Mimetic interactions are relatively common among tropical marine fishes (citations listed by Randall and McCosker, 1993), however, two or more species of fishes mimicking a third fish species is rare. Only seven such complexes are known and only among Indo-Pacific coral-reef fishes (Springer and Smith-Vaniz, 1972; Russell et al., 1976; Smith-Vaniz, 1976). The present report is the first known case of multiple mimic species among Atlantic coral-reef fishes. The only other published records of snappers (Lutjanidae) acting as mimics involves the Pacific Lutjanus bohar whose juveniles mimic several species of Chromis (Russell et al., 1976; Moyer, 1977) and the tropical Eastern Pacific L. inermis mimicking Chromis atrilobata (Allen and Robertson, 1994).

During a survey of coral-reef fishes for large, obvious, external isopods, Anilocra spp., in the Cayman Islands, we were surprised to observe the young of Black Snapper, Apsilus dentatus (Lutjanidae; Fig. 1), apparently mimicking and coexisting with feeding aggregations of Blue Chromis, Chromis cyanea (Pomacentridae), at depths between 12 and 27 m. The only other blue-chromis mimic we are aware of is the Blue Hamlet, Hypoplectrus gemma (Serranidae), which is not known to occur in the Cayman Islands. The blue body color and particularly the black dorsal and ventral margins of the caudal fin of the black snapper juveniles superficially resembled the blue hamlet, although its body shape and width of the black bands made it a better blue-chromis mimic than the blue hamlet. The black snapper is a relatively small snapper (to 65 cm TL, but usually not exceeding 40 cm TL) but is a commercially important deepwater (100–200 m) species. Juveniles occur at shallower depths than do adults, to 37 m but seldom shallower (Thresher, 1978).

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Materials and Methods

We closely observed a total of 9450 blue chromis for external parasites at 21 different sites (200–1000 individuals per site) around all three Cayman Islands on 2–7 August 1998 (Fig. 2). Individual young black snappers were observed associated with blue chromis at one of nine sites on Grand Cayman, 2 of 10 sites on Little Cayman, and zero of two sites on Cayman Brac. One specimen of black snapper was photographed (Fig. 1). Young of the black snapper are commonly observed with blue chromis around all three Cayman Islands (A. Roberts, pers. comm.).

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Results

We observed this association at depths between 12 m and 27 m on the outer reef slope. Young black snappers were slightly larger (approximately 11–14 cm TL) than the blue chromis with which they associated (approximately 10–12 cm TL). Otherwise, they were difficult to distinguish from their “hosts.” If we had not been closely examining the blue chromis for external parasites, we might not have noticed these mimic snappers.

Young black snappers remained with blue chromis even when closely approached by divers. The associations appear to be relatively stable, since, when we forced these mimics out of feeding aggregations for photographs (Fig. 1), they returned to the aggregation as soon as we retreated. One fish remained with the same aggregation along 2–3 m of shelf edge over a 4.5-h period (0812–1242 h).

Discussion

Thresher (1980) suggested that young of black snappers occur as scattered individuals, as we found them in the Cayman Islands. Mimics are expected to be dispersed because common occurrence defeats the benefit of mimicry (Thresher, 1978). Blue chromis and black snapper juveniles predominantly overlap in a depth range between 37 m and 55 m. The rarity of this association above 37 m (or scuba depths) may have prevented this mimicry from being discovered previously. The coloration of young black snappers appears to be an adaption for mimicking blue chromis (compare color photographs in Thresher, 1980:56,148).

The geographic range of the black snapper, the Bahamas and Antilles (Allen, 1985), and the other known blue-chromis mimic, the blue hamlet, southern Florida and Belize (Domeier, 1994), are mutually exclusive. In a recent unpublished survey of reef fishes, 36 individuals of blue hamlets were observed at 24 locations throughout the Bahamas and insular Caribbean (L. Akins, pers. comm.). Some of these observations may have been based on misidentifications of young black snappers. In the survey, black snapper juveniles were only noted once in Belize and once in the Bahamas (L. Akins, pers. comm.).

Three species of snappers in three different parts of the world are mimics. Surprisingly, all are limited to one genus (Chromis) of fish model (Russell et al., 1976; Moyer, 1977; Allen and Robertson, 1994).

We did not observe young black snappers feeding, but they occurred within feeding aggregations of blue chromis, which suggests they may have been feeding on planktonic organisms. A. Acero-P. (pers. comm.) has observed and confirmed with stomach-content analysis that young black snappers in the San Andrés and Providencia Islands, Colombia, feed on zooplankton. Many other juvenile or young snappers feed on crustaceans and change to a diet of fishes when they become adults. The other blue-chromis mimic, the blue hamlet, uses its aggressive mimicry to prey on crustaceans (Thresher, 1978; Domeier, 1994), on the bottom adjacent to blue chromis feeding aggregations, not up in the water column among the blue chromis (Randall and Randall, 1960; J. E. Randall, pers. comm.). These behavioral observations and stomach contents data suggest that black snapper juveniles are social mimics (Dafni and Diamant, 1984; Randall and McCosker, 1993) rather than aggressive mimics and use the blue chromis feeding aggregations for protection from predation. Because the two species are both feeding on zooplankton, the association is almost certainly social mimicry. Even if they were feeding on different kinds of prey, this behavior would still suggest social mimicry.