Grass shrimp, continued 



4 to 1 6%o (Wood 1967). Growth of juveniles is greatest 

 at temperatures between 25° and 32°C and salinities 

 between 16 and 22%°. Below 14°C growth decreases, 

 and is negligible at 11°C (Wood 1967). Breeding 

 temperatures vary with geographic location of the 

 study, and range between 17° to 38°C (Sastry and 

 Vargo 1977, Wood 1967). 



Salinity: The effects of salinity on larval growth and 

 development are unclear and may vary with geo- 

 graphic location and individual populations. Larval 

 survival, however, is generally poor at salinities of less 

 than 15%o (Kirby and Knowlton 1976, McKenney and 

 Neff 1 979). The upper and lower 96 hour LC50 values 

 for larval grass shrimp in laboratory studies occurred at 

 16 and 46%o respectively (Kirby and Knowlton 1976). 

 The optimum salinity for complete larval development 

 is reportedly from 20 to 25%o (McKenney and Neff 

 1979, Knowlton and Kirby 1984). Larval and juvenile 

 grass shrimp are more tolerant of low salinities and 

 high temperatures than of high salinities and high 

 temperatures (Wood 1967). Juveniles and adults are 

 capable of tolerating salinities ranging from to 55%o 

 (freshwater to hypersaline), but are most common in 

 oligohaline to euhaline salinities of 2 to 36% (Wood 

 1 967, Kirby and Knowlton 1 976, Williams 1 984, Ander- 

 son 1985). In southwestern Florida, they were most 

 common from 10 to 15%o in one study (Rouse 1969), 

 and in waters with salinities of <20%o in another (Odum 

 and Heald 1 972). Salinity appears to affect maturation 

 and spawning age, with individuals from higher salinity 

 waters reaching maturity faster than those in lower 

 salinity waters (Alon and Stancyk 1 982). The 96 hour 

 LC50 values for adults is 0.5%° and 44%o (Kirby and 

 Knowlton 1976). 



Dissolved Oxygen (DO): Data on the DO requirements 

 of the grass shrimp are limited (Killam et al. 1 992). It is 

 apparently well adapted to low oxygen conditons, and 

 collections have been made in waters with DO levels 

 that ranged from 2.8 to 1 1 ppm (Welsh 1 975, Barrett et 

 al. 1978, Rozas and Hackney 1984). In laboratory 

 tests, it is able to tolerate DO levels less than 1 .0 ppm 

 (Anderson 1985). Grass shrimp can cope with brief 

 periods of low DO by climbing out of water on Spartina 

 stalks for a few hours, particularly during warm summer 

 nights (Wiegert and Pomeroy 1981). This species is 

 also able to tolerate anoxic conditions by decreasing its 

 oxygen consumption as DO declines (Welsh 1975). 



Migrations and Movements : There is little indication of 

 extensive migrations. The grass shrimp does, how- 

 ever, move to deeper waters with the onset of espe- 

 cially high or low temperatures. The extent of its 

 movements among various depths may be related to 

 the distribution of oyster shell substrates. It tends to 

 migrate in the direction of tidal currents, but avoids fast 



currents (Thorp 1 976, Anderson 1 985). There is some 

 evidence that grass shrimp may be more active at night 

 (Rozas and Hackney 1984). 



Reproduction 



Mode : Sexes in the grass shrimp are separate 

 (gonochoristic). This species is sexually dimorphic 

 and has external fertilization (Burkenroad 1947, 

 Knowlton and Williams 1970). Eggs develop ovipa- 

 rously. 



Mating and Spawning : When females become sexu- 

 ally mature, they molt into breeding-form and become 

 receptive to males (Burkenroad 1 947, Anderson 1 985, 

 Killam et al. 1 992). The breeding-form is characterized 

 by extra setae on the pleopods, enlargement of the 

 abdominal brood pouch, and development of periodic 

 chromatophores and is recognized by males through 

 antennal contact on some part of the female's body 

 (Burkenroad 1947). Mating must occur within 7 hours 

 of the female's molting, and oviposition must occur 

 within 7 hours after transfer of sperm. Spawning 

 usually occurs a few hours after mating (Burkenroad 

 1947). Fertilization is external and occurs with disso- 

 lution of the spermatophore as eggs are released by 

 the female (Burkenroad 1 947, Anderson 1 985). Eggs 

 are extruded onto the female's pleopods and are held 

 there until they hatch, usually in 1 2 to 60 days, depend- 

 ing on temperature. A new brood of eggs is deposited 

 1 to 2 days after hatching of the previous brood 

 (Knowlton and Williams 1 970). The spawning season 

 is from February to October, but may vary with geo- 

 graphic location. Two spawning peaks have been 

 noted in Galveston Bay, Texas, one in the early sum- 

 mer and the other in early fall (Wood 1967). The 

 presence of ovigerous females suggests that spawn- 

 ing occurs throughout the year in southwest Florida 

 (Rouse 1969, Williams 1984, Anderson 1985). 



Fecundity : The number of eggs produced increases as 

 the female grows. Fecundity estimates range from 

 <100 to >700 eggs per female (Welsh 1975, Wood 

 1967, Sikora 1977), but eggs probably number from 

 300 to 500 most commonly (Anderson 1 985, Killam et 

 al. 1992). Females can molt again within a few days 

 after spawning and produce a second brood (Knowlton 

 and Williams 1970, Anderson 1985). Peak egg pro- 

 duction occurs in May and is continuous through the 

 summer months, but begins to wane in September 

 (Knowlton and Williams 1970). 



Growth and Development 



Egg Size and Embryonic Development : Eggs are 0.6 

 to 0.9 mm in diameter (Holthius 1952, Broad 1957) 

 and develop oviparously (Anderson 1 985). Hatching 

 occurs in 12 to 60 days depending on geographical 

 location. The period of incubation is usually shorter in 



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