Pelagic Development Research Articles

Distribution and Synonymy in the Pacific Ocean, and Variation, of the Greenland Halibut, Reinhardtius hippoglossoides (Walbaum)

Long known in the Pacific Ocean from a single specimen from east-central Japan, the very distinctive flatfish genus Reinhardtius has been shown since 1930 to maintain a population of commercial significance from northern Japan to northwestern Bering Sea. It is now found to be common also in eastern Bering Sea, and to occur also just south of the tip of the Alaska Peninsula, in northern and central California, and even at the extreme north of Baja California, Mexico. Japan and the Californias appear to be inhabited by non-breeding expatriates, whose wanderings are attributable to the free-swimming habits and prolonged early pelagic development of the species. Lack of records between the Alaska Peninsula and California may possibly indicate actual absence, attributable to current pattern. Analysis of the only morphometric characters thought to be indicative of subspecific differentiation between the Pacific and Atlantic stocks discloses no basis for such separation, nor do any meristic characters disclose such a basis. The single species Reinhardtius hippoglossoides is therefore regarded as a uniquely undifferentiated amphiboreal taxon. The chief distinction between the ocean stocks lies in the number of vertebrae, but the differentiation (probably genetic) is not beyond the racial level. The difference in vertebral numbers, surprisingly, is not paralleled by a difference in numbers of dorsal or anal rays; nor is there any marked correlation between vertebral and ray numbers. As expected, the numbers of dorsal and anal rays are found to be positively correlated, and the number of precaudal and caudal vertebrae to be negatively correlated. In this flatfish of relatively limited asymmetry the numbers of rays are virtually identical in the two pectoral fins. Two reversed examples of the species are reported.

Abbreviated Pelagic Life of Chilean and New Zealand Oysters

OYSTERS of the genus Ostrea incubate their larvae In O. edulis, for example, veligers are released from the female when 0.15–0.17 mm long, and afterwards undergo pelagic development, usually of 6–14 days, according to water temperature and food supply, before settling and metamorphosing1–3. In O. chilensis Philippi from Chile, however, larvae from incubating females collected by the Lund University Chile Expedition 1948–49 were examined by one of us (R. H. M.), through the courtesy of Dr. T. Soot-Ryen and Prof. H. Brattstrom, and were found to be 0.45–0.49 mm long and to possess the eye spots and foot which other species develop only at the end of their pelagic life. The other (P. J. H.), who is publishing elsewhere a fuller account of the life-history of the New Zealand mud oyster, O. lutaria Hutton, has observed similar larvae in that species. These larvae, when released by the female in laboratory experiments, settled almost immediately. In both species this probably also happens in Nature.

The Development of the Polychaete Haploscoloplos fragilis

ABSTRACT The eggs of Haploscoloplos fragilis, laid in gelatinous cocoons, develop as yolky trochophores which add 4 segments before hatching on the 6th day and two more during a pelagic phase before settling and the onset of feeding on the 10th day. The 6 (embryonic) segments bear a metatroch and 5 amphitrochs. The embryo also has an akrotroch, prototroch, telotroch, and neurotroch. All cilia disappear after settling, when the embryo shows resumption of segment formation. Anterior to the first trunk segment lie the prostomium and mouth region. The latter becomes subdivided by an annulus, but later fuses with the first trunk segment as the peristomium. All other segments become chaetigerous, the 5 embryonic being of thoracic type, the postembryonic abdominal. At the posterior end of the body lies the pygidium, with the segment-forming growth zone immediately in front of it. The development of trunk segments in two series, embryonic and post-embryonic, is characteristic of ariciids, as is the fusion of the first trunk segment with the mouth region to form the peristomium. It is possible that the mouth region incorporates at least one other cephalized segment. Ariciid eggs vary in size among species. No direct relationship exists between egg size and pelagic development. The mode of oviposition also varies. Whether or not a pelagic phase occurs depends on the stage at which the embryo becomes free in the water, which is governed in turn by properties of the cocoon (e.g. jelly viscosity). A pelagic phase is always associated with enhanced ciliation, while in non-pelagic development ciliation is reduced. The basic ciliation (akrotroch, prototroch, metatroch, telotroch, and neurotroch) is usually supplemented by additional segmental bands.

REPRODUCTIVE and LARVAL ECOLOGY OF MARINE BOTTOM INVERTEBRATES

REPRODUCTIVE and LARVAL ECOLOGY OF MARINE BOTTOM INVERTEBRATES