Origin of the trochophora larva

The trochophora larva, which is so well known from the marine plankton, is central to our understanding of the evolution of a large branch of the bilaterians. Two theories for this larval type have been prevalent, the trochaea theory and the theory proposed by Ivanova-Kazas. The embryology, or more precisely the cell-lineage, of these larvae seems to be central for our understanding of their origin, but important details have been missing. According to the trochaea theory, a circumblastoporal ring of blastomeres differentiates to follow the convoluted shape of the conspicuous ciliary bands of the larvae, with prototroch and metatroch around the mouth, forming a filtering system, and telotroch around the anus. According to the Ivanova-Kazas theory, the blastomeres with the ciliary bands develop through specialization of rings of cells of the general ciliation in a lecithotrophic larva. Now, a new cell-lineage study of the gastropod Crepidula has shown that the ring of cells at the edge of the blastopore develops into the band of cells carrying prototroch and metatroch, characteristic of the trochophora. This gives strong support to the trochaea theory.

High genetic similarity between geographically distant populations in a sea anemone with low dispersal capabilities

Samples of the large sublittoral sea anemone Urticina eques (Gosse) were collected from three localities in the northern North Sea and from one locality in the northern Irish Sea. Around the coast the total distance between sampling sites is approximately 1,200 km. The species has a large lecithotrophic larva which may not be planktonic. All samples were screened genetically for 13 loci coding for 11 different enzymes. Results overall indicated a high degree of genetic uniformity over the four populations sampled (FST = 0·025). The data are discussed in relation to current ideas of larval dispersal and results from other similar studies. It is concluded that the lack of genetic differentiation shown by Urticina eques is surprising given the apparently poor dispersive powers of the larva.

Annual spawning cycles and planktonic larvae of benthic invertebrates from Passamaquoddy Bay, New Brunswick

Periods of larval occurrence are reported for 19 polychaete species and 10 other common planktonic larvae. Egg sizes are given for 29 polychaete species with estimates of spawning period for 14 of these based on studies of the adult worms. Spawning periods are shown to correlate better with larval type (e.g. whether planktotrophic or lecithotrophic) than with the zoogeographic distribution of the adults. Among the planktotrophic larvae, the evidence suggests that some larvae (e.g. polynoid larvae) develop successfully only during the spring diatom bloom, and gamete wastage may result from any substantial mismatch between the diatom bloom and spawning period. The lecithotrophic larva of the tunicate Boltenia ovifera, which spawns in midwinter, is described for the first time.

Reproduction , Spawning and Development of the Starfish Patiriella exigua (Lamarck) (Asteroidea : Asterinidae) and Some Comparisons with P. calcar (Lamarck)

P. exigua adults are mature throughout the year. The species can breed in any month of the year, though spawning is most active in the late winter and spring. P. calcar has a definite breeding season in the late spring and summer. P. exigua lays an egg mass through oral gonopores. The 400-�m eggs develop directly, through a yolky brachiolaria stage to a juvenile star, before leaving the egg mass at 22-23 days. P. calcar, with eggs of similar size laid through oral gonopores, has a planktonic lecithotrophic larva. The development of P. exigua is compared with that of other yolky asteroid embryos. Modes of development in the genus Patiriella are discussed.