Topical Guide 299
Mark Dorset
Clams: Life Cycle of
Clams: Sex Life of
They walked the length of Main Street. They stopped across the street from Continental Clam to watch some schoolchildren on a field trip make their way from panel to panel in the stages in the life of a clam. The children seemed fascinated by one panel in particular.
Herb ’n’ Lorna, Chapter 8
Sexual maturity usually is reached at 2 years of age (3 years in many areas in the North Atlantic region). The shell length at this age is between 32 and 38 mm. Size, not age, determines sexual maturity, so that slower growing individuals mature later than 2 years of age. The peak of reproductive potential is reached at 60 mm; larger, older hard clams gradually lose the reproductive capacity (Belding 1931).
The average number of eggs released by a 60-mm female in the wild is about 2 million (Belding 1931). In laboratory tests, the average-sized female released 8 million eggs per season (Davis and Chanley 1956; Ansell 1967a). The fecundity of one large female was 16.8 million eggs, whereas small clams (33 mm) had far fewer eggs (Bricelj and Malouf 1980). About 2,000 spermatozoa are shed for eachovum.
The spherical eggs are 78 μm in diameter with closely packed yolk granules (Belding 1931). [Note: Compare 78 μm (microns) with the diameter of a human hair, which ranges from 17 to 181 μm. MD] A large gelatinous capsule distinguishes the hard clam egg from the eggs of other mollusks. Eggs are released through the excurrent siphon, and the capsule swells after contact with water until it is 3.2 times the diameter of the egg. The gelatinous capsule imparts buoyancy, so that the eggs are pelagic and carried by tidal and coastal currents. Spermatozoa swimming in water come into contact with and penetrate the capsule, fertilizing the egg.
At about 10 hr the embryo developing within the capsule becomes covered with cilia. The lashing of the cilia tears the membrane and gelatinous capsule; the ciliated gastrula escapes into the water. The egg may be carried 2 to 25 km from the spawning site.Jon G. Stanley and Rachael DeWitt, Species Profiles: Life Histories and Environmental Requirements of Coastal Fishes and Invertebrates (North Atlantic) Hard Clam, Fish and Wildlife Service U. S. Department of the Interior Coastal Ecology Group
The hard clam is a protandrous, consecutive hermaphrodite and is dioecious after changing sex (i.e., the clams begin adult life as males, often become females with greater maturity, and require individuals of both sexes for reproduction). Sexual maturity is mainly a function of size. Clams develop functional male gonads at 6–7 mm in shell length in the first or second year of life. Oocytes are sometimes present at this time. After this juvenile male phase definitive sexes are established at a size of about 30 mm shell length. …
Fecundity in hard clams is high. Females can release 16–24 million eggs per spawn, although laboratory studies often have recorded lower values of 1–3 million eggs. With repeated spawns individuals may release up to 60 million eggs over a season. The viability of eggs and subsequent survival of larvae are positively related to egg size, not clam size, but the amount of spawn released increases with increasing clam size. Eggs are 60–85 μm in diameter when released, and covered with a gelatinous membrane which expands in contact with water, further extending the diameter to 163–179 μm. In culture experiments, however, eggs will often pass through a 35 μm mesh; they are retained on a 25 μm mesh. Fertilization occurs in the water column.G. Curtis Roegner and Roger Mann, “Habitat Requirements for Chesapeake Bay Living Resources: Hard Clam Mercenaria mercenaria,” Second Edition June 1991
For discussion: If you were a clam, what would you want your preferred gender pronouns to be?
See also: Clams TG 21; Clams: Cherrystone TG 32; Clams: Fried TG 35, TG 38; Clams: Littleneck TG 32; Clams: Seed TG 32; Clams: Antigona lamellaris and Tapes literatus TG 46; Clams: Babbington Clam Council, The TG 5; Clams: Babbington Clam, the packing plant TG 3; Clams: Clamshell Driveways TG 3
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