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.
II. HABITAT AND DISTRIBUTION
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).
Gammarus mucronatus is commonly encountered throughout the IRL.
III. LIFE HISTORY AND POPULATION BIOLOGY
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.
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.
As with all aphipods, 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
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
The eggs are contained by the female within a thoracic brood pouch and
yound 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).
IV. PHYSICAL TOLERANCES
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).
Gammarus mucronatus is a euryhaline species whose salinity tolerance
ranges from 4 to 35 ppt. (Bousfield, 1973).
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.
V. COMMUNITY ECOLOGY
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.
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
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. FL.
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 moist 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 the
chemically-defended seaweed which is avoided by fish.
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).
Active Gammarus mucronatus may be encountered during daylight and evening hours.
VI. SPECIAL STATUS
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.
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J. Masterson, Smithsonian Marine Station
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