1 MOLLUSCAN AQUACULTURE - INTRODUCTION Mollusks have been cultured since the time of the Romans, who in particular, considered oysters a delicacy. The latest aquaculture production figures from FAO (see Table 1.) show global aquaculture of mollusks resulted in 12,300,000 mt produced in 2003. While this represents almost a third of animal aquaculture, value is only about 20%. About a third of the production is the result of Pacific oyster culture (3,000,000 mt). It should be noted that production figures for mollusks are notoriously inaccurate, in that various countries report either shell or meat weight and some even by individuals. Converting this data to a standard unit of in-shell production is challenging. Also it is very difficult to distinguish between the fisheries catch and the amount produced by aquaculture. Growth of global molluscan aquaculture production is hampered by: a) degradation of prime growing areas b) relative low value of the product c) declining consumer acceptance Molluscan Biology The phylum Mollusca - ~ 65,000 species and is distinguished, in general by: a) Modification of the integument of the dorsal surface, the mantle for the secretion of a protective calcareous shell and b) Adaptation of the ventral surface of the body as a muscular foot for locomotion (see Figure 1.). This foot and consequently locomotion is reduced in the bivalves; clams, oysters, and mussels.
2 Table 1. FAO Aquaculture statistics 2003 summary FISH 27,000,000 mt 64% of total animal aquaculture 49 % of total aquaculture MOLLUSKS (except cephlopods) 12,300,000 mt ~ 29 % of total animal aquaculture; 19% of the value (22 % total aquaculture) Crustaceans 2,800,000 mt ~ 7 % of total animal aquaculture Oysters - 4,500,000 mt (11 % animal aquaculture) Crassostrea gigas 4,400,000 mt (10% of animal aquaculture; 36% of all mollusks; 98% of all oysters) + ~ $ 1.5 billion in pearl production Clams 3,400,000 (29% of molluscan aquaculture) Manila Clam (Japanese carpet shell) 2,600,000 mt Mussels 1,600,000 mt Yesso Scallop 1,200,000 mt Others 1,600,000 mt (including abalone = 3 9,000 mt) Marine Shrimp 1,805,000 mt Others 960,000 mt Crawfish = 34,000 mt ANIMAL AQUACULTURE = 42,300,000 mt; $ 61 billion Marine FW Plant Aquaculture: Seaweeds (& misc. plants) 12,500,000 mt (23% of total aquaculture) Micro-algae production in aquacult. hatcheries?? TOTAL OF ANIMAL AND PLANT AQUACULTURE - 54,800,000 mt
3 Figure 1. Generalized molluscan body plans Figure 2. Generalized view of a razor clam showing siphons (from Figure 18-26 Hickman et al., 1993).
4 Figure 3. Anatomy of the hard clam Mercenaria mercenaria. A. Interior of the right valve. B. Partial dissection, showing some of the internal organs.
5 Figure 4. General anatomy of an oyster.
6 Figure 5a. Cross-section of an oyster gill Figure 5b. Cross section of molluscan gill algal cell concentration.
7 Figure 5c. Cross section of molluscan gill algal cell concentration and movement.
8 Figure 6. Molluscan life cycle. CLAM LIFE CYCLE
Life cycle definitions: Development Molluscs are bilaterally symmetrical eumetazoans. Their coelom is formed by the splitting of embryonic mesodermal masses (schizocoely). Cleavage is determinate and development is protostomous, characteristics shared with annelid (segmented) worms and crustaceans. spermatozoon - haploid gamete produced by the male egg or ovum - haploid gamete produced by the female fertilization - fusion of the sperm with the egg to produce a diploid cell (zygote) capable of giving rise to a complete organism. zygote - the single diploid cell formed as a result of fertilization embryo - early development state that requires a protected environment larva - the developmental stage following the embryonic period, generally bears little resemblance to the adult organism, is not capable of sexual activity, and must undergo metamorphosis in order to become a juvenile. trochophore larva - pear shaped translucent larva with a prominent circlet of cilia in addition to apical tufts. It is found both in molluscs and annelids with primitive embryonic development and is considered on of the lines of evidence suggesting common phylogenetic origin of the two phyla. veliger larva - characteristic of the molluscan classes, Gastropoda and Bivalvia; identified by the presence of a bi-lobed swimming organ known as the velum. pediveliger larva - late veliger stage recognized by the presence of a well developed eye spot and an obvious foot. Sometimes referred to as eyed-larvae. juvenile - developmental stage that resembles a miniature adult, is not capable of sexual activity and must undergo growth and further differentiation to achieve adulthood adult - the stage in an organism s life history that is capable of participating in sexual reproduction. ANS 119 - Molluscan Biology Page 9
Figure 7. Generalized trochophore larva (from Figure 18-6 Hickman et al., 1993). Figure 8. Generalized veliger larva