Chapter 5 – PELECYPODA (EXCLUDING OSTREIDAE)

Tens of thousands of synthetic substances are in existence today and hundreds of new compounds are being introduced every year. Because of the complexity of the physico-chemical interactions between pollutants and the marine environment, the potential toxicity of contaminants can be assessed adequately only by means of bioassays with living organisms. From a practical point of view, a bioassay needs to be sensitive and scientifically valid, yield rapid results at moderate cost, and the organism in question must be readily available.

Ecotoxicological bioassays with bivalve embryos and larvae fulfil these criteria better than most other tests. They have increasingly come into use during the past three decades and are now commonly employed to ascertain the biological effects of pure chemicals, as well as to determine the quality of effluents, coastal waters and sediments sampled in the field. There do not appear to be very great differences between bivalve species with regard to larval sensitivity to toxicants. The principal species for bioassays are oysters (Crassostrea gigas and C. virginica), and mussels (Mytilus edulis and M. galloprovincialis).

Bioassays are conducted with gametes and larvae of ail ages: sperm and unfertilized eggs, embryos, young D-larvae, intermediate umboned larvae, and pediveligers towards the end ofthe pelagic period. Embryos are usually the most sensitive stage. Recent advances now also permit bioassays on metamorphosing pediveligers, a method particularly suited to investigate the effects of adsorbate-contaminated surfaces. There are various criteria for the assessment oftoxic effects, including embryogenesis success (abnormalities), larval growth, mortality, physiology (e.g. feeding or swimming activity), and metamorphosis success.

Chronic toxicity studies may be carried out over periods ofseveral weeks, but larval rearing in the laboratory requires considerable effort (e.g. cultivation of algal food). The method of choice for investigations of acute toxicity and for routine monitoring studies is the embryo bioassay because it is very sensitive, relatively simple, and produces results within 24 or 48 hours. The data obtained by different investigators are often difficult to compare, however, because of differences in methodology. There is no firmly established procedure, and further simplification and standardization of techniques is required.

In bioassays with a single pollutant, the effective toxic concentration may span several orders of magnitude, depending on bioassay procedures, larval stage and choice of response. Tributyl-tin (TBT) is the most toxic compound ever bioassayed with bivalve larvae, with effective concentrations (EC50) as low as a few nanograms per litre (i.e. 10−3 ppb). Heavy metals (particularly mercury, silver and copper) are next in order of toxicity, with EC50 values between a few micrograms per litre (ppb) and several hundred ppb. Chlorine and some organochlorine pesticides may also have EC50 values of less than 100 ppb, while detergents and petroleum products are generally less toxic

The Assessment of Marine Pollution - Bioassays with Bivalve Embryos and Larvae

Controls on isotopic chemistry of the American oyster, Crassostrea virginica: implications for growth patterns

Publisher Summary The chapter discusses the hybridization in the sea. The major intentions of this review are: (1) to point out that hybridization in the sea is not as rare as previously thought; (2) to draw attention to hybridization bias amongst certain groups of marine organisms; (3) to illustrate biogeographic and environmental patterns in the location of hybridization; (4) to compare marine hybridization with hybridization in other environments; and (5) to discuss marine hybridization in the context of fisheries science and conservation. The chapter discusses the five levels of hybridization regardless of the environment. A summary of cases detailing natural hybridization in the sea is provided along with the information concerning the geographic location and biogeographic region of the hybridization event. Protein electrophoresis has proved to be a particularly valuable tool by which hybrids and individuals of mixed ancestry can be identified. This technique permits an objective evaluation of the hybrid status of the individuals, when the two hybridizing taxa are fixed for electrophoretic differences at one or more allozyme loci. The chapter explores that taxonomy plays an important role in the defining of hybrid zones. Factors preventing and promoting marine hybridization are also discussed. Laboratory hybridization experiments have been utilized extensively to confirm the probable hybrid nature of certain individuals by demonstrating that two taxa will interbreed when provided with the opportunity to do so, or that gametes from two taxa can be artificially cross-fertilized. Such information provides a valuable contribution to the study of natural marine hybridization.

Hybridization in the Sea

Phylogeny of Veneroidea (Mollusca: Bivalvia) based on morphology and molecules

Oysters which set early in the spawning season reach sexual maturity and spawn before the end of their first year. The early sexual development in the southern latitudes is the result of both an extended growing season and an increased in stantaneous rate of growth. Widespread spawning of oysters older than one year occurs twice during the reproductive season, with a period of renewed development in between. Spawning by young-of-the-year oysters is of minimal importance to population recruitment during the year they set.

The reproductive cycle of early setting crassostrea virginica (gmelin) in the northern gulf of mexico, and its implications for population recruitment

Chromosome numbers of n = 19, 2n = 38 are reported for Mercenaria mercenaria, M. campechiensis and their F1 hybrids. Meiosis is normal in the hybrids and yields no evidence of chromosome nonhomology or structural rearrangements between the two parents. The hybrids produce functional eggs and sperm which result in normal fertilization and early embryonic divisions in reciprocal backcrosses and at least some F2' s. No gross chromosomal barrier to gene exchange appears to exist between the two species.

Chromosomes of two species of quahog clams and their hybrids

1. Experiments using a circular choice apparatus showed a statistically significant attraction of commensal crabs, Pinnotheres maculatus, to bay scallops, Aequipecten irradians concentricus, and penshells, Atrina rigida.2. The adult males of P. maculatus removed from bay scallops showed a significant attraction to the host.3. When tested for preference between the two hosts, crabs removed from bay scallops showed no preference for one host over the other. The attraction of crabs to both the hosts was statistically significant. Experiments suggested that the crabs removed from scallops are not host-specific.

R. Winston Menzel

Papers

The reproductive cycle of early setting crassostrea virginica (gmelin) in the northern gulf of mexico, and its implications for population recruitment

Chromosomes of two species of quahog clams and their hybrids

Influence of hosts on the behavior of the commensal crab pinnotheres maculatus say