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).
HABITAT AND DISTRIBUTION
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
The reported Florida distribution of Caprella penantis includes the entire IRL system.
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.
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.
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
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 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).
Broad distribution along the east coast of the US from Florida to north of
Cape Cod indicates a broad thermal tolerance for this species.
Widespread distribution in coastal ocean habitats as well as inshore
estuarine habitats demonstrates that Caprella penantis can persist
within a relatively broad salinity range.
Sagasti et al. (2000) indicate that Caprella penantis persists in the face
of frequent summer hypoxic stress (O2 concentrations as low as 0.5 ppm) in
the shallow epifaunal community of the York River, VA. Vassilenko (1991)
calculated weight-based oxygen consumption rates for several Japan Sea
caprellids (including C. penantis) and demonstrated that the
metabolic rate of caprellids is 1.5 times lower than that of gamarideans.
Feeding is accomplished both by scraping and by filter feeding (Caine
1974). Gnathopods are used to scrape encrusting diatoms, epiphytes, and
detritus from the surfaces of the host substratum, while entrapment of
suspended particles occurs via the setose antennae (Caine 1978). Luczkovich
et al. (2002) confirmed epiphyte-grazing as the primary feeding strategy.
Duffy (1990) reports that C. penantis significantly reduced the
growth of epiphytes (primarily diatoms and the filamentous brown alga
Ectocarpus siliculosus) on the brown macroalga Sargassum
filipendula in experimental mesocosms, but left the macroalgal
Caine (1979, 1983) notes a particular instance in which the efficiency of
C. penantis particle capture by direct interception was modified by
the use of an adherent substance on the filtering structures. The adherent
substance was mucus from the associated sea whip Leptogorgia
virgulata. Its presence on the filtering antennae of C.
penantis resulted in improved filtering efficiency and retention of
materials finer than the setal spacing. Laboratory studies by Morgan
(1989) indicate that C. penantis is also at least capable of
facultatively feeding on planktonic crab zoeae.
Intraspecific competition between male Caprella penantis for mates
has been described. Males aggressively defend grasped females from
competing males and often may employ a poison tooth on the second
gnathopods to inflict mortal injury on competitors (Bynum 1978, Caine
The dual ability to scrape food from host substrata and to filter food from
the water column likely minimizes the need to compete for dietary
Phytal amphipods are an important component in the diet of several fish
species associated with macrophytes. Orth and Heck (1980) observed C.
penantis remains in the guts of juvenile (140-165 mm) black sea bass,
Centropristis striata. Teixeira and Musick (1995) record that C.
penantis was among the top three amphipod species consumed by northern
pipefish, Syngnathus fuscus, in Zostera beds of the lower
York River, VA. Caine (1983) lists a number of fish species, including
pinfish, blennies, gobies, and pipefish, observed feeding on the C.
penantis-dominated Leptogorgia virgulata epibiont community in
the Thallassia seagrass beds of northwest Florida. Analysis of
pinfish gut contents revealed that C. penantis was the only
Leptogorgia epibiont consumed.
Predatory invertebrates such as nemertean worms are also capable of preying
on C. penantis (McDermott 1976). Thiel and Kruse (2001) also list C.
penantis as among the amphipod prey of the nemertean Tetrastemma
Caprella penantis is strongly associated with a variety of
macrophyte substrata including seagrasses and macroalgae. The conspicuous
presence of C. penentis within the Florida Gulf Leptogorgia
virgulata epibiont community has been noted previously. Frick et al
(2004) list C. penantis as one of 12 new records of species
encountered as epibionts on loggerhead sea turtles (Caretta
Caprella penantis is described by Boesch et al (1976) as an
opportunistic species with demonstrated capacity to quickly colonize
habitats after a disturbance in response to relaxed biotic pressure. It is
a common inhabitant of seagrass habitats, and in particular has been
observed as the primary epibiont of the sea whip Leptogorgia
virgulata within northwest Florida Thalassia testudinum beds
This amphipod can be found in association with several species of
macroalgae (e.g., Sanchez-Moyano and Garcia-Gomez 1998) in exposed and
protected sites, and has also been recorded as a minor component of the
sand-beach amphipod assemblage on the east coast of Florida (Charvat et al
1990). The stout, short segmented body and periopods of C. penantis
are well-suited for clinging to macroalgae in exposed environments
(Hirayama and Kikuchi 1980).
The species has no direct economic value, but it is an ecologically
important community component, both as a prey resource to other species and
as a potential mediator of the health of phytal substrata. Duffy (1990)
states that the species composition of a phytal amphipod community
determines whether the amphipods increase or decrease the fitness of the
phytal habitat components. Whereas the amphipod Ampithoe marcuzii
consumed host Sargassum Caprella penantis significantly
reduced algel epiphytes without negatively affecting the Sargassum
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