Gammarus mucronatus (Say, 1818)
Family: Gammaridae
Synonyms: Carinogammarus mucronatus
Gammarus mucronatus image
Gammarus mucronatus  

Species Description: Gammarus mucronatus is a highly motile shallow marine gammarid amphipod endemic to the coastal and estuarine waters of the US Atlantic coast. Individuals are typically light green to greenish-brown, with red to brown spots and subtle banding on antennae and appendages. The rounded head has a pair of large, reniform (kidney-shaped) eyes, typical of the genus. Three posterior body segments (pleosomes 1-3) usually possess sharp, posteriorly-directed mucronations (points) that give the species its name. The first pair of antennae are greater or equal to the second in length (Bousfield 1973). As with all gammaridean amphipods, the body is laterally compressed.

The species is sexually dimorphic, with adult males being longer-bodied than females, and with the dactyl of the gnathopod (the anterior-most walking appendage) modified into a hook used to grasp females during precopulation and copulation.

Bousfield (1973) provides a more detailed species description suitable for professional taxonomic purposes.

Potentially Misidentified Species: A great number of epifaunal and/or phytal gammaridean amphipods co-occur with Gammarus mucronatus in throughout its range. In the Indian River Lagoon, the large reniform eye and the mucronate pleopods should be sufficient to allow amateur naturalists to distinguish G. mucronatus from other amphipod species.

Regional Occurrence: Gammarus mucronatus occurs in coastal and estuarine environments along the US Atlantic coast from the Gulf of St. Lawrence south to Florida and the Gulf of Mexico (Bousfield 1969, 1973).

IRL Distribution: Gammarus mucronatus is commonly encountered throughout the IRL.

Age, Size, Lifespan: The field study by Fredette and Diaz (1986a) indicates that mean adult length in the study population ranged from around 1.1 mm to 3.7 mm. Individuals were capable of rapid growth, exhibiting average winter growth rates of 0.04 mm/day and spring growth rates of 0.11 mm/day.

Bousfield (1973) indicates an annual life cycle for this species.

Abundance: Fredette and Diaz (1986a) recorded Gammarus mucronatus population densities in Virginia Zostera marina beds ranging from less than 50 individuals/m2 in the fall to nearly 1,200 individuals/m2 in the summer. These authors also report an early summer peak of 6,800 G. mucronatus individuals/m2 in a macroalgal-based (Ulva and Enteromorpha) community.

Reproduction: As with all amphipods, reproduction is sexual, the sexes are separate, and fertilization is internal.

Reproductive seasonality may vary with geography. Bousfield (1973) indicated ovigerous females are present from April-September in New England populations, but the reproductive season is longer in more southern populations. Fredette and Diaz (1986a) conservatively estimate that approximately 6 cohorts per year can develop before excessively high summer temperatures curtail reproduction.

Data from the York River subestuary of Chesapeake Bay suggest the sex ratio in Gammarus mucronatus is generally close to 1:1 (Fredette and Diaz 1986a). The study also indicated that mature nonovigerous females were rare, with over 90% of all mature females carrying broods. The authors noted that minimum ovigerous size (ca. 1.1 mm) can be attained in as little as 10 days and mean ovigerous size (ca. 2.3 mm) can be attained in as little as 30 days at 23°C. Maturation in warm months and in warmer southern estuaries requires only a couple of weeks.

Egg production is directly correlated to female body size and has been reported to range from a few eggs to more than 200 per brood. Egg size in the Virginia populations examined by Fredette and Diaz (1986a) ranged from 0.308 to 0.532 mm and eggs showed a seasonal decrease in size from winter to summer. The smaller egg sizes develop more quickly (Clarke, 1982).

Embryology: The eggs are contained by the female within a thoracic brood pouch and young hatch out as fully developed juveniles with no free-living larval stage. Mean brood development time has been reported to be just over 30 days at 6°C and 12 days at 10°C, compared to as little as 8.3 days and 4.3 days at temperatures of 17°C and 21°C, respectively (Steele and Steele 1975, Borowsky 1980, Fredette and Diaz 1986a).

Temperature: Gammarus mucronatus is a eurythermal species occurring as far north as the Gulf of St. Lawrence. Field studies in the Virginia Chesapeake Bay region revealed that G. mucronatus populations persisted through an annual temperature cycle that ranged from -1°C to 33°C (Fredette and Diaz 1986a).

Salinity: Gammarus mucronatus is a euryhaline species whose salinity tolerance ranges from 4 to 35 ppt. (Bousfield, 1973).

Dissolved Oxygen: As with most estuarine epifauna, Gammarus mucronatus are capable of persisting in portions of the estuary that experience periodic hypoxic episodes (Sagasti et al. 2000). This highly motile species is also capable of moving to escape conditions of extreme hypoxia.

Trophic Mode: Gammarus mucronatus is a generalist grazer whose diet includes microalgae and detritus, a limited amount of macroalgae, and possibly a small amount of macrofauna (Zimmerman et al 1979).

Laboratory grazing studies by Hauxwell et al. (1998) using the chlorophyte alga Cladophora vagabunda as a food source revealed a G. mucronatus grazing rate ranging between 0.3 mg (low-nitrogen estuary conditions) and 0.8 mg (high-nitrogen estuary conditions) dry wt./individual/day. Howard (1982) indicates that seagrass-dwelling G. mucronatus are important both as grazers of seagrass epiphytes and as detritivores capable of mechanically reducing detrital particle size through the shredding action of its feeding appendages.

Competitors: Experimental studies by Duffy et al. (2001) examining the functional redundancy of several seagrass-associated crustaceans species indicate a high degree of dietary niche overlap between Gammarus mucronatus and co-occurring species, indicating that a degree of trophic competition may exist.

Predators: Gammarus mucronatus is a principle prey item for juvenile and adult fish of several species, and also for large decapods (Young et al. 1976). Fredette and Diaz (1986a) suggested that the spring and summer population decline of G. mucronatus in seagrass habitats is caused by the arrival of migratory predators in the spring and summer. Nelson (1979, 1980) cited fish predation, particularly juvenile pinfish (Lagodon rhomboids), as a key seasonal factor limiting Indian River Lagoon amphipod populations. Stoner (1980), however, noted that some seagrass-associated amphipod populations reach their peak densities during periods of high predator abundance.

Ryer and Orth (1987) report that small size classes of G. mucronatus are a seasonally dominant prey item (spring, summer, fall) of the northern pipefish (Syngnathus fuscus) in the Lower Chesapeake Bay as well. Llanso et al. (1998) also indicate that G. mucronatus was among the preferred prey items of small red drum (Sciaenops ocellatus) in a restored mangrove impoundment in Tampa Bay, Florida.

Duffy and Hay (1994) describe a higher order trophic interaction involving G. mucronatus, the chemically defended brown seaweed Dictyota menstrualis, and predatory fish. The authors note that G. mucronatus is unable to palate D. menstrualis and must expose itself to seasonally abundant fish predators to find suitable food, leading to near-extinction of local populations when predators are most abundant. In contrast, the amphipod Ampithoe longimana preferentially inhabits and consumes D. menstrualis. Experiments confirmed that A. longimana reduces its vulnerability to predation when occupying chemically defended seaweeds, which are avoided by fish.

Habitats: Gammarus mucronatus occurs in a variety of estuarine and coastal habitats including seagrass beds, macroalgal mats, salt marsh, mud and sand flats, sponges, oyster reefs, and open beaches (Watling and Maurer 1972, Bausfield 1973, van Maren 1978, Nelson 1980).

The strong phytal nature of G. mucronatus has been verified experimentally through observation of a greater that three-fold difference in the number of animals associated with algal substrate versus bare substrate in the laboratory (Masterson 1997).

Activity Time: Active Gammarus mucronatus may be encountered during daylight and evening hours.

Economic/Ecological Importance: The species has no direct economic importance, but is a valuable system component from an ecological standpoint. In terms of secondary productivity, populations of seagrass-associated amphipods such as Gammarus mucronatus are extremely important ecosystem components. Fredette and Diaz (1986b) reported G. mucronatus secondary production values of 5-10 g dry wt./m2/year in York River Zostera marina beds, and values of more than 27 g dry wt./m2/year have been reported (Waters and Hokenstrom 1980). As noted, G. mucronatus is also an important prey item to many ecologically and commercially important species.

Experiments conducted by Duffy and Hay (2000) led the authors to conclude that grazing amphipods in general may play important roles in the organization of benthic communities.

Borowsky B. 1980. Reproductive patterns of three intertidal salt-marsh gammaridean amphipods. Marine Biology 55:327-334.

Bousfield EL. 1969. New records of Gammarus (Crustacea: Amphipoda) from the Middle Atlantic Region. Chesapeake Science 10:1-17.

Bousfield EL. 1973. Shallow-water gammaridean Amphipoda of New England. Corell University Press, Ithaca, New York. 312p.

Clarke A. 1982. Temperature and embryonic development in polar marine invertebrates. International Journal of Invertebrate Reproduction 5:71-82.

Duffy JE and ME Hay. 1994. Herbivore resistance to seaweed chemical defense: The roles of mobility and predation risk. Ecology 75:1304-1319.

Duffy JE and ME Hay. 2000. Strong impacts of grazing amphipods on the organization of a benthic community. Ecological Monographs 70:237-263.

Duffy EJ, MacDonald KS, Rhode JM, and J. Parker. 2001. Grazer diversity, functional redundancy, and productivity in seagrass beds: An experimental test. Ecology 82:2417-2434.

Fredette TJ and RJ Diaz. 1986. Life history of Gammarus mucronatus Say (Amphipoda: Gammaridae) in warm eemperate estuarine habitats, York River, Virginia. Journal of Crustacean Biology 6:57-78.

Fredette TJ and RJ Diaz. 1986. Secondary production of Gammarus mucronatus Say (Amphipoda: Gammaridae) in warm temperate estuarine habitats, York River, Virginia. Journal of Crustacean Biology, Vol. 6, No. 4, (Nov., 1986), pp. 729-741.

Hauxwell J, McClelland J, Behr PJ, and I Valiela. 1998. Relative importance of grazing and nutrient controls of macroalgal biomass in three temperate shallow estuaries. Estuaries 21:347-360.

Llanso RJ, Bell SS, and FE Vose. 1998. Food habits of red drum and spotted seatrout in a restored mangrove impoundment. Estuaries 21:294-306.

Masterson JW. 1997. Investigation of the effects of macrophyte structure, food resources and health on habitat selection and refuge value in vegetated aquatic habitats. Unpublished Ph.D. Dissertation. 145 p.

Nelson WG. 1979. Experimental studies of selective predation on amphipods: Consequences for amphipod distribution and abundance. Journal of Experimental Marine Biology and Ecology 38:225-245.

Nelson WG. 1980. The biology of eelgrass (Zostera marina L.) amphipods. Crustaceana 39:59-89.

Ryer CH and RJ Orth. 1987. Feeding Ecology of the Northern Pipefish, Syngnathus fuscus, in a Seagrass Community of the Lower Chesapeake Bay. Estuaries 10:330-336.

Sagasti A, Schaffner LC, and JE Duffy. 2000. Epifaunal communities thrive in an estuary with hypoxic episodes. Estuaries 23, No. 4:474-487.

Steele DH and VJ Steele. 1975. The biology of Gammarus (Crustacea, Amphipoda) in the northwestern Atlantic. XI. Comparison and discussion. Canadian Journal of Zoology 53:1116-1126.

Stoner AW 1980. The role of seagrass biomass in the organization of benthic macrofaunal assemblages. Bulletin of Marine Science 30:537-551.

van Maren MJ. 1978. Distribution and ecology of Gammarus tigrinus Sexton, 1939 and some other amphipod Crustacea near Beaufort (North Carolina, USA). Bijdragen tot de Dierkunde 48:46-56.

Waters TF and JC Hokenstrom. 1980. Annual production and drift of the stream amphipod Gammarus pseudolimnaeus in Valley Creek, Minnesota. Limnology and Oceanography 25:700-710.

Watling L, and D Maurer. 1972. Marine shallow water amphipods of the Delaware Bay area, USA. Crustaceana, Suppl. 3:251-266.

Young DK, Buzas MA, and MW Young. 1976. Species densities of macrobenthos associated with seagrass: A field experimental study of predation. Journal of Marine Research 34:577-592.

Zimmerman R, Gibson R, and J Harrington. 1979. Herbivory detritivory among gammaridean amphipods from a Florida seagrass community. Marine Biology 54:41-47.

Gammarus mucronatus image
Gammarus mucronatus