Species Description: 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 enlarged chelipeds.
Potentially Misidentified Species: Tanaid taxonomy and positive identification of specimens to species level is generally beyond the scope of amateur naturalists.
Regional Occurrence: Hargeria rapax occurs from the Bay of Fundy south to both Florida coasts, and along the Gulf of Mexico into Texas (GBIF undated).
IRL Distribution: Hargeria rapax is a common inhabitant of the Indian River Lagoon (Stoner 1983).
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.
Abundance: 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.
Reproduction: 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 observed for other members of the family Paratanaidae (Biickle-Ramirez 1965, Highsmith 1983, Masunari 1983).
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 March.
Embryology: 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 1989).
Temperature: 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.
Salinity: 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.
Trophic Mode: 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.
Competitors: 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.
Predators: 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 invertebrates.
Associated Species: 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).
Habitats: 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 environmental perturbation.
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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.
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