) has ctenes at the site of the auricles, but is has a mucll more complex gastrovascular canal systeln thall has L. tetragona. Cryptolobata primitiva described by Moser (1910), a possible larval lobatean form, and a larva of Ocyropsis spec. reported by Dawydoff (1948), are also medusoid in shape. In the former nominal species, the course of the gastrovascular canal is quite different from that in L. tetragona. In the latter it has a manubrium-shaped peduncle, though neither tentacle nor tentacle sheath exists in its cydippid larval stage (Dawydoff, 1948; and personal observation). These features are different from those in L. tetragona, though Dawydoff mentioned nothing about gastrovascular canals and gonads in the medusoid larva.
　　The presence of four gonads on the subpharyngeal meridional canal also has been recorded in Savangia (only testis), and in Thalassocalyce (Harbison & Madin, 1982) the gonads are formed on the subtentacular meridional canals instead of on the subpharyngeal meridional canals. The formation of gonads in the subpharyngeal meridional canals is usually seen only early in reproduction, in dissogony. Therefore, it might be possible to consider the existence of four gonads in the present species as a neotenous character. The presence of typical lobatean characters in L. tetragona (though rudimentary developed), such as "primitive" oral lobes and ctenes at the presumptive site of the auricles, might indicate that it is an undescribed larval form of a lobatean ctenophore. In Lobata, however, the characteristic lobe formation appears early in development and metamorphosis takes place at a body size much smaller than in L. tetragona. In Bolinopsis vitrea L. Agassiz, 1860 and Mnemiopsis leidyi A. Agassiz, 1865, oral lobes and auricles appear in larvae of 7-10 mm body length (Mayer, 1912). As mentioned previously, L. tetragona is quite different from any medusoid larval form described so far; and the two available specimens did not show any sign of morphogenesis during rearing in the laboratory for 15 and 41 days, respectively (see introduction). Therefore, the author considers the present specimens to represent an adult form of a previously unknown species, genus and family.
　　Although it is difficult to deduce the adequate generic and family criteria from the diagnosis of the new species, the author wishes to establish a new monotypic genus Lobatolampea and a new family Lobatolampeidae, tentatively, to hold the present new form. However, the author wants to refrain himself from establishing a new order, pending further studies on variation and morphogenesis of features defining higher taxa of Ctenophora.
　　Etylmology.− The generic and family names express that the new species is provided with oral lobes (though in the "primitive" state) as in Lobata, while the body shape is freely changeable as in Lampea (Cydippida). The specific name refers to the four conspicuous gonads.