Mieko Komatsu

REPRODUCTION AND DEVELOPMENT OF THE HERMAPHRODITIC SEA-STAR, ASTERINA MINOR HAYASHI

1. The breeding season of Asterina minor is estimated to be during the month of May in Kushimoto, Japan. A. minor shows a characteristic breeding assemblage and the eggs are laid on the substratum in a mass spawning. The eggs are not protected by the adults.2. A. minor is a spatial hermaphrodite, where ovaries and testes in an individual become mature simultaneously. Isolated individuals are capable of self-fertilizing and the self-fertilized eggs develop normally.3. The spawned eggs are spherical, yellow, and 437 µm in average diameter. They attach to the substratum with a sticky jelly layer. Cleavage is total and radial.4. Eggs through the wrinkled blastula stage develop into a pear-shaped brachiolaria bearing three brachiolar arms within the fertilization membrane.5. About four days after spawning, the brachiolariae hatch from the fertilization membrane and creep on the substratum with well-developed brachiolar arms. There is no evidence of pelagic life in the present species.6. One day after hatching,...

On the development of the sea-star, Astropecten latespinosus Meissner.

1. The entire process of development in the sea-star, Astropecten latespinosus, is reported, especially with regard here to the external morphology and the skeletal system.2. The eggs are medium-sized, about 300 µ in diameter. They develop into free-swimming larvae through a wrinkled blastula stage by holoblastic, radial cleavage.3. The free-swimming larva has a peculiar barrel shape, being neither bipinnaria nor brachiolaria. Such a larva has not previously been reported.4. Metamorphosis takes place while the larva is pelagic, and there is no feeding at this stage. Five days after insemination, metamorphosis is completed and the resulting juveniles bear 2 pairs of tube-feet and a terminal tentacle in each arm.5. The present observations are compared with those studied by other workers, and are discussed, with special reference to the type of development of sea-stars.

Reproduction and Development of the Conspicuously Dimorphic Brittle Star Ophiodaphne formata (Ophiuroidea)

Ophiodaphne formata is a conspicuously dimorphic ophiuroid; the disk diameters are approximately 1 mm for males and 5 mm for females. The dwarf male clings to the larger female, with the oral surfaces and bursae of the paired ophiuroids closely appressed. Moreover, the female of each pair adheres aborally to the oral surface of a host sand dollar, Astriclypeus manni. Spawning and external fertilization occur in August, at Tsuruga Bay, Sea of Japan. Development of the dimorphic brittle star O. formata is described for the first time, from spawning through metamorphosis, with special attention to the formation of the skeletal system and the external morphology of early juveniles. Fertilized eggs are about 90 μm in diameter, pale pink, and negatively buoyant. The embryos undergo equal, total, and radial cleavage, and the larval skeleton first forms as a pair of tetraradiate spicules. Larval development proceeds to an 8-armed planktotrophic ophiopluteus, with skeletal elements that consist of a body rod and two recurrent rods. Three weeks after fertilization, all the pluteal arms, except for the postero-lateral arms, are absorbed, and the metamorphosing larvae sink to the bottom. Metamorphosis is completed 21.5 days after fertilization, and the resulting juvenile is pentagonal and approximately 270 μm in diameter. The smallest specimen (480 μm in disk diameter) collected by field sampling exhibited male features on the skeletal plates of the jaw and disk. Sexual dimorphism, the peculiar pairing behavior, and the close relationship with the host sand dollar may have evolved as distinct reproductive characteristics in this ophiuroid with its typical ophiopluteus larvae.

Development of a True Ovoviviparous Sea Star, Asterina pseudoexigua pacifica Hayashi

Asterina pseudoexigua pacifica is a true ovoviviparous asteroid in that its development and metamorphosis occur within the spatial hermaphroditic gonad. From the middle of June to the middle of July, the gonad contains numerous embryos and juveniles in various stages through metamorphosis. The opaque, greenish yellow mature ovum is 450 μm in diameter. Development is direct. Embryos develop through wrinkled blastula and gastrula stages into a pear-shaped brachiolaria with three arms. The general process is similar to that of asteroids having direct development. Newly metamorphosed juveniles are released from the gonopores. Peak release occurs in the middle of July. The maximum number of juveniles released from an adult is about 1300. The juvenile is 900 μm in diameter and has two pairs of tubefeet in each arm; the skeletal plates are well developed. The present results are compared with those of other true viviparous echinoderms.

Development of the nervous system in the brittle star Amphipholis kochii.

There are several studies of neural development in various echinoderms, but few on ophiuroids, which develop indirectly via the production of pluteus larvae, as do echinoids. To determine the extent of similarity of neuroanatomy and neural development in the ophiuroids with other echinoderm larvae, we investigated the development of the nervous system in the brittle star Amphipholis kochii (Echinodermata: Ophiuroidea) by immunohistochemistry. Immunoreactive cells first appeared bilaterally in the animal pole at the late gastrula stage, and there was little migration of the neural precursors during A. kochii ontogeny, as is also the case in echinoids and holothuroids. On the other hand, neural specification in the presumptive ciliary band near the base of the arms does occur in ophiuroid larvae and is a feature they share with echinoids and ophiuroids. The ophiopluteus larval nervous system is similar to that of auricularia larvae on the whole, including the lack of a fine network of neurites in the epidermis and the presence of neural connections across the oral epidermis. Ophioplutei possess a pair of bilateral apical organs that differ from those of echinoid echinoplutei in terms of relative position. They also possess coiled cilia, which may possess a sensory function, but in the same location as the serotonergic apical ganglia. These coiled cilia are thought to be a derived structure in pluteus-like larvae. Our results suggest that the neural specification in the animal plate in ophiuroids, holothuroids, and echinoids is a plesiomorphic feature of the Ambulacraria, whereas neural specification at the base of the larval arms may be a more derived state restricted to pluteus-like larvae.

Functional evolution of Ets in echinoderms with focus on the evolution of echinoderm larval skeletons

Convergent evolution of echinoderm pluteus larva was examined from the standpoint of functional evolution of a transcription factor Ets1/2. In sea urchins, Ets1/2 plays a central role in the differentiation of larval skeletogenic mesenchyme cells. In addition, Ets1/2 is suggested to be involved in adult skeletogenesis. Conversely, in starfish, although no skeletogenic cells differentiate during larval development, Ets1/2 is also expressed in the larval mesoderm. Here, we confirmed that the starfish Ets1/2 is indispensable for the differentiation of the larval mesoderm. This result led us to assume that, in the common ancestors of echinoderms, Ets1/2 activates the transcription of distinct gene sets, one for the differentiation of the larval mesoderm and the other for the development of the adult skeleton. Thus, the acquisition of the larval skeleton involved target switching of Ets1/2. Specifically, in the sea urchin lineage, Ets1/2 activated a downstream target gene set for skeletogenesis during larval development in addition to a mesoderm target set. We examined whether this heterochronic activation of the skeletogenic target set was achieved by the molecular evolution of the Ets1/2 transcription factor itself. We tested whether starfish Ets1/2 induced skeletogenesis when injected into sea urchin eggs. We found that, in addition to ectopic induction of mesenchyme cells, starfish Ets1/2 can activate some parts of the skeletogenic pathway in these mesenchyme cells. Thus, we suggest that the nature of the transcription factor Ets1/2 did not change, but rather that some unidentified co-factor(s) for Ets1/2 may distinguish between targets for the larval mesoderm and for skeletogenesis. Identification of the co-factor(s) will be key to understanding the molecular evolution underlying the evolution of the pluteus larvae.

Neural architecture of the brachiolaria larva of the starfish, Asterina pectinifera.

The nervous system of the brachiolaria larva of the starfish, Asterina pectinifera, was characterized using immunohistochemistry with the neuron‐specific monoclonal antibodies 1E11 and 1F9 and an anti‐serotonin antibody. The antigen recognized by 1F9 was determined by immunoprecipitation, peptide identification by mass spectrometry, and cDNA cloning as a novel START (steroidogenic acute regulatory protein [StAR]‐related lipid transfer) domain‐containing protein. Nerve cells are prominent in the brachiolar arms, ciliary bands, and adult rudiment. The brachiolar arms contain sensory‐like nerve cells in the adhesive papillae, flask‐shaped nerve cells in the adhesive disk, and bundles of fibers with branches interconnecting them. In the ciliary bands, nerve cells are interconnected with axon bundles along the ciliary bands and some neurons send fibers toward the oral and aboral epidermis. These neural components of the ciliary bands are regionally modified to form masses such as lateral and oral ganglia. The future aboral epidermis of the adult rudiment forms a nerve plexus with cell bodies enriched over spicules. Serotonergic nerve cell bodies are found throughout the nervous system except in the adhesive disk, the bipinnaria arms, and the adult rudiment. In addition, there are neural components in the esophagus and in the coelom where nerve fibers or bundles have distinct orientations with respect to the muscle fibers. The neuroanatomy of the brachiolaria suggests how it may function in controlling larval physiology and identifies intriguing problems on the origin of larval and adult nerves. J. Comp. Neurol. 509:271–282, 2008.

DEVELOPMENT OF THE SEA-STAR, ASTERINA BATHERI GOTO

Using natural spawning and artificial fertilization, the entire process of development from eggs to juveniles was observed in the sea‐star, Asterina batheri Goto.

Close Relationship between Asterina and Solasteridae (Asteroidea) Supported by Both Nuclear and Mitochondrial Gene Molecular Phylogenies

Abstract Phylogenetic relationships among asteroids remain to be extremely controversial in spite of many morphological and molecular studies have been applied to this issue. In the present study, especially focusing on resolving the relationship of Asterina and Solasteridae, we reconstructed the molecular phylogenetic tree of asteroids using nuclear 18S rDNA. A close relationship between Asterina and Solasteridae, which has been suggested from analyses of mitochondrial 12S rDNA and 16S rDNA, is supported here by the nuclear 18S rDNA dataset. The support is even stronger when the sequences of mitochondrial rDNAs and nuclear 18S rDNA are combined as a total dataset. The independent support from both nuclear 18S rDNA and mitochondrial rDNAs strongly argues for a close relationship between the Asterina and Solasteridae.

Adhesive papillae on the brachiolar arms of brachiolaria larvae in two starfishes, Asterina pectinifera and Asterias amurensis, are sensors for metamorphic inducing factor(s)

It has been hypothesized by Barker that starfish brachiolaria larvae initiate metamorphosis by sensing of metamorphic inducing factor(s) with neural cells within the adhesive papillae on their brachiolar arms. We present evidence supporting Barkers hypothesis using brachiolaria larvae of the two species, Asterina pectinifera and Asterias amurensis. Brachiolaria larvae of these two species underwent metamorphosis in response to pebbles from aquaria in which adults were kept. Time‐lapse analysis of A. pectinifera indicated that the pebbles were explored with adhesive papillae prior to establishment of a stable attachment for metamorphosis. Microsurgical dissections, which removed adhesive papillae, resulted in failure of the brachiolaria larvae to respond to the pebbles, but other organs such as the lateral ganglia, the oral ganglion, the adhesive disk or the adult rudiment were not required. Immunohistochemical analysis with a neuron‐specific monoclonal antibody and transmission electron microscopy revealed that the adhesive papillae contained neural cells that project their processes towards the external surface of the adhesive papillae and they therefore qualify as sensory neural cells.