Grass shrimp, continued 



areas with warmer water than in cooler locations. 



Age and Size of Larvae : Newly hatched larvae are 2.6 

 mm. They go through 3-11 zoeal stages (molts), 

 ending at about 6.3 mm. The zoeal stages last from 

 1 1 days to several months depending on environmen- 

 tal conditions including the amount of food (Broad 

 1957). In a study conducted in Georgia, it was 

 suggested that settlement from the plankton by ad- 

 vanced zoeal stages and metamorphosis to the 

 postlarva stage is triggered when larvae enter veg- 

 etated habitats (Kneib 1987b). 



Juvenile Size Range : Growth to maturity in Texas is 

 reported to take 2 to 3 months in summer and 4 to 6 

 months in winter. Females are mature at a size of 

 approximately 18-24 mm TL (total length) and males 

 at approximately 1 5 mm TL (Broad 1 957, Wood 1 967, 

 Knowlton and Williams 1 970, Alon and Stancyk 1 982). 



Age and Size of Adults : The life span of this species 

 is 6 to 13 months. The older overwintering shrimp 

 usually spawn early in the year as adults, and 

 postlarvae that survive the winter spawn the following 

 spring. In South Carolina, habitats with consistently 

 higher salinities (>20% o ) may provide more optimal 

 conditions, resulting in faster growth and earlier spawn- 

 ing, than fluctuating, lower salinity habitats (<20%o) 

 (Alon and Stancyk 1982). Reported maximum sizes 

 for males and females are 33 mm and 50 mm TL, 

 repectively (Holthuis 1952). . 



Food and Feeding 



Trophic mode : This species is an opportunistic, om- 

 nivorous feeder (Anderson 1 985, Kneib 1 987a, Nelson 

 and Capone 1990). It probably uses tactile cues and/ 

 or chemoreceptors on its legs in order to find relatively 

 sedentary benthic prey, but may rely on the sensitivity 

 of its compound eyes to detect nektonic prey (Kneib 

 1987a). 



Food Items : Planktonic larvae feed on zooplankton, 

 algae, and detritus. Juveniles and adults eat a variety 

 of animal and plant matter including detritus, polycha- 

 etes, meiofauna, blue crab megalopae, larval fish, 

 algae and dead animal matter (Heard 1 979, Anderson 

 1985, Kneib 1987a, Nelson and Capone 1990, Olmi 

 1990). Grass shrimp are known to consume the 

 epiphytic organisms attached to seagrasses while 

 living in this habitat (Morgan 1980). When epiphyte 

 abundance is high, grass shrimp are capable of using 

 them to completely satisfy their dietary needs. 



Biological Interactions 



Predation : Wading birds such as the clapper rail (Rallus 

 longirostris) utilize the grass shrimp as food (Heard 

 1 982). It has also been found in the stomach contents 



of juvenile American alligators (Piatt et al. 1990). 

 Piscine predators include: longnose gar (Lepisosteus 

 osseus), blue catfish (Ictalurus furcatus), gafftopsail 

 catfish (Bagre marinus), hardhead catfish, gulf killifish, 

 yellow bass (Morone mississippiensis), largemouth 

 bass (Micropterus salmoides), snook, gray snapper, 

 silver perch, Atlantic croaker, spotted seatrout, sand 

 seatrout, red drum, black drum, pinfish, sheepshead, 

 bighead searobin (Prionotus tribulus), Spanish mack- 

 erel, king mackerel (S. cavalla), and southern flounder 

 (Gunter 1945, Kemp 1949, Miles 1949, Darnell 1958, 

 Harrington and Harrington 1961, Linton and Rickards 

 1965, Boothby and Avault 1971, Diener et al. 1974, 

 Bass and Avault 1975, Danker 1979, Levine 1980, 

 Overstreet and Heard 1 982, Rozas and Hackney 1 984, 

 Perschbacherand Strawn 1 986, Morales and Dardeau 

 1987, Peters and McMichael 1987, Hettler 1989). 

 Penaeid shrimp may also prey upon juvenile grass 

 shrimp (Kneib 1987b). Blue crabs in Florida are known 

 to occasionally prey on grass shrimp during the winter 

 (Laughlin 1982), and small juvenile blue crabs have 

 been observed capturing and consuming grass shrimp 

 when both were held in aquaria set up with marsh 

 habitats (Pattillo pers. obs.). 



Factors Influencing Populations : 

 Temperature and salinity are considered to be the 

 major factors affecting the distribution of grass shrimp 

 (Wood 1 967, Killam et al. 1 992). Although this species 

 can tolerate wide ranges of these two parameters, 

 reproduction, optimal growth, and survival can be 

 negatively affected by extreme conditions. Grass 

 shrimp abundance can be affected by habitat alter- 

 ations that destroy vegetation on which this species 

 depends (Trent et al. 1 976, Anderson 1 985). The loss 

 of vegetation also results in a reduction of detrital input 

 into surrounding systems which can cause a decrease 

 in grass shrimp abundance. Palaemonetes pugio is 

 not as tolerent to higher salinities as some of its sister 

 species, and this may contribute to its replacement in 

 high salinity waters by P. vulgaris and/or P. interme- 

 dius (Williams 1985). Predation by fishes can have a 

 major influence in the distribution and longevity of 

 grass shrimp (Alon and Stancyk 1982, Kneib 1987b). 

 Displacement of grass shrimp from their preferred 

 habitats of submerged macrophytes makes them more 

 vulnerable to predation (Anderson 1 985). Adult grass 

 shrimp prey on the larvae of killifish (Fundulus sp.) and, 

 by so doing, contribute to the control of one of their 

 principal predators (Kneib 1 987a). Diseases and para- 

 sites do not appear to have any major effect on the 

 abundance and growth of grass shrimp in the Gulf of 

 Mexico (Anderson 1985). 



84 



