Hargeria rapax is a common tanaidacean crustacean of the western
Atlantic and Gulf of Mexico (Modlin and Harris 1989). The species has an
elongate, cylindrical body. The first pair of thoracic appendages are
modified into maxillipeds and the second pair are chelipeds (pincers). As
in all tanaids, the first two thoracic segments are fused with the head and
are covered by a short carapace (Rupert and Fox 1988). The animal is pale
whitish and partly translucent.
The sexes are dimorphic, with males being more robust and having greatly
Potentially Misidentified Species
Tanaid taxonomy and positive identification of specimens to species level
is generally beyond the scope of amateur naturalists.
HABITAT AND DISTRIBUTION
Hargeria rapax occurs from the Bay of Fundy south to both Florida
coasts, and along the Gulf of Mexico into Texas (GBIF undated).
Hargeria rapax is a common inhabitant of the Indian River Lagoon (Stoner 1983).
LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan
Modlin and Harris (1989) note that reproductive individuals in their
Dauphin Island field study ranged between 2 and 4 mm in length. Stoner
(1983) relates that the average length of individuals from Indian River
Lagoon seagrass beds was just under 2 mm.
Hargeria rapax can be a highly abundant member of infaunal
communities in which it occurs. An Indian River Lagoon field study by
Stoner (1983) revealed it to be the most abundant infaunal animal taken in
cores of Halodule seagrass beds as well as bare sand habitats,
comprising 54-61% of the peracaridan fauna in Halodule.
A study of an Apalachicola Bay, FL, Halodule wrightii infaunal
community by Sheridan and Livingston (1983) revealed H. rapax to be
the numerical dominant species during one year of monthly sampling. The
authors report an average monthly density of more than 6,300 individuals
per square meter (range: 394-18,303 individuals per square meter).
A study of Dauphin Island, AL, tidepool populations of H. rapax by
Modlin and Harris (1989) indicates that population size may change
dramatically over the course of a season. The authors recorded peak summer
densities in excess of 10 individuals per square centimeter, followed by a
population crash by August resulting in densities of less than 1 animal per
square centimeter for the remainder of the year. Gaston et al. (1988)
similarly note that abundant H. rapax in Calcasieu Lake, LA, occur
less consistently than many other numerical dominants, but occasionally
increase dramatically in population size.
Female Hargeria rapax brood eggs and young within a marsupium
located between the pereiopods (Kneib 1984).
Laboratory- and field-based based observation by Modlin and Harris (1989)
reveals that Hargeria rapax utilizes a primarily protogynous
reproductive strategy in which most mature individuals function initially
as reproductive females and then as reproductive males. Size-frequency
analysis suggest small numbers of gonochoristic (primary) males were born
into the populations under study, but the majority of males encountered
appeared to be larger, secondary males. The authors demonstrated that
under laboratory culture conditions and in the absence of males, a small
percentage of primary females would molt into secondary males. A similar
reproductive strategy has been (elucidated ?observed) for other members of
the family Paratanaidae (Biickle-Ramirez 1965, Highsmith 1983, Masunari
In the study by Modlin and Harris (1989), males were few in number and
sporadic in occurrence, and male to female ratios averaged 1:8. In
Apalachicola Bay, Sheridan and Livingston (1983) observed gravid females in
all months except September, with peak abundance occurring in February and
Hargeria rapax exhibits prenatal parental care in the form of
females that brood eggs and early larval (manca I) stages in marsupial
brood pouches. Young emerge as manca II stage instars, progress through
larger neutrum stages, and finally molt into primary females or
(infrequently) males. A significant positive relationship exists between
female size and the number of eggs and larvae brooded (Modlin and Harris
The widespread distribution of Hargeria rapax (Bay of Fundy to
Texas) is indicative of a broad thermal tolerance for the species as a
whole. Within specific populations, eurythermal tolerance also appears to
be the rule. Modlin and Harris (1989) studied a Dauphin Island, AL,
population that persisted at temperatures ranging from 9°C in winter
to 34°C in late summer and early autumn.
Heard (1981) suggests that Hargeria rapax is euryhaline in nature,
although Modlin and Harris (1989) contend the species may prefer mesohaline
habitats and this preference may explain its near-disappearance as summer
salinities rose from 5.5 ppt to 29 ppt in study tidepools. These authors
also indicate that reproductive success appeared to coincide with
mesohaline salinity conditions.
A review of the literature suggests that Hargeria rapax is a
generalist detritivore apparently capable of switching between deposit- and
suspension-feeding modes. Odum and Heald (1972) found Hargeria sp.
to be detritivorous. Luczkovich et al. (2002) group H. rapax within
the suspension-feeding trophic guild, and Gaston and Nasci (1988) and
Gaston et al. (1988) also list H. rapax as a suspension feeder.
Rader (1984) reports H. rapax feeds on benthic diatoms, ostracods,
and assorted gammaridean amphipods. Heard (1981) posits detritus and
benthic diatoms as the principal dietary resources, while Highsmith (1982)
suggests H. rapax likely captures and consumes the larval stages of
a variety of benthic invertebrates.
Information is lacking with regard to competition in Hargeria rapax.
The generalist detritivore trophic tendencies of the species likely serve
to minimize dietary resource competition with other species.
Hargeria rapax is a prey item for several fish species as well as
penaeid shrimp and grass shrimp, Palaemonetes pugio (Overstreet and
Heard 1978, Henwood et al.,1978, Heard 1981, Kneib 1988, McTigue and
Zimmerman 1998). They are particularly easily grazed in exposed benthic
habitats like mud flats.
Rozas and LaSalle (1990) list H. rapax as among the most important
prey of Gulf killifish, Fundulus grandis, sampled from a Mississippi
brackish marsh. Smaller size classes of mummichods, Fundulus
heteroclitus, also make heavy use of H. rapax when available
(Kneib et al. 1980, Kneib 1986).
Kneib (1994) notes that predation may be a partial determinant of
small-scale distribution patterns in H. rapax. Most of the
population biomass of H. rapax in a Georgia salt marsh population
examined by the author was concentrated near the mean high water line where
tidal fluctuations limit the foraging time of predatory fish and
Rey and Stoner (1984) report a statistically significant, non-obligate
positive association between H. rapax and egg masses of the sea hare
Aplysia brasiliana. The authors suggest the association is fostered
by the abundance of fine-grained sediment and detrital material typically
found on the egg masses. Hargeria has also been cited in
association with snails of the family Hydrobiidae (Heard, 1979).
Hargeria rapax is a tubiculous (tube-dwelling) infaunal/meiofaunal
species found in several protected intertidal to subtidal estuarine
habitats including seagrass beds, mangrove shorelines, saltmarshes, sand
flats, in fouling communities, and shoreline tidal pools (Odum and Heald
1972, Hoese et al., 1972, Heard 1981, Sheridan and Livingston 1983, Stoner
1983, Modlin and Harris 1989).
A survey of the literature suggests Halodule wrightii (= wrightii) is the
preferred seagrass habitat of this species, but it also inhabits
Thalassia testudinum and Syringodium filiforme meadows
(Livingston et al. 1982, Sheridan and Livingston, 1983, Stoner 1983, Lewis
1984). Stoner (1983) speculates that the dense rhizome mats of
Halodule offer a better predation refuge than those of
Thalassia and Syringodium.
A study by Flint and Younk (1983) from Corpus Christi Bay, TX, placed H.
rapax into a faunal group that appeared to occur at higher densities
during periods of dredging, indicating a degree of resiliency to
No information is available at this time
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Lake, Louisiana: Community and trophic structure. Estuaries 11:192-200.
Gaston GR and JC Nasci. 1988. Trophic structure of macrobenthic communities
in the Calcasieu estuary, Lousiana. Estuaries 11:201-211.
Global Biodiversity Information facility (GBIF). Undated. Species:
Hargeria rapax (Harger, 1879). Available online.
Heard RW. 1981. Guide to common tidal marsh invertebrates of the
northeastern Gulf of Mexico. MASGP-79-004, Mississippi-Alabama Sea Grant
Consortium. 88 p.
Highsmith RC. 1982. Induced settlement and metamorphosis of sand dollar
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Hoese HD, Nelson WR, and H Beckert. 1972. Seasonal and spatial setting for
fouling organisms in Mobile Bay and eastern Mississippi Sound, Alabama.
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Kneib RT. 1986. The role of Fundulus heteroclitus in salt marsh
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Livingston RJ, Sheridan PS, McLane BG, Lewis FG, III, and GG Kobylinski.
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