Palaemonetes paludosus
riverine grass shrimp

Palaemonetes paludosus, commonly known as glass shrimp, eastern grass shrimp, or ghost shrimp, is naturally found in freshwater ponds, lakes, and streams in the coastal plain of North American east of the Allegheny Mountains, from Florida to New Jersey. The California Department of Fish and Game introduced Palaemonetes paludosus in the lower Colorado River, it is now established in various locations in California. It is also found in Louisiana, Texas, and Oklahoma, where it has most likely been introduced. (Beck and Cowell, 1976; Lowe and Provenzano, 1990)

Eastern grass shrimp are primarily found in freshwater habitats. They have been found in brackish water, but there is no evidence that they persist there. They reside in some sort of aquatic cover and are most abundant in dense beds of submerged vegetation. Eastern grass shrimp are not common in areas of little or no vegetation. In summer months these shrimp are abundant among semi-aquatic grasses and water hyacinths. Preferred water temperatures range from 10°C to 35°C. (Beck and Cowell, 1976)

Eastern grass shrimp are transparent. Individuals grow to be approximately 47 mm long. Adults usually do not exceed 50 mm long. Males and females are dimorphic. Males are distinguished from females by differences in the first and second pleopods. Males can be separated from females by the presence of the appendix masculina attached to the appendix interna on the endopod of the second pair of pleopods. Also, the endopod of the first pleopod is larger in males than in females.

Several features distinguish caridean shrimp in the genus Palaemonetes from penaeidean shrimp. The pleura of the second abdominal somite overlap those of the first and the third walking legs and do not have claws in caridean shrimp. Other morphological features include the following: a well developed rostrum bearing both dorsal and ventral teeth, a smooth carapace and abdomen, rounded abdominal pleura, and well developed eyes with globular pigmented corneas. Eastern grass shrimp have six to eight teeth on the dorsal surface of the rostrum, three to four on the ventral surface, and none on the tip.

Seasonal variation in abundance has been reported. Palaemonetes paludosus abundance increases from May to August and then decreases throughout the remainder of the year. Maximum density of these shrimp is usually in August, minimum density is in April. Increased abundance in August can be explained by the presence of large numbers of juveniles in the population. Biomass is largest in the months of November, March, and August, respectively. Biomass is lowest in May. Growth of immature stages most abundant in August results in the large biomass peak in November, when populations contain many adults. The peak in March is a result of the abundant large, ovigerous females in the population. (Beck and Cowell, 1976)

Larval eastern grass shrimp hatch from eggs after an incubation period of approximately 12 to 14 days at 26 to 28°C. Development from hatching to maturity takes two to three months when water temperatures exceed 26° C, cooler temperatures delay maturation. Eastern grass shrimp larval development is short, consisting of three stages. They first undergo metamorphosis 5 to 10 days after hatching at 15 to 31°C. In the first larval stage (the first zoea), eastern grass shrimp range from 3.7 to 3.9 mm in size. The rostrum is straight with dorsal humps near its base and usually lacks spines. The abdomen has six segments, the last two being fused with the telson. The larvae are yellowish in color and the yolk is a dark green mass under the carapace posterior to the eyes. Second zoeae range from 3.8 to 4.1 mm in size. The main difference between first and second zoea is that the eyes are separated from the carapace, and a small pair of median telson spines are present. The rostrum has a single spine, and the carapace has a pair of supra-orbital spines and a pair of antero-ventral spines. Third zoeae range from 3.8 to 4.4 mm. The differentiating characteristics are the presence of uropods and a second dorsal spine on the rostrum. In the post larva stages, there is a loss of function on the thoracic appendages, elongation of the antennal flagellum, and the presence of setae on the pleopods. The rostrum has three dorsal spines. The pleopods are biaramus and aid with swimming. (Dobkin, 1963)

Eastern grass shrimp reproduce sexually. In pre-spawning females, their ripening ovaries take on a greenish color. Females become receptive to males after molting, during which time their exoskeleton is soft. Copulation occurs within seven hours of molting. The male only recognizes the female as a potential mate if physical contact is made with her exoskeleton. During copulation, the ventral surfaces of the partners are positioned so that their genitals are close together. A spermatophore is extruded by the male and transferred to the female, where it remains until oviposition. Oviposition occurs within seven hours after sperm transfer. Part of the spermatophore dissolves and the spermatozoa are released. Ova are fertilized externally as they are extruded, then adhere to the pleopods and setae on the ventral surface of the female's abdomen. (Beck and Cowell, 1976)

The breeding season for Palaemonetes paludosus varies with location and temperature of the water. Breeding usually occurs between early-February and mid-October at water temperatures of 18 to 33°C. However, glass shrimp breed year-round in Florida, where water temperatures are warmer. In pre-spawning females the ripening ovaries take on a greenish color. Many females carrying eggs often have ripe ovaries, suggesting two broods per female are possible. Females that hatch in early spring reproduce in late summer as small adults (20 to 24 mm long) and again in late winter as large adults (25 to 43 mm long). Females typically produce 8 to 85 eggs during their life cycle and carry them for up to 2 months. (Beck and Cowell, 1976)

Like many crustaceans, eastern grass shrimp females carry developing eggs on their abdomen. Females carry the eggs for up to 2 months. (Beck and Cowell, 1976)

Eastern grass shrimp are confined to a one year life cycle. Post spawning mortality occurs from April to October. (Beck and Cowell, 1976; Beck and Cowell, 1976)

Eastern grass shrimp are more active at night than during the day. They remain close to the bottom of a stream or lake during the day. This decreases their predation risk from visual predators. Because this species of shrimp is relatively large, it would be to their advantage to restrict daytime movement in order to avoid being spotted by visual predators. At night, however, any movement associated with foraging would be much less likely to result in attacks by visual predators. (Turner, et al., 1975; Wessel, et al., 2001)

The home range size of eastern grass shrimp is unknown.

There is little available information on communication or perception in Palaemonetes paludosus.

The diet of eastern grass shrimp is dominated by algae (diatoms and green algae), but they also consume vascular plants, detritus, aquatic insects, and other benthic coarse particulate organic matter. Diatoms that glass shrimp eat include species in the genera Fragilaria, Nivicula, Stephanodiscus, Gomphonema, Synedra, and Cymbella. Examples of green algae consumed include species in the genera Cosmarium, Closterium, and Scenedesmus. In laboratory conditions, these shrimp were observed feeding on aquatic weeds. Insects they feed on include mayfly nymphs (Baetidae) and dipteran larvae in the families Chironomidae, Heleidae, Chaoboridae, and Culicidae. (Beck and Cowell, 1976)

Largemouth bass (Micropterus salmoides) and other fish feed on Palaemonetes paludosus. Vulnerability to predators is often inversely related to habitat complexity. With more complex plant cover, glass shrimp have more protection from predators. Eastern grass shrimp may remain close to the bottom during the day as a means of avoiding predation from visual predators, such as wading birds, waterfowl, and sunfishes and bass (Centrarchidae). Their transparent bodies may help to camouflage them. (Wessel, et al., 2001)

Eastern grass shrimp are important for energy flow and turnover of detritus in freshwater habitats and are important links between freshwater organisms found in benthic and water column habitats. Interactions among eastern grass shrimp, benthic predators, and nektonic omnivores have strong direct and indirect effects on benthic densities and community composition. Omnivorous shrimp are important organizers of community structure and play a key role in reducing sediment cover on rock substrates, which increases algal populations.

The parasitic isopod Probopyrus pandalicola uses Palaemonetes paludosus, among other shrimp, as hosts. These parasites sterilize female hosts by preventing ovarian maturation, although they do not effect external sex characteristics. Growth of external sex characters of male host are effected. The growth of chelae is accelerated, while the growth of pleopod structures is slowed. The parasite causes these changes though a combination of nutritional drain and hormonal interference. (Beck, 1980; Lowe and Provenzano, 1990; Turner, et al., 1975)

Because of their unique physical appearance, Palaemonetes paludosus is a common aquarium pet.

There are no known adverse affects of Palaemonetes paludosus on humans.

Palaemonetes paludosus is not currently listed as threatened or endangered.

The amino acid levels in Palaemonetes paludosus are more similar to levels found in marine decapods than to other freshwater invertebrates, such as crayfish like Astacus pallipes. Some suggest that how recent the freshwater invasion by a taxon can be determined by similarity in amino acid concentration of tissue to that of marine species. If this is true, eastern grass shrimp may be relatively recent invaders of freshwater habitats. (Turner, et al., 1975)