Shin Kubota

Tiny Sea Anemone from the Lower Cambrian of China

Background Abundant fossils from the Ediacaran and Cambrian showing cnidarian grade grossly suggest that cnidarian diversification occurred earlier than that of other eumetazoans. However, fossils of possible soft-bodied polyps are scanty and modern corals are dated back only to the Middle Triassic, although molecular phylogenetic results support the idea that anthozoans represent the first major branch of the Cnidaria. Because of difficulties in taxonomic assignments owing to imperfect preservation of fossil cnidarian candidates, little is known about forms ancestral to those of living groups. Methods and Findings We have analyzed the soft-bodied polypoid microfossils Eolympia pediculata gen. et sp. nov. from the lowest Cambrian Kuanchuanpu Formation in southern China by scanning electron microscopy and computer-aided microtomography after isolating fossils from sedimentary rocks by acetic acid maceration. The fossils, about a half mm in body size, are preserved with 18 mesenteries including directives bilaterally arranged, 18 tentacles and a stalk-like pedicle. The pedicle suggests a sexual life cycle, while asexual reproduction by transverse fission also is inferred by circumferential grooves on the body column. Conclusions The features found in the present fossils fall within the morphological spectrum of modern Hexacorallia excluding Ceriantharia, and thus Eolympia pediculata could be a stem member for this group. The fossils also demonstrate that basic features characterizing modern hexacorallians such as bilateral symmetry and the reproductive system have deep roots in the Early Cambrian.

Early Cambrian pentamerous cubozoan embryos from South China.

Background Extant cubozoans are voracious predators characterized by their square shape, four evenly spaced outstretched tentacles and well-developed eyes. A few cubozoan fossils are known from the Middle Cambrian Marjum Formation of Utah and the well-known Carboniferous Mazon Creek Formation of Illinois. Undisputed cubozoan fossils were previously unknown from the early Cambrian; by that time probably all representatives of the living marine phyla, especially those of basal animals, should have evolved. Methods Microscopic fossils were recovered from a phosphatic limestone in the Lower Cambrian Kuanchuanpu Formation of South China using traditional acetic-acid maceration. Seven of the pre-hatched pentamerous cubozoan embryos, each of which bears five pairs of subumbrellar tentacle buds, were analyzed in detail through computed microtomography (Micro-CT) and scanning electron microscopy (SEM) without coating. Results The figured microscopic fossils are unequivocal pre-hatching embryos based on their spherical fertilization envelope and the enclosed soft-tissue that has preserved key anatomical features arranged in perfect pentaradial symmetry, allowing detailed comparison with modern cnidarians, especially medusozoans. A combination of features, such as the claustrum, gonad-lamella, suspensorium and velarium suspended by the frenula, occur exclusively in the gastrovascular system of extant cubozoans, indicating a cubozoan affinity for these fossils. Additionally, the interior anatomy of these embryonic cubozoan fossils unprecedentedly exhibits the development of many new septum-derived lamellae and well-partitioned gastric pockets unknown in living cubozoans, implying that ancestral cubozoans had already evolved highly specialized structures displaying unexpected complexity at the dawn of the Cambrian. The well-developed endodermic lamellae and gastric pockets developed in the late embryonic stages of these cubozoan fossils are comparable with extant pelagic juvenile cubomedusae rather than sessile cubopolyps, whcih indicates a direct development in these fossil taxa, lacking characteristic stages of a typical cnidarian metagenesis such as planktonic planula and sessile polyps.

Sexually Dimorphic Cubomedusa Carybdea sivickisi (Cnidaria: Cubozoa) in Seto, Wakayama, Japan

The small but distinctive cubomedusa Carybdea sivickisi has been reported from a range of tropical, subtropical, and mild temperate localities in the Pacific. In Japan, it has only definitively been documented in the subtropical region of Okinawa. However, in 1970 Uchida noted that three specimens from Seto, Wakayama, which he had referred to as Tamoya alata in 1929, were really C. sivickisi. The presence of this species as far north as Wakayama was questionable because of the confusion about the identity of the specimens and the fact that C. sivickisi had never been observed subsequently in this region. In August 2006, we discovered a population of this tiny species nearby the Seto Marine Biological Laboratory in Shirahama, Wakayama. In this paper we comment and clarify the geographical range, developmental morphology, and sexual behaviour of this species.

Occurrence and detrimental effects of the bivalve-inhabiting hydroid Eutima japonica on juveniles of the Japanese scallop Mizuhopecten yessoensis in Funka Bay, Japan: relationship to juvenile massive mortality in 2003

In November 2003, we first observed prevalent occurrence of a hydroid, Eutima japonica, on soft body tissues of age zero Japanese scallop (Mizuhopecten yessoensis) juveniles cultured in large areas of Funka Bay, Hokkaido. The occurrence coincided with massive death of juvenile scallops. A major objective was to clarify ecological relationships between the symbionts, and to infer the relationship between symbiosis and the massive mortality. To do this, we investigated distributions of association rates of hydroids with juvenile scallops at 15–34 sites over 3xa0years (2003–2005), with age one adult scallops at 24 sites in 2003, and with mussels at 13 sites in 2004. We studied seasonal changes in association rates with juvenile scallops, and numbers of polyps per juvenile scallop at three sites from November 2003 to June 2004. We also quantified the hydroid impacts on juvenile scallop shell length growth and triglyceride accumulation in the digestive gland. The association rate of E. japonica polyps with juvenile scallops was high in large areas of Funka Bay in 2003, and overlapped the distribution of mussels bearing polyps. Association rates with age one adult scallops were very low in November 2003, even at the sites where polyps were very common on juvenile scallops. Levels of hydroid occurrence in juvenile scallops varies by year. We found that hydroids presence in juvenile scallops declined drastically in 2004 and 2005. The association rates with juvenile scallops, and numbers of polyps per juvenile scallop declined during winter, until they disappeared completely in the following June. Since polyps were rare in adult scallops, we believe that infection of juvenile scallops was probably initiated from the planulae produced by medusae released from polyps growing on Mytilus spp., especially M. galloprovincialis. Subsequently, the inhabitation spread intraspecifically and interspecifically. In juvenile scallops, inhabitation of polyps reduced shell length growth by 43%, and triglyceride accumulation in digestive glands by 24–47%. Inhabitation of E. japonica on juvenile scallop is best regarded as parasitism, rather than inquilinism or commensalism. Occurrence of polyps was probably not a direct lethal factor for juvenile scallops, because there were some sites where association rates were high, but mortalities were low. Massive mortalities in 2003 may have resulted from simultaneous impacts of heavy polyp load and stresses caused by the way in which the animals were handled (transferred from cages for pre-intermediate culture to cages for intermediate culture), because the massive mortality occurred within a month of the transfer. The presence of polyps in juvenile scallops does not affect the quality of the product in Funka Bay, because market size scallops are hydroid-free.

Influences of temperature and salinity on asexual budding by hydromedusa Proboscidactyla ornata (Cnidaria: Hydrozoa: Proboscidactylidae)

At the head of Tanabe Bay, on the Pacific coast of Japan, dense populations of Proboscidactyla ornata medusae were observed at 24°C, 34 psu from July 2001 to September 2002 (maximum 472.4 individuals m −3 ). In the laboratory, 288 immature medusae of P. ornata collected from Tanabe Bay in May–June 2006 were cultured individually and fed with asphyxiated Artemia nauplii under 12 experimental conditions (four different temperatures (17°C, 20°C, 23°C and 26°C) combined with three different salinities (28 psu, 31 psu and 34 psu)). For 30 days, 94% of the medusae survived at all conditions. The maximum number of clones produced was 0.32 clones medusa −1 day −1 at 20°C, 34 psu; the number of medusae increased 10.5 times in 30 days. The maturation percentage at 23°C (54.2–58.3%) was higher than at 17°C (12.5–25.0%) and was positively correlated with the number of nauplii ingested. Temperature-specificity of energy usage was observed: 20°C for asexual budding and 23°C for sexual reproduction. The direct relationship between budding and temperature is demonstrated among hydromedusae for the first time in this study. Additionally, the influence of salinity on budding suggests a hypothesis on an advantage of budding at offshore and high saline waters.

Synchronous Mass Release of Mature Medusae from the Hydroid Halocordyle disticha (Hydrozoa, Halocordylidae) and Experimental Induction of Different Timing by Light Changes

The timing mechanism for synchronous mass release of mature medusae of Halocordyle disticha was studied, using colonies from Shirahama, Wakayama, Japan, which were kept in a 450 I aquarium tank. In near natural conditions medusa release is correlated with sudden drop of light intensity such as occurs around sunset. Timing could be manipulated by controlling light intensity. Artificial sunset 2 hours earlier than normal caused mass release of medusae earlier than under natural conditions, whereas sunset artificially delayed 3 hours later than normal caused continuous release of medusa after the onset of darkness. The spawning of gametes of H. disticha is almost simultaneous with medusa release, and since the medusa has an ephemeral planktonic existence, synchrony of mass medusa release and also spawning of gametes may maximize fertilization

Notes on the morphology, ecology and life cycles of Fukaurahydra anthoformis and Hataia parva (Hydrozoa, Athecata)

Fukaurahydra anthoformis and Hataia parva are solitary athecate hydroids occurring in northern Japan. New information on the external morphology, nematocysts, ecology, and life cycles of these species is presented. It is noteworthy that H. parva bears stenoteles, which are generally not found among the families of Filifera. Neither species produces free medusae. The eggs are fertilized in the female gonophores, from which unciliated larvae are released. These larvae do not swim and soon attach to a substrate. After attachment the larvae become covered by a sheath to form cysts. The cysts rest on a substrate without any outer change for several months. As the water temperature drops in autumn to early winter the cysts begin to hatch, forming tiny polyps after the larva creeps out from the chitinous sheath. Cyst formation proves to be common also in other solitary hydroids, most of which are inhabitants of cool or cold waters.

Fluorescence distribution pattern allows to distinguish two species of Eugymnanthea (Leptomedusae: Eirenidae)

The auto-fluorescence patterns of the medusae observed under a fluorescent microscope with blue light excitation allows to distinguish two species of Eugymnanthea , this even when they are still attached to the hydroid as small medusa buds despite the occurrence of a sex-dependant pattern in E. japonica . A total of four distribution patterns of green fluorescence, including non-fluorescence, could be found. Three of them are found in E. japonica , called ‘subumbrellar fluorescence type’ except for non-fluorescence, while another type is found in E. inquilina , called ‘umbrellar margin fluorescence type’. During the short life of the medusa the latter type remained invariable for up to six days in E. inquilina , while the pattern observed for up to seven days in E. japonica changed sometimes, but it always remained distinguishable from the pattern found in E. inquilina . Therefore, the fluorescence pattern is a reliable taxonomic character. Fluorescence was not found in unfertilized eggs, planulae 2–8 days old, parthenogenetically produced larvae, or in the hydroids of the two species. The auto-fluorescent and possible bioluminescent tissues of these Eugymnanthea medusae could have some unknown biological significance.

A New Form of the Bivalve-Inhabiting Hydrozoan Eutima japonica (Leptomedusae, Eirenidae) in Japan

Further intraspecific variability of Eutimajaponica is described and illustrated. The newly matured medusa of this so-called transitional form, the hydroid of which is associated with the bivalve Barbatia virescens from Kashima Island off Kyushu, southern Japan (53.8% prevalence), is more developed than the medusa of the intermedia form, but is less developed than the medusae of the southern and northern forms. The major morphological features of the new forms newly matured, 7to 14-day-old medusae of both sexes are: a very short peduncle of only 0.3-1.5 mm in length, four (61.7% ofindividuals) or eight (34.0%) tentacles, a medium-sized umbrellar diameter (4.8-7.4 mm), and a large but variable number (0-67) of lateral cirri. The life-span of the medusae in the laboratory at 22 ± l oc was relatively short, about one month.

Redescription of Stylactaria conchicola (Yamada, 1947), comb. nov. (Hydrozoa: Hydractiniidae) from Hokkaido, Japan

Stylactaria conchicola (Yamada), comb. nov., a highly polymorphic athecate hydroid, is redescribed based on specimens collected from the type locality (Muroran) and a new locality (Oshoro) in Hokkaido, Japan. Three new types of zooids (thick-gastrozooid, non-hypostomed gonozooid and tentaculozooid) are described in addition to the two types of known zooids (slender-gastrozooid and normal gonozooid). The non-tentaculate gonozooid originally described as the third zooid type is considered a normal gonozooid being degenerated under rearing conditions. Aberration of gastrozooids is also described and briefly discussed in terms of zooid differentiation. Intercolonial variation of zooid composition is presented and its significance in species identification is discussed. Nematocyst composition and morphology of gametes are also described. S. conchicola is substrate specific; all of the 78 colonies examined were found on the shells of living gastropods, Homalopoma amussitatum (Gould, 1861).