American oyster, continued 



Growth and Development 



Egg Size and Embryonic Development Egg develop- 

 ment is oviparous. Fertilized eggs are pear shaped 

 (55-75 u.m long and 35-55 ^m wide), and contain 

 numerous oil droplets. These droplets are important 

 for providing energy and nutrients to the developing 

 embryo. The eggs hatch 6 hours after fertilization at a 

 temperature of 24°C, and progress through blastula 

 and gastrula stages, developing into a trochophore 

 larvae in 6 to 9 hours (Galtsoff 1 964, Loosanoff 1 965, 

 Bahr and Lanier 1 981 , Burrell 1 986, Lee and Heffernan 

 1991). 



Age and Size of Larvae : Larvae remain in the water 

 column 2 to 3 weeks after hatching, passing through 

 several developmental stages (trochophore, 

 prodissoconch I, prodissoconch II orpediveliger). The 

 final larval stage, the eyed pediveliger, is approxi- 

 mately 300 urn in length. At this stage the larval oyster 

 uses its eyespot and foot to find a suitable substrate for 

 settlement. In Galveston Bay, Texas, setting was first 

 seen about 2 months after spawning when the larvae 

 were approximately 0.2 mm in length (Hopkins 1931). 

 Upon attachment, the larval foot and eyespot are lost 

 and the newly settled, sessile juveniles are referred to 

 as spat (Ritchie and Menzel 1 969, Palmer 1 976, Manzi 

 et al. 1 977). Spat-fall on the Gulf coast typically occurs 

 from March until mid-November (Hopkins 1931, Ingle 

 1951, Hopkins 1955). 



Juvenile Size Range : Juveniles (spat) develop when 

 larvae cement themselves to the substrate. Growth of 

 spat varies with location of settlement site with an 

 average monthly growth rate of approximately 1 to 4 

 mm (Palmer 1 976, Manzi et al. 1 977). Fastest growth 

 for juveniles occurs during the first 3 months, and 

 decreases as they increase in size (Bahr 1 976). Func- 

 tional gonads may be present at 2-3 months of age and 

 a size of only 1 cm (Bahr and Lanier 1981). 



Age and Size of Adults : In the Gulf of Mexico, sexual 

 maturity may be reached as soon as 4 weeks after 

 attachment (Menzel 1951), but generally 18 to 24 

 months is normal (Quast et al. 1988). Butler (1954) 

 reports growth for the Gulf oysters to be approximately 

 50 mm/year. Gunter (1 951 ) gives growth rates of 0.26- 

 0.30 mm/day in the first 3 months, 60 mm in the first 

 year, 90 mm in the second year, and 1 1 5 mm in the third 

 year. Growth coefficients in Louisiana are highly 

 variable, fluctuating from 0.42 to 0.86 mm/day (Gillmore 

 1982). Growth is greatest in August and September, 

 after spawning when glycogen reserves are restored 

 (Loosanoff and Nomejko 1949, Price et al. 1975). 

 Mortality rates for adult oysters generally increase with 

 their size and age (Quast et al. 1 988). In the absence 

 of predation and fishing, 98% of all individuals die 

 before they reach 6 years of age with the lowest 



mortality occurring in salinities below 15%o and even 

 10%o (Hopkins 1 955, Mackin 1961 , Quast et al. 1988). 

 The maximum adult size is approximately 300 mm. 



Food and Feeding 



Trophic Mode : Larvae are planktivorous with large 

 umbo stage larvae able to ingest particles from 0.2 to 

 30 u.m (Davis 1953, Guillard 1957, Loosanoff 1965, 

 Bahr and Lanier 1981, Burrell 1986, Baldwin et al. 

 1989). Juveniles and adults are suspension filter 

 feeders that filter large quantities of brackish water, 

 and are particularly effective at removing particles 

 around the 3-4 urn range (Haven and Morales-Alamo 

 1970, Stanley and Sellers 1986). The rate of filtration 

 varies with water temperature, with the volume filtered 

 almost 1500 times the volume of the oyster's body 

 (Stanley and Sellers 1986, Berrigan et al. 1991). 



Food Items : Food is obtained from suspended par- 

 ticles entering through the ventral inhalent siphon and 

 passed to the gills. The particles are sorted in the gills, 

 and large particles are rejected. The rejected material 

 is voided as pseudofeces through the inhalent siphon 

 (Barnes 1 980). Larvae feed on microscopic algae and 

 naked flagellates (Davis 1 953, Guillard 1 957, Loosanoff 

 1 965, Bahr and Lanier 1 981 , Burrell 1 986, Stanley and 

 Sellers 1986). Naked flagellates are preferred by 

 adults. Bacteria are sometimes consumed, presum- 

 ably because they are attached to detritus particles, 

 but bacteria are generally a minor component of the 

 diet. Oysters have variable uptake of carbon from 

 Spartina altemiflora crude fiber ranging from less than 

 1% in Chesapeake area to over 20% in the southeast 

 region, primarily due to differences in crude fiber con- 

 centrations in the seston (Crosby et al. 1989). 



Biological Interactions 



Predation : Larvae are susceptible prey to a variety of 

 filterfeeders such as ctenophores, coelenterates, tuni- 

 cates, barnacles, molluscs, and and fishes (Hofstetter 

 1977, Berrigan et al. 1991). Ciliated protozoans also 

 prey on larvae, and are able to ingest as many as six 

 larvae at a time. Among sessile oysters, the predatory 

 oyster drill, Thais haemastoma, is responsible for the 

 majority of mortalities in Louisiana, Mississippi and 

 Alabama (Chapman 1959, Gunter 1979). In Missis- 

 sippi, rocksnails can destroy up to 50% of the oysters 

 on a productive reef, and up to 1 00% of the oysters on 

 a nonproductive reef. It is also a serious predator in 

 high salinity areas of Texas bays (Hofstetter 1977, 

 Soniat et al. 1989). All sizes of oysters are potential 

 prey for the rocksnail, but spat are particularly vulner- 

 able (Butler 1 954, Chapman 1 959). A single snail can 

 consume up to 4 spat per hour, or up to one adult oyster 

 every 8 days (Butler 1954, Gunter 1979). Rocksnails 

 open oysters by a combination of chemical dissolution 

 of the shell and drilling (radular rasping) (Stanley and 



25 



