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Carriker, M. R. 1961. 



Interrelation of functional morphology, behavior, and autecology in early 

 stages of the bivalve Mercenaria mereenaria. J. Elisha Mitchell Sci. Soc. 

 77(2): 168-241. 



Clam larvae were obtained by inducing spawning in the laboratory. In 6 

 years, of 2,890 clams used, 12% spawned. Most spawning occurred in early 

 summer and was more difficult to induce as the season advanced. Spawning 

 could be delayed by keeping clams at low temperatures. The most vigorous 

 and rapidly growing larvae, with least mortality, were progeny of fully 

 spawning, vigorous females, who discharge their ova rapidly. Reared larvae 

 survived gradual temperature fluctuations ranging from 13°-30°C." Larvae 

 were fed cultures of Chlorella and Nannochloris with mixed microorganisms 

 and pablum extract. Larvae grew best in seawater hauled from the bay, next 

 best in seawater pumped into the lab, then heat-treated seawater, and grew 

 worst in charcoal treated seawater. The minimum time for larvae to reach 

 setting size was 8 days. Several early stages of hard clam are described 

 by differences in structural features and behavior: Non-shelled planktonic 

 stage, straight-hinged veliger, umboned veliger, prodissoconch I, pro- 

 dissoconch II, pediveliger, byssal plantigrade, and juvenile plantigrade. 

 Prodissoconch stages have smooth shells. The curvature of the primary 

 and secondary shell ridges, formed in the byssal plantigrade, anchor the 

 clam in sediment, but offer a minimum of resistance to locomotion through 

 sediment. Pediveligers and newly set plantigrades locomote by ciliary action 

 on the foot, but change to muscular crawling so the clam can burrow or emerge 

 from sediment. During plantigrade stage, before loss of the velum, the 

 byssus gland becomes functional and remains functional until the clam is 

 about 9 mm. Clams attach quicker when buried in sediment, than on an open 

 surface. The byssal thread can be dislodged to move to another site. The 

 time between dislodgement and reattachment ranged from a few minutes to 

 several hours in the lab. During plantigrade stage, mantle edges fuse, 

 dividing gap between valves into exhalant and combination pedal and inhalant 

 openings. A valvular membrane forms at the rim of the exhalant opening 

 which controls and directs the exhalant current. An exhalant siphon with 

 terminal tentacles forms at the base of the valvular membrane. The inhalant 

 siphon develops as an extension of the mantle edge. Most of valvular 

 membrane is reabsorbed with time. Tips of the two siphons are pointed away 

 from each other. Tentacles form protective grids over siphons which exclude 

 larger sedimentary grains from entering when the plantigrade is buried deep 

 in sediment. Clams can emerge from a depth of sediment at least five times 

 their shell length. In the lab, trochophore and early straight-hinged 

 veligers concentrate in surface water layers of culture jars and swim 

 strongly. Later stage veligers were distributed evenly in water column 

 and remained suspended throughout planktonic life if they were healthy, 

 sick veligers remained on the bottom. Lower Little Egg Harbor, New Jersey, 

 retains relatively homogeneous seawater in which a large number of larvae 

 remain for at least the veliger stage. For the period 1929-1951, estimates 

 of annual yield to clammers ranged from 40 to 160 million clams. Distribution 

 of veligers was studied in the bay. The largest and densest swarms appeared 

 during July, but the size of the spawning varied from summer to summer. Of 

 the 67 spawnings observed, 73% occurred during a period of two or three 

 days of rising median daily temperature, the rest during periods of falling 

 median temperatures. Overall average median daily spawning temperature was 

 25.7°C with a range of 22°-30°C. Spawning occurred more frequently, and 

 produced higher density swarms of veligers, during neap tides than spring 

 tides. Higher temperatures during neap tides probably triggered spawning. 

 Time of spawning was probably around ebb tide when temperatures are maximum. 

 Survival of veligers was about 2.6% during periods of little rainfall and 

 median to low tidal amplitudes (minimal flushing and exchange) . During 

 periods of high rainfall and spring tides, even large veliger swarms 

 disappeared from the water. Veligers grew to a setting size of about 200 

 microns in eight days from fertilization in Little Egg Harbor where median 

 daily temperatures ranged from 20.4°-26.2°C and salinities from 30.4-31.4°/°°. 

 Horizontal distribution of veligers was uneven but widespread. Maximum 



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