Species Description: The daggerblade grass shrimp, Palaemonetes pugio, is a small transparent shrimp with a well-developed rostrum bearing several dorsal as well as three distinct ventral teeth, a smooth carapace and abdomen, and two pairs of chelate (claw-bearing) walking legs, the second pair more robust than the first. It has well-developed eyes with globular pigmented corneas and some slight yellow pigmentation in the eyestalks. The back is straight and the telson has two pairs of well-developed dorsal spines and also two pairs of posterior spines (Anderson 1985, Kaplan 1988, Rupert and Fox 1988).
Potentially Misidentified Species: Palaemonetes pugio may be confused with several co-occurring shrimp species in the IRL. The relatively small size and the lack of chelae (claws) on the third pair of walking legs is sufficient to distinguish the caridean shrimps (including P. pugio) from the familiar penaeid shrimp species.
Members of the genus Palaemonetes can be differentiated from other palaemonid shrimp (e.g., Palaemon, Macrobrachium) by the absence of mandibular palps, although this usually requires detailed examination beyond the scope of amateur naturalists (Anderson 1985). Likewise positive differentiation of P. pugio from co-occurring congeners requires close examination of subtle differences in the chelae, rostra, antennules, and other external features (see included figure, adapted from Anderson 1985).
Other members of genus Palaemonetes from the IRL are P. intermedius, P. paludosus, and P. vulgaris.
Regional Occurrence: Palaemonetes pugio is a widely distributed western Atlantic and Gulf of Mexico species occurring from Canada to Texas (Kaplan 1988).
IRL Distribution: Palaemonetes pugio occurs in seagrass beds and other suitable habitats throughout the IRL system.
Age, Size, Lifespan: Adult Palaemonetes pugio reach a length of around 5 cm (Kaplan 1988). The life span of P. pugio is 6 to 13 months (Alon and Stancyk 1982).
Abundance: Palaemonetes spp. shrimp are among the most abundant and widely distributed benthic macroinvertebrates of Atlantic and Gulf coast estuaries (Sikora 1977, Anderson 1985).
Reproduction: Palaemonetes pugio mature at 1.5 to 2 months of age and 15-18 mm length (Anderson 1985). The duration of the spawning season varies with geographical location. In the Gulf of Mexico, the season extends from approximately March through October and animals may spawn more than once in a season. Overwintering older P. pugio typically spawn early in the year and die before the next winter (Alon and Stanyck 1982). In contrast, early arriving young-of-the-year can spawn late in the year as adults (Anderson 1985). In the colder northern waters of Rhode Island, P. pugio spawns only once during a shorter season lasting only from May-July (Welsh 1975).
Fecundity in P. pugio is reportedly variable depending on geographic location. Females collected from Rhode Island in June averaged 486 eggs per female, while in Texas the average was only 372 eggs/female and only up to 247 in South Carolina (Welsh 1975, Wood 1967, Sikora 1977). A significant positive correlation exists between the length of ovigerous females and egg number (Wood 1967).
Burkenroad (1947) describes the female premating condition, mating and spawning for P. vulgaris. Just prior to mating, the female undergoes molting. Copulation occurs within 7 hours of female molting and involves the paired shrimp positioning themselves so that their genital apertures are close to each other. The male transfers a spermatophore onto the genital sternites of the female and remains there until oviposition occurs around 7 hours later. Prior to oviposition, spermatozoa are released through a weakened (probably by female enzymatic secretions) spermatophore. Eggs undergo external fertilization as they are extruded from the female genital aperature. The fertilized eggs are then manually transferred and adhered to the pleopods and ventral setae of the female's abdomen where they are incubated.
Embryology: Eggs hatch 12-60 days after fertilization, depending on the species and location. Free-swimming larvae are released from eggs with the aid of osmotic swelling of the inner membrane, undulation of the ventilating appendages of the female, and struggling of the larvae within the eggs (Davis 1965).
Larval P. pugio are approximately 2.6 mm at hatching and around 6.3 mm at metamorphosis (Broad 1957, Anderson 1985). The duration of multi-stage (7-11 larval stages) larval development in P. pugio ranges from 11 days to several months, depending on the environment (Floyd 1977). The final larval stage metamorphoses into a postlarval form closely resembling the adult shrimp.
The planktonic larvae feed on zooplankton, phytoplankton, and detrital material (Anderson 1985).
Temperature: P. pugio is a eurythermal species. While optimum growth appears to occur at around 30°C, animals thrive at temperatures ranging from 5-38°C (Wood 1967, Christmas and Langley 1973). Sastry and Vargo (1977) reported breeding in P. pugio from Rhode Island when water temperatures ranged between 22° and 27°C, while Wood (1967) indicated breeding in a Texas population between 17° and 38°C.
Wood (1967) reports that P. pugio may migrate to deeper water to avoid both seasonal high and low temperature conditions.
Salinity: Palaemonetes pugio occupies estuarine habitats that experience broad fluctuations in salinity. Adult P. pugio may briefly tolerate salinities as low as 0 ppt and as high as 55 ppt, but in the wild they are typically found within a narrower range of around 2 to 36 ppt (Wood 1967, Christmas and Langley 1973, Morgan 1980).
Although Broad and Hubschman (1962) experienced low larval survival at salinities of less than 10 ppt, McKenny and Neff (1979) achieved nearly 50% larval survivorship at salinities as low as 3 ppt. Salinities between 20 and 25 ppt appear optimal for larval development (McKenny and Neff 1979, Knowlton and Kirby 1984), and Floyd (1977) notes that larvae mature faster and often pass through fewer larval stages when reared near optimal salinity conditions. Kirby and Knowlton (1976) report LD50 values of adult P. pugio of 0.5 ppt and 44 ppt.
Individuals mature and spawn at a younger age in habitats with a relatively high salinity (Alon and Stancyk 1982), and specimens collected in low salinity waters were smaller than those from more saline waters (Wood 1967).
Dissolved Oxygen: Barrett et al. (1978) commonly encountered P. pugio in Louisiana waters with DO concentrations ranging from 6-11 ppm, but field and laboratory studies indicate some survivorship after limited exposure (a tidal cycle) to hypoxic conditions as low as 0.1 ppm (Anderson 1985). Under conditions of hypoxia, the rate of P. pugio oxygen uptake decreases (Welsh 1975, Dillon 1983), and individuals have even been observed to escape oxygen-deficient conditions by climbing out of the water for brief periods (Pomeroy and Wiegert 1981).
Trophic Mode: Like most grass shrimp, Palaemonetes pugio is a generalist forager that can consume a variety of dietary items depending on availability. They may forage as primary consumers, secondary consumers, and detritivores (Morgan 1980, Anderson 1985).
Despite their association with seagrasses and other benthic aquatic vegetation, grass shrimp consume little to no actual macrophyte biomass. Rather, they primarily consume the epiphytic microalgae growing on the surfaces of seagrasses and other aquatic macrophytes (Morgan 1980).
Grass shrimp also function as predators on meiofauna and small infauna including ostracods, nematodes, polychaetes, and oligochaetes (Bell and Coul 1978, Chambers 1981). Morgan (1980) also indicates that grass shrimp are capable of preying on motile fauna such as mysids. Epibenthic predation along with associated disturbance of sediments by grass shrimp is capable of altering infaunal community structure (Bell and Coul 1978, Knieb and Stiven 1982).
Grass shrimp function as detritivores by aiding in the mechanical breakdown of seagrasses and other refractory plant materials. They also assimilate the microfloral and fungal biomass that colonizes and enriches the detritus and cycles that energy through the estuarine food web (Adams and Angelovic 1970, Anderson 1985).
Grass shrimp also derive a substantial fraction of their nutrition from dissolved organic matter adsorbed onto fine (clay-sized) particles (Odum and Heald 1972).
Competitors: Evidence for interspecific and intraspecific competitive interactions involving Palaemonetes pugio comes from published field and laboratory studies.
Studies reveal that Palaemonetes vulgaris can displace P. pugio from preferred habitats (e.g., oyster reefs). Lab experiments by Chambers (1981) with P. pugio and P. vulgaris showed females were dominant over males and large shrimp were dominant over smaller ones. P. vulgaris also generally dominated P. pugio.
Predators: Grass shrimp are an important item in the diets of a large number of estuarine species including economically important commercial and recreational fishery species (Overstreet and Heard 1982, Heard 1982, Anderson 1985). Grass shrimp are also consumed by killifishes and other forage fish that are themselves important prey items for larger piscivores (Harrington and Harrington 1972, Kneib and Stiven 1982). Abundant as they are, grass shrimp are an extremely important conduit in the transfer of energy from the producer and decomposer levels up to the higher consumer levels of the trophic pyramid (Anderson 1985).
Grass shrimp minimize their exposure to predators through their association with benthic macrophytes such as seagrasses and marsh grass or other protective physical structure (e.g., oyster reefs). Displacement from these preferred refugia increases the predation rates (Thorp 1976, Coen et al. 1981, Heck and Thoman 1981).
Parasites: The published literature indicates that grass shrimp are hosts for a number of parasites, including coccidians, microsporidians, trematodes, isopods and leeches (Overstreet and Weidner 1974, Anderson 1977, Overstreet 1978, Solangi and Overstreet 1980). Grass shrimp are also frequently parasitized by mated pairs of the bopyrid isopod Probopyrus pandalicola which form a conspicuous blister on the carapace of the shrimp (Rupert and Fox 1988). Nevertheless, parasites are not considered to be limiting to grass shrimp abundance and overall population health (Anderson 1985).
Habitats: Palaemonetes pugio is a common inhabitant of seagrass beds and oyster reefs (Thorp 1967, Anderson 1985). Grass shrimp typically inhabit shallow coastal and estuarine environments, but they have been collected from depths exceeding 14 m (Williams 1965).
Field surveys in Florida showed that P. pugio is most abundant in habitats characterized by aquatic macrophyes, relatively high turbidities, and low salinities (Livingston et al 1976). Weaver and Holloway (1974) report that densely vegetated habitats supported an abundance of grass shrimp far higher than adjacent habitats where macrophyte cover was lese dense.
Grass shrimp are tolerant of relatively high turbidities, such as in tidal creeks and marshes, and this may afford the animals a degree of protection from predators in areas where submerged macrophyte cover is lacking (Livingston et al. 1976, Anderson 1985).
Siroka (1977) observed that grass shrimp in tidal creeks migrate seaward or drift with the current during ebb tides and migrate upstream into tidal creeks during incoming tides.
Activity Time: Movement and distribution in Palaemonetes pugio may be influenced by photoperiod, although the effect of tidal cycles is possibly of greater importance. Shenker and Dean (1978) report that some P. pugio are buried in the sediments during daylight, but active individuals are also encountered by day.
Economic Importance: While the value of Palaemonetes pugio and other caridean shrimp as food or bait species is minimal, their importance in terms of the higher trophic levels which they support is difficult to overstate. Grass shrimp are an important trophic link, transferring energy and nutrients through estuarine food webs among several trophic levels including primary producers, grazers, decomposers, carnivores, and detritivores (Welsh 1975, Morgan 1980, Anderson 1985).
P. pugio has been utilized as a bioassay organism in studies measuring the toxicity of petroleum hydrocarbons, cadmium, antifouling biocides, and a variety of pesticides including DDT, parathion, kepone, heptachlor, toxaphene, and others (Anderson 1985). They are also of potential value as an indicator of sediment quality in coastal areas impacted by pollution (Lewis and Foss 2000).
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