Amphipod (Ampelisca abdita)

Description

Ampelisca abdita, a gammarid, is a small shrimp-like amphipod with multiple pairs of immobile eyes. Its body is laterally compressed, lacks a carapace, and is typically white with red or purple spots (Weiss 1995). A. abdita construct tubes of fine sand grains approximately 3.5 centimeters in length and 2 to 3 millimeters wide. Most of the tube is below the substrate with approximately 1 centimeter above the surface.

Background

Distribution. A. abdita are found in the shallow marine waters from Maine to Louisiana (Mills, 1967) and along the entire U.S. Pacific coast (Grosse et al. 1986). A. abdita are typically observed in the fine sands of estuaries (Caracciolo and Steimle 1983, Franz and Harris 1988), and are generally limited to approximately 27 meters in depth (Mills 1967).

Feeding. These amphipods feed with their ventral surfaces facing upwards from the tops of their tubes. This positioning allows them to detect movement around the tube, which initiates feeding activity. Suspended particles (including algae, sediment grains, and organic detritus; Redmond et al. 1994) are accumulated at the midline portion of the body and are then brought to the mouth. A. abdita can also set up a current by using its pleopods and second antennae to direct sand grains and detrital material to its midline (Mills 1967).

Fishery. Though there is no commercial fishery for amphipods, A. abdita and other gammarids are an invaluable food source for many economically important fish and invertebrate species. Jamaica Bay winter flounder populations have been shown to depend heavily on A. abdita as a primary food source (Franz 1990). That study highlighted the importance of this amphipod for commercially valuable flatfish species. They are also important environmental quality indicators, since they have limited mobility and are susceptible to pollution. Additionally, amphipods are efficient benthic scavengers and are thought to process more organic debris than any other benthic organisms (Grosse et al. 1986).

Life Cycle

Ampelisca life span is approximately 10 months, with two distinct generations (winter and summer) dominant in a population. The winter generation begins its breeding season from mid-April to May in response to an increase in water temperature to 8 degrees Celsius. Males mature prior to the breeding season and are numerous in the population; then most die shortly after breeding, leaving oviferous females dominant in the winter generation by late spring. Although the specific mating process is not known for A. abdita, it is assumed to be similar to other amphipods (Mills 1967). Mature males encounter females at random and attach via copulatory hooks. If a female is ready to mate, she will maintain contact with the male in a position known as amplexus, or mating embrace. The male carries the female during this precopulatory period that will last for hours or days. During this time the amphipods abandon their tubes and swim freely. Following precopula, the female molts and begins to lay one to five eggs in a pouch located on her ventral thoracic region (Reish & Barnard 1979). The male then positions his body in order to make contact with the brood chamber and fertilize the eggs. After fertilization, the pair may remain together for some time, but the male may mate with multiple females. At the end of the breeding season, males leave the population to die and females return to the substrate with young in the brood chamber. Approximately two weeks later, adult-like juveniles (1.5 millimeters in length) leave the brood pouch and the winter generation females die. Growth of the summer generation juveniles is rapid, including 12 molts every 20 to 30 days, and by mid-summer they approach sexual maturity. Peak breeding of the summer generation is in July, followed by a decrease in numbers until October. The summer generation’s young become the next winter generation, maturing slowly over the winter season. Sizes of winter generation amphipods are typically 2 to 7 millimeters in length (Mills 1967, Grosse et al. 1986), and females are significantly larger than the males and mature more rapidly.

Environmental Influences

Salinity. A. abdita has been found in salinities ranging 10 to 35 practical salinity units (psu; Redmond et al. 1994). Adults are more tolerant to salinity changes in this range than juveniles and embryos (Reish and Barnard 1979, Grosse et al. 1986), and typically respond by burrowing into the sediment. Without this ability to respond behaviorally to salinity stress, their tolerance range decreases (Reish and Barnard 1979).

Predation. A. abdita is an important food source for many juvenile and adult bottom-feeding fish (Franz 1990, Redmond et al. 1994) such as winter flounder (Pleuronectes americanus) and scup (Stenotomus chrysops).

Pollution. Amphipods are generally absent from areas of high pollution, leading to their recognition as an environmental indicator. Specifically, amphipods are sensitive to oil pollution and related compounds (Grosse et al. 1986), and experiments have shown that amphipods exposed to petrochemicals respond with avoidance behavior (Reish and Barnard 1979). Additionally, Reish and Barnard (1979) found that slight increases in organic matter will boost amphipod abundance, but beyond a threshold level, their numbers decrease. This trend is not surprising since these amphipods feed on detritus. In addition to pollution, dredging can also negatively impact amphipod populations since they live at or near the sediment-water interface. Recolonization of a borrow area or spoil site is dependent mostly on larval dispersion and recruitment, and may take some time (Reish and Barnard 1979).

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