Fu-Shiang Chia

How Distribution and Abundance Influence Fertilization Success in the Sea Urchin Strongylocentotus Franciscanus

Many organisms reproduce by releasing gametes into the environment. How- ever, very little is known about what proportion of released eggs become fertilized. We examined the influence of spawning group size, degree of aggregation, position within an aggregation, and water flow, on in situ fertilization in the sea urchin Strongylocentrotus franciscanus. This study was conducted at a depth of 9 m on the west coast of Vancouver Island, British Columbia, Canada. Males were simulated by syringes filled with sperm; females were simulated by sperm-permeable containers filled with eggs. Individuals were placed 0.5 or 2.0 m apart within a 2 x 2 or 4 x 4 (group size of 4 or 16 individuals) experimental array. The results indicate that group size, degree of aggregation, position within a spawning group, and water flow all affect fertilization success. Fertilization success ranged from 0 to 82%. Increases in group size and aggregation, decreases in flow velocity, and central and downstream positions within an aggregation all lead to increases in fertil- ization success. Thus, individual reproductive performance is dependent on, and highly sensitive to, population parameters and environmental conditions.

Kinetics of fertilization in the Sea Urchin Strongylocentrotus franciscanus : interaction of gamete dilution, age, and contact time

Determining fertilization success of free spawning organisms in the field requires knowledge of how eggs and sperm interact under varying encounter frequencies and durations. In the laboratory, we investigated the relative influence of sperm concentration, egg concentration, sperm-egg contact time, and sperm age on fertilization in the sea urchin Strongylocentrotus franciscanus. Our results indicated that sperm concentration, sperm-egg contact time, sperm age, and individual variability were sequentially the most important factors influencing fertilization success. Egg concentration was not significant over the range tested. A theoretical model of fertilization (Vogel-Czihak-Chang-Wolf model) was used to estimate the two rate constants of fertilization kinetics: the rate constant of sperm-egg encounter and rate constant of fertilization. This model explained 91% of the variation in fertilization success, provided estimates of the rate constants involved in fertilization, and indicated the proportion (3%) of sperm-egg contacts that result in fertilization. Estimates of sperm swimming velocity and egg diameter were used to independently calculate the rate of sperm-egg encounter and confirm the predictions of the model. This model also predicts the non-significant effect of egg concentration on fertilization success found empirically.

Drifting and dispersal of small bivalves and gastropods with direct development

Quantitative evidence that several marine benthic invertebrates lacking a planktonic larval stage disperse as juveniles and small adults by drifting in the water column has been obtained using off-bottom intertidal collectors. These collectors were mimics of a habitat commonly used by small molluscs in intertidal seaweed communities: red turf algae. Taxa caught in collectors include three brooding bivalves [Musculus sp., Lasaea sp., Transennella tantilla (Gould, 1853)] and small gastropods that produce benthic egg capsules and crawl-away juveniles (Barleeia spp.). Dispersal is accomplished by juveniles—and even sexually mature adults in the case of small-sized species — drifting with water currents using mucous threads in at least some of the taxa found. Some of these molluscs have also been caught in off-bottom collectors placed away from the shore, in the middle of a bay. Findings indicate that, contrary to what is suggested by their mode of development, these invertebrates have alternative and likely effective dispersal mechanisms (at least at a local scale). It is probable that frequent drifting excursions in these taxa enhance rafting opportunities, which in turn may favor long distance dispersal. Large numbers of juveniles of several other species of molluscs whose life cycles include a planktonic larval stage were also caught in off-bottom collectors. These results, coupled with those from previous studies, show that postmetamorphic drifting is a common feature of the life history and ecology of many marine bivalves and gastropods, regardless of their mode of development.

Microhabitat-associated variability in survival and growth of subtidal solitary ascidians during the first 21 days after settlement

Newly settled juvenile ascidians were transplanted into shaded and unshaded sites at two subtidal depths to test the hypothesis that photonegative settlement behavior enhances juvenile survival. Silt, filamentous algae and grazing gastropods (Margarites pupillus) were identified as sources of mortality associated primarily with non-cryptic microhabitats. Silt inflicted heavy mortality on all six species tested. Algal overgrowth was important in shallow water, where it increased mortality and depressed growth rates over a 14-d period. Laboratory experiments demonstrated that gastropods can remove juvenile ascidians from the substratum. The sources of post-settlement mortality investigated may help determine distributional patterns of ascidians in the San Juan Islands, Washington, USA, and may also represent selective pressures that maintain negative phototaxis in the behavioral repertoires of ascidian tadpoles.

LABORATORY EVIDENCE FOR DELAY OF LARVAL SETTLEMENT IN RESPONSE TO A DOMINANT COMPETITOR

In nature, the marine bryozoan, Bugula pacifica, is consistently the ‘loser’ when competing for space with the compound ascidian, Diplosoma macdonaldi. We provide laboratory evidence to show that Bugula larvae will delay settling in either ‘Diplosoma-conditioned water’ or an alcohol Diplosoma extract. It is reasoned that the delay response is elicited by an inhibitory factor released continually from Diplosoma. This response is reversible, non-pathological, and species specific. The possible ecological significance of larval avoidance behavior in this and other communities is discussed.

Factors controlling spatial distribution of the sea cucumber Psolus chitonoides: Settling and post-settling behavior

Substratum selection experiments were carried out in situ with larvae of the holothurian Psolus chitonoides Clark in the rocky subtidal of the San Juan Islands, Washington. The experiments indicated, in agreement with laboratory findings, that pentactula larvae settle gregariously, either on the adult, or (when the bivium is obscured by epizooites) by clustering around the margin of the adult sole. Late larvae and early juveniles are strongly photonegative. Following metamorphosis, juveniles migrate from the adults into nearby shaded habitats, where they may take up residence indefinitely. Although adults retain the capacity to move, they relocate only when their living conditions become unsuitable; in most circumstances they are effectively sessile. Small-scale spatial pattern of the adults, which was documented quantitatively on subtidal rock walls, boulders and cobbles, correlates well with microtopographical features. The aggregation observed in the field is primarily due to post-settling migration rather than larval substratum choice at settlement.

Fecundity and egg size are mediated by food quality in the polychaete worm Capitella sp.

Abstract Fecundity, egg size, and energetic investment·egg −1 were studied in two populations of Capitella sp.: one population inhabited an intertidal mudflat and fed on detritus and the other resided inside of and fed on squid egg capsules subtidally. Animals of the subtidal population were larger and produced smaller eggs than animals of the intertidal population but subtidal worms compensated by producing more eggs. Laboratory experiments (altering the food types between sibling offspring from each population) indicated that fecundity, egg size, and egg energy content are mediated by food.

Fine structural studies of the nervous system and the apical organ in the planula larva of the sea anemone Anthopleura elegantissima

The nervous system of the planula larva of Anthopleura elegantissima consists of an apical organ, one type of endodermal receptor cell, two types of ectodermal receptor cells, central neurons and nerve plexus. Both interneural and neuromuscular synapses are found in the nerve plexus.

Characterizing temperate rocky shores from the perspective of an early juvenile snail: The main threats to survival of newly hatched Nucella emarginata

Mortality factors most likely to constitute substantial selective pressures for early juvenile gastropods on temperate rocky shores were identified by examining the vulnerability of hatchlings of an intertidal snail, Nucella emarginata, to heat stress, desiccation, and predation in 1992 and 1993. The highest temperature of substrata measured at tidal heights colonized by N. emarginata in Barkley Sound, British Columbia, Canada, was 28.5°C. This temperature was not lethal to hatchlings in laboratory tests. In laboratory and field desiccation experiments, all hatchlings died within 6 h of emersion. Early juveniles could not survive direct exposure to even moderate drying conditions for the duration of a low tide. Hence, intertidal microhabitats which dry up even for short periods during low tides would prove lethal. Of 45 intertidal animal species to which hatchlings were exposed in the laboratory, small decapod crustaceans were the only organisms to cause substantial hatchling mortality. Of these, Pagurus hirsutiusculus and Hemigrapsus nudus were by far the most abundant in the field, and are probably the only important predators of early juvenile N. emarginata at most sites. Total predator densities in the field were as high as 438 individuals m−2, suggesting that predation pressure may be intense. Desiccation and predation by decapod crustanceans appear to be the most significant threats to early juvenile N. emarginata. These factors commonly occur on most temperate rocky shores and undoubtedly constitute major selective agent influencing population parameters and shaping life-history strategies and early juvenile traits of intertidal invertebrates.

Development and metamorphosis of the planktotrophic larvae of Rostanga pulchra (Mollusca: Nudibranchia)

Rostanga pulchra MacFarland, a small (1 to 2 cm) dorid nudibranch, lays an average of 7000 eggs in the laboratory during a period of 30 days in the summer. The veligers hatch 15 to 16 days after oviposition and it takes another 35 to 40 days to become competent for metamorphosis at a temperature of 10° to 15°C. Larval cultures were maintained initially at a concentration of 500 veligers per 100 ml of filtered sea water (antibiotics added). During the planktotrophic phase of development, the veliger grows from 150 to 300 μm in shell length. Although the veligers are generalists in their food preference, the best result (faster growth) was achieved by feeding them with a combination of Monochrysis lutheri and Isochrysis galbana. The concentration of food cells was kept at 104 cells per ml of culture media and was supplied every 2 to 3 days. A veliger which is competent to metamorphose is identifiable morphologically by its propodium, eyespots, rhinophores, and spiculated dorsal papillae. The entire metamorphic process lasts 24 h when a suitable substrate such as the food sponge Ophlitaspongia pennata is provided. The competent veliger is able to delay metamorphosis for at least 3 weeks. Juveniles were kept in the laboratory for 70 days and, during this period, grew to a length of 4.5 mm.