Species Description: Alpheus heterochaelis, the bigclaw snapping shrimp, belongs to the snapping shrimp or pistol shrimp family (Alpheidae), whose members possess a single, large chela (claw) that has been modified such that it is capable of producing a distinct snapping or popping sound. The acoustic claw snap of Alpheus heterochaelis is important as a means of stuning/killing prey, in defense, and as part of a theat display in agonistic intraspecific encounters as well (Herberholz and Schmitz 1998)
Individuals are typically dark translucent green with orange and blue tipped uropods. The rostrum is small and the carapace edge is smooth and spineless and large snapping claw is strongly notched on both the upper and lower margins at the base of the fingers. The snapping claw can be either the left or right claw, and it attains a length nearly half that of the body. The opposite paired claw remains an unmodified pincer. The snapping claws of male A. heterochaelis are larger and broader than those of females of equal size (Nolan and Salmon 1970, Herberholz and Schmitz 1998).
A. heterochaelis is the largest and most colorful of the snapping shrimps of the southeastern United States (Rupert and Fox 1988).
Sound-production in Alpheus heterochaelis had long been believed to be the result of either rapid closure of the oversized pistol claw or of the separation of two smooth disk areas at the base of each of the fingers of the large claw. The true basis for snapping shrimp sound production was described in 2000 by Versluis et al. The authors explain that the very rapid claw closure emits a high-velocity (25m/s) jet of water whose speed exceeds cavitation conditions to create a small (expanding from miscroscopic to around 3.5mm at maximum size), extremely short-lived cavitation bubble. High-speed imaging revealed that the characteristic popping sound was actually produced by the rapid (<300µs after formation), violent collapse of the cavitation bubble.
Lohse et al. (2001) studied this phenomenon further and found that as the snapping claw cavition bubble collapses a very short, intense flash of light is emitted. The authors conclude from this observation that the collapsing bubbles experiences extremely high internal pressures and temperatures exceeding 5,000°C.
Curiously, auditory organs have not been identified in A. heterochaelis and may be absent. Mechanoreception and chemosensory reception may therefore be the most important means for individuals to analyze signals from conspecifics (Herberholz and Schmitz 1998). This subject is taken up in more detail below.
Potentially Misidentified Species: Alpheus heterochaelis is the largest and likely the most abundant of at least 11 Alpheus species documented from the Indian River Lagoon. The other known IRL snapping shrimp are: Alpheus armillatus, A. bouvieri, A. cristulifrons, A. floridanus, A. formosus, A. normanni, A. nuttingi, A. paracinitus, A. thomasi, and A. viridari.
Regional Occurrence: Alpheus heterochaelis is native to the western Atlantic Ocean, occurring from North Carolina and Bermuda to Brazil and the West Indies (Kaplan 1988).
IRL Distribution: Alpheus heterochaelis occurs in suitable habitats throughout the IRL system.
Age, Size, Lifespan: Adult Alpheus heterochaelis may reach a length of between 3.0 and 5.5 cm (Kaplan 1988), but individuals encountered are often considerably smaller, e.g., 6-20 mm carapace length (Nolan and Salmon 1970).
Abundance: Despite their tendency to be heard and not seen, field sampling studies indicate that Alpheus heterochaelis is generally among the numerical dominant decapods within its preferred habitats. Lift net surveys conducted in intertidal oyster reefs of Mosquito Lagoon by Boudreaux et al. (2006), for example, revealed A. heterochaelis to be the most abundant motile species, with more than 2,400 individuals collected over a single sampling season.
Working in Beaufort, NC, Nolan and Salmon (1970) noted a seasonality to A. heterochaelis abundance in which population densities were highest in early summer but became scarce after the end of July.
Reproduction: Like most other alpheid shrimp, Alpheus heterochaelis individuals are most often encountered as mated pairs and social monogamy appears to be a widespread phenomenon. A study by Rahman et al. (2003) suggests that mate guarding by the male appears to be the key factor leading to social monogamy in A. heterochaelis, i.e., rather than shared territoriality or biparental care of young. The authors indicate that the female molt cycle is highly cryptic and that females are only receptive for a few hours immediately after molting. Mate guarding through male-female pairing is therefore advantageous to the male because it maximizes mating opportunities and also to the female because it minimizes the need to search for a mate during the vulnerable soft-bodied receptive phase.
Nolan and Salmon (1970) indicate that A. heterochaelis collected in their study were most often (~65% of animals collected) captured as male-female pairs, and that such pairs could be found at all times during the study (April-August). Individuals in the majority of pairs collected from the field differed by 2 mm or less in carapace length, with a non-significant trend for females to be slightly larger than males.
Male-female interactions under laboratory conditions begin similar to same sex agonistic interactions (see below). In a nontrivial number of instances, however, aggressive face-off and antennulation between a male and female gave way to mutual aquiescence and pair formation, typically in under an hour. Nolan and Salmon (1970) noted that the mechanisms underlying sexual discrimination had yet to be elucidated, but predicted that chemical cues played a role. In earlier experiments conducted by Hazlett and Winn (1962), exposure to extract from crushed shrimp of the opposite sex elicited a less aggressive response than did extracts from individuals of the same sex.
Embryology: Knowlton (1973) notes in his description of early development of Alpheus heterochaelis from North Carolina that total brood gestation time from laying to hatching was approximately 28 days at 25°C.
This species is unusual in that it exhibits an abbreviated larval life cycle consisting of just three larval stages. At 22-25°C, the first instar lasts only 1-2 hours, the second instar for approximately 28 hours, and the entire larval period lasts just 4-5 days (Knowlton 1973).
All A. heterochaelis larval stages are non-feeding, and developing animals appear to rely entirely on large yolk and oil reserves for nutrition during this period. Feeding commences at the onset of the postlarval stage once the mouthparts are fully developed (Gross and Knowlton, 1991).
With the first postlarval stage, the young animals begin to resemble adults in most aspects. Notably, however, the cheliped asymmetry typical of the family does not arise until some time later (Knowlton 1973).
Temperature: The northern distribution limit for Alpheus heterochaelis has been reported to be in the vicinity of Cape Hatteras, NC (Nolan and Salmon 1970), and this limit is likely dictated by thermal tolerance. Winter low temperatures average around 4.4-6.6°C, but historic low temperatures of less than -14°C have been recorded.
Salinity: Alpheus heterochaelis appears to have a salinity tolerance sufficient to allow it to persist in shallow subtropical estuarine habitats susceptible to wide seasonal fluctuations. Matheson et al. (1999) collected individuals from Florida Bay mud banks experiencing salinities that ranged from mesohaline (12.8 ppt) to hyperhaline (49 ppt).
Trophic Mode: The concussive force of the snapping shrimp claw snap is sufficient to stun and even kill small prey at close distance. In light of recent findings that cavitation bubbles are the true source of the concussive pop, the potency of the claw snap attack is now better understood. Cavitation is the phenomenon of formation and subsequent implosion of cavities or bubbles in a flowing liquid in a region where the pressure of the liquid falls below its vapor pressure. Cavitation bubbles formed by the action of boat propellers implode with such force as to be capable of damaging the propeller. Small prey located within a few millimeters of the tip of the snapping claw are exposed to similar destructive forces that are short-lived but considerable (Versluis et al 2000).
Worms, small shrimp, crabs and other crustaceans, and even small fish such as pearlfish and gobies may be stunned or killed by the concussive claw snaps of Alpheus heterochaelis (Hazlett, 1962, Herberholz and Schmitz 1998).
Nolan and Salmon (1970) report the observation of grazing in laboratory-maintained A. heterochaelis picking algae off a shell and passing it to the mouth parts, but how representative this is of natural behavior is unknown.
Competitors: Nolan and Salmon (1970) speculate that some interspecific competition for shelter may occur in areas where multiple species of snapping shrimp co-occur, but also note that microenvironmental preference differences among species may moderate such interactions.
Most competitive interactions in this species are likely agonistic intraspecific interactions, including competition for refuge areas and mates. When two shrimp of the same sex interact in an experimental laboratory setting, one shrimp will become dominant over the other. This occurs after an initial face-off period followed by mutual contact between the second antennae of the opposing animals and aggressive posturing and claw snapping (Herberholz and Schmitz 1998, Nolan and Salmon 1970).
Studies by Herberholz and Schmitz (1998) have greatly increased understanding of how aggressive signals (e.g., claw snaps) are sensed by conspecifics. Mechanoreceptive setae on the snapper claw allow individuals to detect the rapid water jet produced by the claw snap of a conspecific facing it in close proximity. Agonistic intraspecific encounters of this sort may convey a warning to other snapping shrimp that a shelter or territory is occupied.
Male A. heterochaelis are more aggressive than females in their agonistic intraspecific snapping behavior, both in terms of snap frequency and the water velocities produced. Herberholz and Schmitz (1998) suggest that shrimp receiving water jet signals should be able to discern the sex of their opponent. Visual, chemical, and tactile cues may also be important in sex recognition in A. heterochaelis. Distant chemoreception, for example, plays a role in sex recognition in the cogener Alpheus edwardsii. Hughes (1996a, b) experimentally confirmed a visual component to size assessment in A. heterochaelis intraspecific interactions, and also demonstrated that chemical cues could mediate the response of individuals to visual signals.
In intraspecific encounters, the snap does not injure the opponent because the interaction distance (9 mm on average) is sufficient to avoid implosion danger (Versluis et al 2000). This non-lethal aggression may represent an instance of stereotyped fighting, in which aggressive motivation is conveyed without damaging physical contact occurring. Stereotyped fighting has been suggested for a number of decapod species, including mantis shrimp, hermit crabs, fiddler crabs, and others (Dingle and Caldwell 1969, Hazlett 1966, Crane 1966, Nolan and Salmon 1970).
Associated Species: Sillman et al (2003) describe a commensal association between Alpheus heterochaelis and the black-clawed mud crab (Panopeus herbstii), a benthic, burrow-building east coast mud crab. The authors report that 11% of occupied P. herbstii burrows they examined also housed snapping shrimp which were unable to construct burrows of their own. Mud crabs did not prey on the snapping shrimp although they regularly consume similarly sized crustaceans. The authors conclude that this association may allow A. heterochaelis to expand its intertidal habitat range. Note that this observed inability of A. heterochaelis to construct burrows on its own is at odds with earlier observations by Nolan and Salmon (1970) who reported the species was able to manipulate and excavate in certain substrata to construct their own shelters or improve existing rudimentary refuge.
In the IRL and elsewhere, Alpheus heterochaelis and other members of the genus are often parasitized by the bopyrid isopod Probopyria alphei (Rupert and Fox 1988).
Habitats: Alpheus heterochaelis is a benthic species inhabiting a variety of shallow marine habitats and typically residing within some manner of physical protective structure. It is common on reefs and oyster beds, in and around seagrass beds, and in salt marshes and mudflats. Nolan and Salmon (1970) note that A. heterochaelis lived in and among clumps of oyster shells, presumably burrowing into the mud underneath (but see above). Heck and Orth (1980) report that otter trawls from vegetated (Zostera marina) habitats contained A. heterochaelis, although the number of animals collected in this study was very small.
Activity Time: Early field surveys recording Alpheus spp. snapping levels noted a night-time increase in shrimp noise, suggesting elevated activity (Knudsen et al. 1948, Knowlton and Moulton 1963). Another historic study revealed a crepuscular rhythm in which snapping shrimp sound production peaked shortly after sunset and again shortly before sunrise (Johnson et al. 1947).
Laboratory investigations by Nolan and Salmon (1970) revealed that A. heterochaelis spent the greater part of its time during the day under or partly under cover, and increased the amount of activity away from shelter at night. Field surveys by these authors also confirmed an increase in sound production at night as well as discernable crepuscular peaks.
Population-wide snapping activity declines during the day, but does not disappear. This is likely reflective of the multiple roles of snapping behavior, i.e., to subdue prey, to defend territory, and to facilitate intraspecific agonistic interactions.
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