Blue crab, continued 



5% to the first crab stage, compared with 22% in the 

 control group (Bookout et al. 1980). Juvenile blue 

 crabs exposed to Kepone were shown to have a 96 

 hour LC50 at concentrations greater than 210 ppb 

 (Schimmel and Wilson 1 977). Mirex has been reported 

 to be toxic to blue crab zoeae at concentrations of 1 .0 

 and 10 ppb, whereas 0.01 and 0.1 ppb were sublethal 

 (Lowe et al. 1971, Bookout and Costlow 1975). DDT 

 and its derivatives tend to accumulate in the hepato- 

 pancreas of adult crabs (Sheridan 1975) and have 

 been demonstrated to cause high mortalities when 

 combined with low temperatures in natural habitats 

 (Koenig et al. 1 976). Juvenile blue crabs (27 mm CW) 

 died within a few days exposure to DDT concentrations 

 greater than 0.5 ppb (Lowe 1 965). Mass mortalities of 

 blue crab occurred in South Carolina, North Carolina, 

 and Georgia in 1966, and it was speculated that 

 pesticides were responsible (Newman and Ward 1 973). 

 Lipid-rich blue crab eggs may serve as a route for 

 exporting lipophilic compounds such as kepone (Rob- 

 erts and Leggett 1980). 



Ecological : The blue crab performs a variety of func- 

 tions in the estuarine ecosystem, and plays an impor- 

 tant role in trophic dynamics (Van Den Avyle and 

 Fowler 1984). At different stages in its life cycle, it 

 serves as predator and prey to plankton, small inverte- 

 brates, fish, and other crabs. It has been characterized 

 as an opportunistic benthic omnivore whose food hab- 

 its are governed by availability of food items (Darnell 

 1959). 



Range 



Overall : The blue crab is a cosmopolitan species found 

 in coastal waters, primarily in bays and brackish estu- 

 aries. It occurs occasionally from Nova Scotia, Maine, 

 and northern Massachusetts to northern Argentina, 

 and also Bermuda and the Antilles (Millikin and Will- 

 iams 1984, Williams 1974, Williams 1984). It is found 

 north of Cape Cod only during favorable warm periods 

 that allow it to move into these waters. This species 

 has also been introduced into coastal waters of Europe 

 and Japan. 



Withinthe Study Area : This species is abundant through- 

 out the nearshore and estuarine areas of the Gulf of 

 Mexico (Table 5.11) (Millikin and Williams 1984, Will- 

 iams 1 974, Williams 1 984). For the purposes of Table 

 5.11, all zoeal and megalopal stages are considered 

 together as "Larvae". 



Life Mode 



The blue crab spends most of its life in estuaries and 

 nearshore Gulf waters. Eggs are carried externally by 

 the female for approximately two weeks. Egg-bearing 

 females are commonly known as sponge or berry 

 crabs. Eggs hatch near the mouths of estuaries, and 



the zoeal larvae are carried offshore. Zoeae are 

 planktonic, and remain in offshore waters for up to one 

 month. Metamorphosis to the megalopal stage follows 

 the seventh zoeal molt. Re-entry to estuarine waters 

 occurs during the megalopal stage. Juveniles and 

 adults tend to be demersal and estuarine. Adult males 

 spend most of their time in low salinity waters; females 

 move into these lower salinities as they approach their 

 terminal molt to mate. After mating, females move to 

 higher salinity areas of estuaries and nearshore envi- 

 ronments for spawning (Dudley and Judy 1 971 , Millikin 

 and Williams 1984, Van Den Avyle and Fowler 1984, 

 Williams 1984). 



Habitat 



Type : The blue crab is dependent on estuaries during 

 portions of its life. Depending on the life stage, indi- 

 viduals can be neritic, estuarine and/or riverine. Zoeae 

 are found in oceanic habitats (Williams 1 984), and they 

 are positively phototropic (Costlow et al. 1959). The 

 megalopae swim freely and may be found in the surf 

 area near the bottom in nearshore or lower estuarine 

 high-salinity areas. In Tampa Bay, the primary habitat 

 that megalopae use for settlement appears to be 

 seagrass or vegetated bottom (Killam et al. 1992). In 

 the northern Gulf of Mexico, megalopae move into 

 nearshore marshes where molt to the first crab stage 

 occurs (Perry pers. comm.). Within an estuarine sys- 

 tem, habitat is partitioned for use by blue crabs based 

 on size class, and may be related to food availability, 

 predator avoidance, nutritional requirements, repro- 

 ductive success, and growth (Steele and Bert 1994). 

 Juveniles have been found in greatest numbers in low 

 to intermediate salinities characteristic of upper and 

 middle estuarine waters (Steele and Perry 1 990). They 

 prefer seagrass as nursery habitat but also utilize salt 

 marsh habitat (Thomas et al. 1 990, Killam et al. 1 992). 

 Juveniles and adults tend to be demersal and estua- 

 rine. Adult males spend most of theirtime in low salinity 

 water and females move from higher to lower salinities 

 as they approach their terminal molt in order to mate 

 (Dudley and Judy 1971, Millikin and Williams 1984, 

 Van Den Avyle and Williams 1984, Williams 1984). 

 Although juvenile and adult blue crab distributions are 

 affected by salinity (Killam et al. 1 992, Steele and Bert 

 1994), other factors such as substrate type and food 

 availability also play a major role (Steele and Perry 

 1990). 



Substrate : Juveniles and adults are found on muddy 

 and sandy bottoms. Juveniles have been found in 

 greatest abundances in association with soft mud 

 bottoms (Van Engel 1958, Perry 1975, Perry and 

 Mcllwain 1986). 



Physical/Chemical Characteristics : Environmental re- 

 quirements affecting the growth, survival, and distribu- 



99 



