Brown garden eels were observed during ten encounters with large-bodied fish at three reef sites (Fig. 1; Suppl. Fig. 2, Table 1, Video 1), and showed variable responses to five different species. This suggests garden eels have the ability to discriminate between organisms, and react according to relative predation risk. The most considerable and longest lasting reaction was in response to an encounter with two cubera snappers (Lutjanus cyanopterus), predators of garden eels. Similarly, encounters with a sand tilefish (Malacanthus plumieri) and queen triggerfish (Balistes vetula), also predators, elicited noticeable responses with a prolonged and full retraction of the eels into their burrows. Multiple encounters at two sites with two species of sharks, both improbable predators, resulted in less pronounced reactions. The reactions to sharks were consistent across encounters but varied with proximity. An encounter with a non-predator, saucereye porgy (Calamus calamus), resulted in the mildest response, despite very close proximity.
Five of the ten encounters were attributed to Caribbean reef sharks (Carcharhinus perezi; estimated size range = 150 to 180 cm total length [TL]), an improbable predator of garden eels, which passed approximately between 50 to 600 cm above the garden eels (Fig. 1A; Suppl. Video 1). On the four occasions sharks passed between 50 and 220 cm above the eels, the eels fully retracted ~2 s before the shark’s arrival, and then reemerged immediately after the sharks had passed. When the shark passed over the eels at a height of ~600 cm, no response was elicited. One encounter was attributed to a great hammerhead shark (Sphyrna mokarran; ~300 cm TL), which passed over ~300 cm above the eels (Suppl. Fig. 2A, Video 1). In this instance, the eels partially retracted ~5 s before the shark’s arrival, and then re-extended immediately after it had passed.
Large sharks that do not commonly feed in the benthos on small bony fish are improbable predators of garden eels. Other elasmobranchs, ray species that do feed regularly in the benthos, have been suggested and observed as predators of garden eels. As such, the observed response to the elasmobranchs in these encounters may be precautionary. Additionally, sharks of this size are often accompanied by teleosts, such as remoras and jack species, which are primarily piscivores. It is possible, therefore, that the response of the eels may be precautionary for potential commensal predator presence associated with sharks. Encounters with Caribbean reef sharks at two different sites, and multiple encounters at one site, elicited similar responses each time, suggesting that adjusted responses are consistent with probability of predation. Responses to two shark encounters, one with a Caribbean reef shark and one with a great hammerhead shark, at a large distance of separation, elicited a consistent response of partial retraction while remaining exposed throughout the encounter. The consistency between these responses, and the contrast to the Caribbean reef shark encounters at much closer proximity, suggests recognition of predation risk relative to immediate proximity.
The only encounter with a large-bodied bony fish that was a non-predator resulted in the smallest reaction observed. This supported the concept that garden eels can discriminate between larger fish with varying predation risk and adjust their responses accordingly. This encounter was with a saucereye porgy (estimated size ~40 cm TL), which passed approximately 30 cm above the garden eels (Fig. 1B; Suppl. Video 1). During this encounter the eels partially retracted at the point of closest proximity, but remained exposed throughout. Saucereye porgy exclusively feed on crustaceans, presenting no known predation risk to garden eels. The mild response observed probably constituted physical avoidance, rather than predator avoidance behaviour. Continued exposure from their burrows, despite the close proximity throughout the encounter, indicated a clear ability to discern between fish of similar size with varying associated predation risk.
The largest response elicited from all ten encounters with large-bodied organisms was in response to the greatest apparent predation risk, again indicating an ability to adjust predator-avoidance response relative to risk. In this encounter, a pair of cubera snappers (estimated size of both ~50 cm TL) passed approximately 30 cm above the eels (Fig. 1C; Suppl. Video 1). The eels responded by fully retracting ~5 s before arrival of the snappers, and then remained submerged for 28 s after they had passed. Cubera snappers are tenacious predators, with bony fish forming a large part of their diet. This strong response was probably relative to the high predation risk associated with close proximity of these active redators.
The second largest response resulted from encounter with a sand tilefish (estimated size ~30 cm TL), which are considered benthically-associated predators, with bony fish as their main dietary component. The tilefish passed approximately 20 cm above the eels, which reacted by fully retracting immediately before its arrival, and remained submerged for 17 s after it had passed (Suppl. Fig. 2B, Video 1). In this encounter, the tilefish moved directly over the garden eel colony and hovered in close proximity to the burrow locations, as if searching for food. It is, therefore, logical that the eels would fully retract and remain submerged for an extended period after the tilefish passed.
Close encounter with a queen triggerfish (estimated size ~30 cm TL) also resulted in an extended reaction from the garden eels. The triggerfish passed approximately 5 cm above the eels, which reacted by fully retracting ~5 s before its arrival and remained submerged for ~8 s after it had passed (Suppl. Fig. 2C, Video 1). Queen triggerfish are considered to primarily feed on sea urchins and other invertebrates, however, bony fish have been observed in their stomachs. Indeed, queen triggerfish are one of the few recognized predators of garden eels, having been observed chasing garden eels into their burrows, then ‘dive-bombing’ the substrate to dig out and consume them. Prolonged retraction into their burrows for an extended period after the triggerfish left the area, again indicates recognition of direct predation risk and an adjusted response to the encounter with this species.