The order Decapoda consists of shrimps, crayfishes, lobsters, and crabs. The members of this group have ten legs and are distinguished from other crustaceans by a well-developed carapace that covers the head and thorax. The decapod crustaceans of the ACE Basin study area are ecologically, recreationally and commercially important. Species such as blue crab and penaeid shrimp are important to commercial and recreational fisheries, while others such as grass shrimp are important to the trophodynamics of the estuary (Welsh 1975).
It appears that decapod crustaceans play a critical role in metabolizing and controlling the flow of energy in estuarine ecosystems. Decapods are preyed upon by a variety of predators from alligators to fishes. Depending on its intensity, predation is a factor in controlling population density, as well as structuring species assemblages within a habitat. Decapods are also important predators themselves, consuming phytoplankton, benthic algae, and macrobenthos (Coull and Bell 1983). Decapod particulate feeders consume detritus derived from Spartina and feces, thereby making detritus available to several different trophic levels and processing particles in such a way that substrate is enhanced for accelerated growth by diatoms and bacteria (Field 1983). While there is some information on the role of specific decapod species in aquatic systems, our understanding of decapod crustacean populations and communities is limited. As more information is collected on life histories, demography, and species interactions, a better understanding of the role of decapods in ecosystems will emerge.
The ACE Basin study area estuarine system is similar to other systems throughout the state in that it supports a diverse assemblage of decapods and provides both a seasonal habitat for adults and juveniles and a permanent year-round habitat for resident species. Subtidal and intertidal estuarine habitats provide a refuge from predation and a source of food for many decapods. These habitats are exploited by year-round residents such as grass shrimp (Palaemonetes sp.), or seasonally abundant species such as blue crab, Callinectes sapidus, and the penaeid shrimps, white shrimp Penaeus setiferus (recently changed to Litopenaeus setiferus), brown shrimp Penaeus aztecus (recently changed to Farfantepenaeus aztecus), and pink shrimp Penaeus duorarum (recently changed to Farfantepenaeus duorarum) (Kneib 1984; Weinstein 1979). Although decapod species are conspicuous inhabitants of the ACE Basin study area and many are economically important, few comprehensive studies of the decapod community have been done in that region. No information is available on the decapod community from shallow marsh, oyster reefs, or impoundments in the ACE Basin study area; however, information on decapods from these habitats is available for other coastal areas of South Carolina. Most of the information on decapod species assemblages has come from trawl surveys of the rivers in and near the ACE Basin study area. These data are the most comprehensive to date for assessing spatial and temporal changes in species composition, diversity, and biomass.
The coastal zone, defined by Struhsaker (1969) as extending from the sounds and estuaries out to depths of 18 m (59 feet), is characterized by low relief sand and mud bottom among which is interspersed "Alive-bottom" reefs. These reefs are distinguished from the surrounding sand biotope by supporting a diverse assemblage of sessile invertebrates as well as numerous motile species which are inhabitants of the complex microhabitats of the reefs. The decapod community from the nearshore coastal zone, outside the geographic boundaries of the ACE Basin study area, was described in several papers. Wenner and Read (1981, 1982) described decapod crustacean assemblages from open shelf and live bottom habitats between Cape Fear, North Carolina and Cape Canaveral, Florida. They collected 184 species from 38 families. The inner shelf assemblage contained few species with high abundance. The inner shelf deca pod community at depths of 4-20 m (13 - 66 ft) was further described by Wenner and Wenner (1988). They collected 60 species of decapods, many of which occur in trawl samples from the ACE Basin study area. Trachypenaeus constrictus, the hardhead shrimp; Callinectes similis, the lesser blue crab; the portunid crabs, Portunus gibbesii, P. spinimanus, Ovalipes stephensoni and O. ocellatus; and the white (Penaeus setiferus) and brown (Penaeus aztecus) shrimps were the most abundant decapods encountered. The coastal assemblage appears to be composed of physiologically adaptable species which are not stressed by variable salinity due to large amounts of river runoff or seasonal changes in bottom temperature. Several of these nearshore species, such as the blue crab, lesser blue crab, portunids (Portunus spp. and Ovalipes spp.), white and brown shrimp, hardhead shrimp, and rock crab (Cancer irroratus) have been encountered in trawl tows in the ACE Basin study area (Decapod trawl survey ).
Sampling of decapod crustaceans in nearshore areas is continuing as part of the Southeast Area Monitoring and Assessment Program (SEAMAP) . The SEAMAP trawl survey covers the coastal zone between Cape Hatteras, North Carolina and Cape Canaveral, Florida. Multi-legged cruises are conducted in spring, summer and fall during which stations from 24 inner strata, delineated by the 4-10 m (13-33 ft) depth contours, are sampled. Ten outer strata off South Carolina, Georgia and Florida at depths of 10-19 m (33-62 ft) are also sampled in spring, while seven outer strata off North Carolina were sampled in fall. As many as 50 decapod species have been collected since the survey began in 1989. Catches have consistently been dominated by white and brown shrimp, the portunid crabs Portunus gibbesii and Ovalipes stephensoni, and the lesser blue crab Callinectes similis (J. Boylan. 1999. SCDNR. pers. comm.).
Decapod crustaceans are an important component of estuarine subtidal rivers and tidal creeks in the ACE Basin study area. The first comprehensive trawl survey describing fluctuations in the distribution and abundance of decapod crustaceans in the ACE Basin study area was conducted from 1973-1975 (Wenner et al. 1991). A total of 38 species were collected from eight stations in the North Edisto River, while 30 species were taken from four stations in the South Edisto. Mesh size, net dimensions and towing speed influence trawl catches. The small size of the trawl used in the ACE surveys makes them highly selective for juvenile stages. The dominant decapod species in terms of abundance and biomass were white shrimp (Penaeus setiferus), brown shrimp (Penaeus aztecus), blue crab (Callinectes sapidus) and sea bob (Xiphopenaeus kroyeri). These species together comprised almost 94% by number and > 97% by weight of the total decapod catch. As indicated for finfish assemblages, the decapod assemblages consisted of transient species, stenohaline marine species, and estuarine endemics. (See related section: Fish .) The faunal diversity was controlled primarily by salinity in the South Edisto River, while the North Edisto lacked a distinct halocline and had a fairly uniform distribution of species among stations. In the South Edisto, the estuarine endemic species were separated spatially from euryhaline marine transients and stenohaline marine species.
More recently, a routine monitoring of decapod crustaceans was begun in 1995 as part of a long-term survey of the three major rivers in the ACE Basin (See NERR trawl stations ). This monthly trawl survey in the ACE Basin National estuarine Research Reserve (NERR) has collected a total of 43,319 individuals representing 28 species (Decapod trawl survey ). Greater number of species (26) was collected in the South Edisto, while 23 species were collected from the Ashepoo River and 17 were taken from the Combahee River (Species by river ). This difference probably reflects influence of the strong salinity gradient in the South Edisto which encourages invasion by stenohaline marine species from the coastal zone. Within all rivers, stations in the lower part yielded the most species. Station E001 in the South Edisto was especially diverse with 22 decapod species. The stations that were characterized as mesohaline, based on salinity conditions at the Ashepoo , the Combahee , and the South Edisto trawl stations over the sampling period, generally yielded more individuals than other stations. Fewest individuals were collected at station E019 in the South Edisto which has the lowest salinity of any stations sampled . As noted by Wenner et al. (1991), it appears that many estuarine endemic forms were spatially separated from the euryhaline transients and the stenohaline marine species. The overlap in distribution of the transient and stenohaline decapods contributed to increased species richness at stations nearest the mouth. 0
The white shrimp Penaeus setiferus, the brown shrimp Penaeus aztecus, and the blue crab Callinectes sapidus were the numerically dominant species for all three rivers. These three species constituted >90% of the total number of individuals collected. Penaeus setiferus and C. sapidus, respectively, were the top-ranked species in terms of weight.
Seasonal changes in abundance are common for many decapods. Wenner et al. (1991) noted that the numerically-dominant decapod crustaceans in the North and South Edisto displayed similar patterns of seasonal abundance in both rivers. Recent trawl surveys in the ACE Basin NERR indicated that Penaeus setiferus was most abundant in summer and fall. Abundance was fairly consistent among stations in the Combahee and Ashepoo, although fewest white shrimp occurred at stations furthest upriver. In the South Edisto, stations E001 and E019 yielded the greatest abundance. .
Temporal and spatial abundance patterns of white shrimp are related to life history of the species and the size at which white shrimp are vulnerable to the trawl gear. White shrimp are recruited migrate) as postlarvae to the estuaries and sounds of the ACE Basin study area in late spring and early summer, grow rapidly to juvenile and subadult stages in estuarine nursery areas, and emigrate to coastal waters in fall (See related subsection: Zooplankton: Macrozooplankton ). Postlarvae seek habitats in shallow tidal creeks with muddy/sand substrate and plentiful organic debris. Juveniles prefer oligohaline and mesohaline areas but may be found along the entire estuarine salinity gradient (Wenner and Beatty 1993). During mild winters, white shrimp can overwinter in deep, high salinity areas but their survival is dependent on temperature minima being >6EC (Farmer et al. 1978).
Brown shrimp Penaeus aztecus are highly seasonal in the ACE Basin study area with peak catches occurring in June or July. Brown shrimp were collected at every station in the trawl survey of the ACE NERR, but very few were collected at station E019 located furthest upriver in the South Edisto (See NERR trawl stations ).
Abundance patterns of brown shrimp are closely related to its life history in the southeastern United States. Brown shrimp generally recruit to South Carolina estuaries as early as February, with peak ingress of postlarvae occurring in March-April (Wenner et al. 1990). Postlarvae and juveniles are associated with shallow vegetated habitats over a wide range of salinities (Wenner and Beatty 1993). Brown shrimp grow rapidly until late summer when much of the population moves from estuaries to offshore areas causing a rapid decline in abundance within the estuaries. Emigration usually occurs in July and August in the Carolinas, with a corresponding peak recruitment to the coastal shrimp fishery (South Atlantic Fisheries Management Council 1981).
The blue crab Callinectes sapidus occurred during every season and at every station sampled, indicating its general ubiquity within the ACE Basin study area estuarine areas (60-70% of all trawls in the three rivers captured this species). Greatest densities of juveniles and subadults occurred in fall and winter, although this varied depending upon station and river.
Blue crab are recruited as megalopae to estuaries of South Carolina in late summer and early fall (Mense and Wenner 1989; Boylan and Wenner 1993). Following settlement and molting to the first crab stage, blue crab occupy a variety of estuarine habitats including tidal creeks, marsh rivulets and oyster shell banks. Juveniles and subadults occupy the mid to upper reaches of estuaries. Spring and winter peaks in juvenile blue crabs have been reported for coastal South Carolina. Mense and Wenner (1989) found that the greatest abundance of juveniles occurred during January and September in the Charleston Harbor system. This winter cohort of juveniles most likely results from a late summer spawn of larvae, Wenner et al. (1990) hypothesized that the increased abundance of juvenile blue crabs in early winter may reflect increased utilization of estuarine nursery areas from which predators have emigrated due to decreasing water temperature.
Salt marshes provide habitat for several decapod species in coastal South Carolina. Rapid fluctuations in water quality variables such as salinity, temperature, and dissolved oxygen restrict the number of decapods that occur in the salt marsh. In spite of these harsh conditions, marshes are highly productive in terms of organic matter. The protection afforded by marsh grass stem structure and the abundant food supply of salt marshes make them important nursery habitats for larval and juvenile stages of decapod species such as blue crab, white shrimp, and grass shrimp. Subadult stages move into intertidal marshes along the creek edge on incoming tides and penetrate the interior marshes during flood tide (Kneib and Wagner 1994). Resident species such as fiddler crabs (Uca spp.) burrow preferentially in sediments with intermediate densities of Spartina root mats (Bertness and Miller 1984) ## . Fiddler crabs and grass shrimp are important prey of piscine, avian, an d mammalian marsh inhabitants.
Although no specific information is available on the decapod community from shallow marsh habitats in the ACE Basin study area, decapod crustaceans common to salt marshes of the nearby Port Royal Sound were studied by Vernberg and Sansbury (1972). A total of eight decapod species was collected from five marsh sites. The fiddler crab, Uca pugnax, was most common in their samples, although U. pugilator was frequently collected near high tide in sandy substrates. The mud crabs, Panopeus herbstii and Eurypanopeus depressus, were common around areas where oyster shell occurred. Other species common at their marsh sites included Sesarma reticulatum and S. cinereum. Knott et al. (1997) sampled salt marshes near Charleston, South Carolina and found that grass shrimp Palaemonetes pugio dominated collections from the marsh surface. Other dominant species were white shrimp and the mud fiddler crab, Uca pugnax.
Oyster reefs are a major habitat in the intertidal zone of the ACE Basin study area and harbor numerous decapod species. These reefs provide the only intertidal three-dimensional structural relief in an otherwise unvegetated, soft-bottom, benthic habitat Klemanowicz (1985) sampled motile and non-colonial invertebrates from oyster reefs in the Coosaw River and found that the mud crabs, Eurypanopeus depressus and Panopeus herbstii, were the dominant decapods. More recently, Wenner et al. (1996) provided a list of decapod and fish species associated with intertidal oyster reefs in the vicinity of Charleston Harbor. Several decapod species such as the mud crabs, Panopeus herbstii and Eurypanopeus depressus, and the stone crab, Menippe mercenaria, occupy the many crevices found on intertidal oyster reefs. The association of E. depressus with oyster reefs in intertidal area s enables this species to avoid dessication as well as potential predators (Grant and McDonald 1979). Thus, oyster reefs provide a refuge from predation and also harbor smaller invertebrates upon which many decapods feed. In turn, decapods on these reefs are prey for finfish species such as oyster toadfish, sheephead and blennies that inhabit the reefs at flood tide. Other decapod species move on and off the reefs with the tide. These include grass shrimp (Palaemonetes spp.), blue crab and penaeid shrimp.
Impoundments are a man-made alteration to wetlands that can affect the community structure of organisms that inhabit them. Impounding wetlands produces changes in vegetation that may affect the relationship between the natural distribution of salt-marsh plant communities and associated animal populations. Because tidal flooding expands the surface area of the marsh and its utilization by natant macrofauna, predator-prey interactions, competition, disturbance, and physical stresses may be intensified in impoundments that are subject to prolonged flooding. Despite the potential consequences on macroinvertebrate populations, little information exists on community assemblages from South Carolina. The most comprehensive treatment of the decapod community from coastal wetland impoundments was provided by Wenner et al. (1986) for the Santee delta. The most abundant species in impoundments were grass shrimp (Palaemonetes pugio and P. vulgaris), white shrimp (Litopenaeus setiferus), brown shrimp (Penaeus aztecus), pink shrimp (F. duorarum) and blue crab (Callinectes sapidus). These species were also dominant in trawl collections from an adjacent tidal creek. Abundance and biomass of these dominant species declined in summer and early fall, likely due to stressful water quality conditions occurring in impoundments. Overall, the faunal assemblages in the impoundments were distinct from those in the adjacent creek which was attributed to sampling bias as well as real differences in the habitats themselves.
The degree to which decapod species are able to inhabit impounded areas depends on the timing of recruitment and the water exchange schedules of the impoundment management strategy (Olmi 1986). Planktonic decapod crustaceans occurred in estuarine impoundments of the Santee Delta from May to November. Penaeid shrimp postlarvae recruiting to impoundments were brown shrimp, pink shrimp, white shrimp, and hardhead shrimp (Trachypenaeus constrictus). Megalopae of the blue crab recruited to impoundments from July through November. As the decapod species grew within the impoundments, the water-control structures probably acted as barriers to their emigration. Blue crab and penaeid shrimp had restricted access to the creek during periods when they would normally emigrate to offshore areas. Greater abundance of mature female blue crab within impoundments during months when spawning migrations to high-salinity waters occur suggested that crabs were retained within impoundments. Su b-adult white and brown shrimp were collected in the impoundments during fall, the season when emigration to nearshore coastal areas occurs (Wenner 1986).
The decapod community in freshwater portions of the ACE Basin study area has not been scientifically investigated, in spite of the importance of tidal freshwater wetlands as transition zones between salt-brackish marshes and non-tidal freshwater areas. Consequently, many decapods that occur in oligohaline zones of estuaries may also occur in freshwater areas of the ACE. There has been some information on community composition and nursery function of low-salinity wetland habitats along the US Atlantic coast. Odum et al. (1984) reviewed the community composition and nursery function of low-salinity marshes and noted that clear differences exist in the invertebrate communities of tidal freshwater marshes versus salt marshes. It has been hypothesized that few crustacean species inhabit freshwater marshes because of physiological difficulties that result from inhabiting low salinity areas. Species impoverishment of tidal freshwater marshes has also been attributed to general lack of habitat diversity (Odum 1988). Recent studies in tidal freshwater wetlands of Virginia indicated that invertebrate species composition of marsh surface residents was similar in tidal freshwater and salt marshes, despite physicochemical differences in the habitats. Many of the shrimp and crabs occurring in freshwater marshes have well-developed osmoregulatory capabilities and are not stressed by low and often variable salinities that occur in oligohaline conditions (Moore 1992). Rozas and Hackney (1983, 1984) documented the importance of oligohaline marshes as nursery areas for grass shrimp and blue crab. Freshwater shrimp of the genus Macrobrachium and the mud crab, Rhithropanopeus harrisii, occurred but were rare in their collections.
Given the baseline information collected on decapod crustaceans from the ACE Basin study area, it appears that this system is similar to others that have been studied throughout the state in that it supports a diverse assemblage of decapods and provides both seasonal habitats for adults and juveniles and permanent year-round habitats for resident species. Given differences in life history, sizes and behaviors of decapod crustaceans, further efforts to understand their community structure in the ACE Basin study area should focus on shallow marsh and freshwater areas where there is a paucity of information. A state-wide monitoring program being initiated in 1999 by the South Carolina Department of Natural Resources may provide some additional data to compare the communities in the ACE Basin study area with other areas in the state.
E. Wenner, SCDNR Marine Resources Research Institute
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