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Species Name: 
Caprella penantis Leach, 1814

Common Name:
None.

Species Description:
Caprella penantis is a common, cosmopolitan caprellid amphipod. Members of the genus Caprella, commonly referred to as skeleton shrimps, typically have a slender, cylindrical body and three rear pairs of morphologically specialized periopods (walking legs) that allow them to cling to various substrata such as algae, hydroids, and seagrasses (Aoki 1997). The animal possesses large hooked gnathopods and setose antennae, and their abdominal segments are greatly reduced.

Caprella penantis is relatively robust-bodied and has a small, sharp rostrum on the front of the head that is useful for distinguishing it from other caprellids (Rupert and Fox 1988), although identification to species is generally difficult. Bynum (1900) and Caine (1989) report morphological variation between C. penantis collected from exposed versus protected habitats, with animals from exposed sites being relatively more robust than those from protected sites.

McCain (1968) describes morphological adaptations suiting the species to gorgonian host substrata, namely, a reduction in periopod setae and a reduction in the size and number of grasping spines. Typically drab in coloration, C. penantis may gain color camouflage by incorporating the pigments of the biotic substrata on which it feeds (Caine 1998).

Potentially Misidentified Species:
A number of other caprellid amphipods belonging to genus Caprella and Paracaprella co-occur in the IRL with C. penantis. Differentiation of these animals is generally beyond the scope of amateur naturalists and typically involves morphometric analysis of a number of body parts. Caine (1989) has demonstrated a high degree of environment-mediated morphological plasticity (e.g., body and appendage robustness), and questions the reliability of using body and appendage proportions for taxonomic purposes.

Caprella equilibra, the most common congener in the southeastern US, is typically larger than C. penantis (around 2 cm, versus 1.3 cm) and has no rostrum. Paracaprella tenuis, another common caprellid, typically reaches only 1 cm and lacks a rostrum as well as regularly arrayed setae on the antennae (Rupert and Fox 1988).

II.  HABITAT AND DISTRIBUTION 

Regional Occurrence:
Caprella penantis is a cosmopolitan species, widespread in tropical, subtropical, and temperate waters of the world's oceans (Vassilenko 1991). Along the east coast of the US, Caprella penantis occurs from Florida (both coasts) northward to north of Cape Cod, MA (Fox and Bynum 1975).

IRL Distribution:
The reported Florida distribution of Caprella penantis includes the entire IRL system.

III. LIFE HISTORY AND POPULATION BIOLOGY

Age, Size, Lifespan:
Caprella penantis is fairly large, around 1.3 cm in length.

Bynum (1978) suggests that female C. penantis from North Carolina live for approximately one year.

Abundance:
Caprella penantis can be encountered in abundance. Bartholomew (2002) lists it as the fourth most abundant mobile invertebrate colonizing artificial seagrass plots in experiments conducted in York River, VA, Zostera beds. More than 5,000 individuals were collected from the plots during two one-week sampling periods.

Marsh (1973) reported C. penantis as the 12th most abundant epifaunal species in the York River. In the Zostera beds he examined, C. penantis occurred in 37 of 48 collected samples and represented more than 2% of the total fauna collected.

Fox and Bynum (1975) list C. penantis as common and widespread in NC estuaries, and very abundant on pilings and exposed open beaches. Marsh (1973) noted that Caprella penantis was common in the York River, VA, Zostera beds only during the colder months of the season. Williams and Bynum (1972) indicate that C. penantis was present in North Carolina samples from all months except October, and was most abundant in May-June.

Reproduction:
Bynum (1978) studied aspects of the reproduction of Caprella penantis in North Carolina. Males grasp females with the posterior periopods, in contrast to male gammaridean amphipods which employ the first gnathopods for this purpose. Males defend grasped females from competing males.

Breeding occurred year-round with a peak in the spring and a lesser peak in the late summer and early fall. Females probably live for one year and produce several broods during their life (Bynum 1978).

Vassilenko (1991) reports that female C. penantis in his studies from Japan had broods of approximately 16-21 eggs and that they ranged between 0.25-0.30 mm in diameter. Egg-bearing females ranged between approximately 15-30 mg in weight.

Embryology:
Embryology is typical of peracarid crustaceans, with fertilized eggs carried in the brood pouch of the female and direct-developing young emerging as crawl-away juveniles with no free-living planktonic larval stage (Aoki 1997).

Although prolonged co-habitation of mother and young emerged from the brood pouch has been observed in some caprellid amphipods (Lim and Alexander 1986, Aoki and Kikuchi 1991), Aoki (1997) reports that C. penantis shows no such interaction between mother and emerged young in experimental trials. Instead, newly emerged juveniles immediately dispersed onto the experimental substratum (algal thalli or hydroid branches).