Our video shows a “conspecific, non-lethal encounter” of moray eels. The two fishes we had filmed were already locked by bites near their heads when they were seen (Fig. 1). They were waving, bending, and rearranging the rest of the bodies for a better position (Fig. 2). This is called “knotting”, a behavior also observed during feeding (Miller 1987, 1989; Barley et al. 2016; Deep Sea Fishing 2021). Both eels in our video had clear dorsal bite marks near the head or even near the eyes (Figs. 1 and 2a-i, white arrows). The way they were biting each other suggested a combination of attack and defense in both animals: Bites were placed where flesh could be ripped out of the opponent to hurt and weaken the other fish, but also and maybe more so to control the other’s ability to bite. This had resulted in a stand-off situation, where both had to maintain their bites to prevent the other one to reattack, and the main action was reduced to pushing against each other. Eventually, the eel in the background lost its grip (Fig. 2j, black arrow), and both eels immediately rose into the water column to reattack, more widely pushing and curling around each other, seeking out new places to bite, but failing to secure a new hold (Fig. 2k-n). Within seconds, the weaker animal disengaged and swam off, while the winner settled on the ground, looking exhausted (Fig. 2o). Both animals had been of similar size, but coincidentally the winner was the eel with the deeper dorsal wound.
We looked into possible motives for the observed fight – the opponent as rival, as mate or as food. We immediately excluded sexual reproduction as context for our video. When mating, eels may join by jaw-gripping, which we did not observe (Loh and Chen 2012). Instead, we saw deep wounds. Further, we estimated the size of the eels as around 30 to 40 cm in length, whereas the maximum length of this species is 1 m (Froese and Pauly 2022). Further, the black spots on the bodies of the animals in our video did not yet have the connecting lines that are present in adults (Kendall 2014; Froese and Pauly 2022; Fig. 1b). The eels’ size and the patterns on their bodies thus suggested that the observed eels were juveniles. Moreover, we could not see any reproductive materials being released when the eels reared up into the water column and excluded mating behaviour. We also thought it unlikely that the fight was food-motivated, and the aggressive behavior did not end with one eel eating the other. The main diet of E. nebulosa consists of crustaceans and occasionally of small fish (Shao 2022; Froese and Pauly 2022; Table 1). E. nebulosa has been described as temperamental (Shao 2022), but also as not aggressive (Mazza and Beltramini 2022), and several individuals can easily be kept in aquaria without risking cannibalism as long as regularly fed (e.g. Tropical Fish Hobbyist Magazine 2020; Swannyson7 in Axelrod 2022). Accounts of “cannibalism” in moray eels exist, but they mostly seem to refer to morays eating any morays in general, not apparently of the same species (Clipperton 2014; Ebner 2017; Young and Winn 2003). However, the snowflake eel is territorial and shelters in dens it steadily occupies at least for a while (Mazza and Beltramini 2022; Shao 2022). Overall, we thus interpreted the presently observed behavior as a rival fight between two juvenile moray eels with possible territorial motive, rather than courtship or foraging behavior.
To set this into a more general context for moray eels, we retrieved further videos. Including our own video, we found 34 films in which two eels were attacking each other, more or less ferociously, sometimes with one opponent being killed and eaten (Table 1). Aggressive behavior of moray eels against other moray eels is common, and we found videos for 12 different species (Table 1). The genus Gymnothorax was represented best and contributed 8 different species and made up 71% of the videos. Gymnothorax javanicus stood out by appearing in almost half of the videos (15 videos, 44%). Echidna nebulosa was the next-common species with about 1/4 of the videos (8 videos, 24%). Echidna xanthospilos, Myrichthys and Gymnomuraena species only occurred in one video each.
In these videos we saw similar behavioural patterns as we had found in our own video, regardless of whether the encounter was between the same or different species. Where videos showed that eels met or sought each other out, they stopped and gaped their mouths at each other, apparently trying to warn or intimidate the opponent. The gaping behaviour was more pronounced in conspecific encounters, while between different species an attack was more immediate. In conspecific encounters, the individuals often circled each other, occasionally pushing each other around with their long bodies, or rarely aligning their bodies. This behaviour may possibly help gauging the opponent’s strength, and it was always conducted with slightly opened to gaping mouths. Occasionally, it was accompanied by nipping, maybe as a further warning. When the eels charged, they attempted to bite and lock onto a muscular part of the opponent’s body. This was not normally possible when charging the broader side, unless the attacker was significantly larger than the attacked. Gripping and biting was much more easy when attacking the slimmer dorsal or ventral parts of the other fish, in most cases the dorsal region. Thus, most injuries were inflicted dorsally, and a number of videos showed individuals with dorsal irregularities, where injuries of previous fights had scarred over. Where eels locked in combat, they curled their bodies against each other and “knotted”, a method that provides leverage for ripping out the piece of flesh they had bitten. This behavior is also known from attacking prey (Miller 1987, 1989; Barley et al. 2016; Deep Sea Fishing 2021). When disengaging, the opponents circled each other again, looking for another place to bite, and again twisted into a knot of curled bodies. Eventually, the weaker or disadvantaged animal gave up and tried to swim away, with scrapes or gaping white flesh showing positions of inflicted wounds. In same-species encounters, the winner would let the opponent leave, but in different-species attacks, the attacker would sometimes use its advantage to kill the other. Size gave a contestant an advantage, and the larger animal appeared to be more likely to win (Table 1).
In 56% of the 34 videos we found, the eels fought in open terrain (Fig. 3a). In the other 44% of the cases, one eel was attacked in its den. In conspecific encounters we interpreted fights near dens as possible indicator for territorial rivalry. In different-species fights at a den, the behaviour of the aggressor and the common death of the attacked eel rather suggested that this was evidence for foraging behaviour, not a fight over space. Overall, conspecific fights were much more common, suggesting rivalry (Table 1). Only in 6 of the 34 videos (18%; Fig. 3b), different species fought with each other, and the aggressor was always a Gymnothorax species and larger than the attacked species. However, most filmed fights were between specimens that were roughly of the same body size (Fig. 3c), which again seems to indicate a higher likelihood for rivalry than for another motive. Fights incurred a risk of injury. However, the winners were only hurt in 15% of the videos, while the losers had a 50:50 risk to be injured (Fig. 3d-e). Hurt or not, the losers survived in 76% of the observed cases, and only 15% were killed by the attacker (Fig. 3f). Further, the killer eel was always a species with fish as its main diet and appeared to have actively sought out its victim (Table 1; Fig. 4a). Where videos showed pre-fight scenes, the aggressor was moving purposely through the area, checking cavities and crevices for potential prey, providing further evidence for an interpretation of this behavior as predatory foraging (see also Fishelsen 2013). Where we saw morays eating non-moray fishes, this behavior was the same (e.g. Deep Sea Fishing 2021). We found publications that mentioned “cannibalism” in moray eels, but none of them explained clearly whether that meant intra- or interspecific cases of eel-eating-eel, and we think that these authors probably meant two-species encounters (Devadoss and Pillai 1979; Young and Winn 2003; Clipperton 2014). In the videos, the attacks against eels of another species were usually swift and violent: The smaller eels were pulled from their dens, then severely bitten and rapidly ingested by the larger eel, dragging the victim or parts of them down the oesophagus by use of their front jaws and pharyngeal jaws (Mehta and Wainwright 2007, 2008; Mehta and Donohoe 2021). In only one video for two-species encounters the victim was able to flee and to survive (of 6 videos, 17%). All videos in which one eel was killed included different species, and none of the videos with conspecific encounters ended in death (Fig. 4b). It was more likely that the victim that was killed was of the same size or smaller than the aggressor (Fig. 4c). Winners were more likely to be injured if they were of the same size or smaller than the attacked fish (Fig. 4d). More losers were injured if they were of the same or smaller size than the winners, but within the larger specimens there were more injured than non-injured losers (Fig. 4e). This may have been due to smaller animals giving up sooner, thereby avoiding serious injury.
Videos with conspecific encounters between moray eels were most difficult to interpret, but we regarded them all as aggressive fights due to rivalry. They showed mouth-gaping and/or biting, most commonly involving individuals of a similar size (Table 1). The opponents often displayed deep, V-shaped wounds where flesh had been torn out of them (e.g. our own video; Fig. 1). Only in one video two eels aligned in parallel and rubbed against each other, not gaping their mouths, and this may have started out as courtship behavior (Netscript 2016). However, at some point the two individuals suddenly attacked each other violently, and one of them fled, which we took as an indication that at least at the end of the video it was a fight. Observations of moray reproductive behaviour are extremely rare, and we did not know whether courtship can occasionally look aggressive as in this video. Courtship can involve jaw-gripping (Loh and Chen 2018; Wen-Chien Huang pers. comm). One of the videos listed in Table 1 showed two yellow-edged morays (Gymnothorax flaviamrginatus) struggling with each other, and one held the other below its jaw. However, the opponents had deep wounds that made it unlikely that this was a display of rough mating, and no release of reproductive material was seen (as e.g. reported by Moyer and Zaiser 1982). Therefore, we interpreted all the retrieved videos with conspecific fights as aggression. Conspecific losers were neither eaten, nor chased by the winner when they fled, and thus the attack was not likely motivated by predatory foraging. An inherent, permanent hostility against same-species individuals can possibly be ruled out, because a number of aquarists’ reports stated that several individuals of Gymnothorax, Gymnomuraena and Echidna species can be kept together in aquaria without fighting, and especially those species that prefer to eat crustaceans (e.g. Tropical Fish Hobbyist Magazine 2020; Swannyson7 in Axelrod 2022). After these considerations and viewing the videos, we judged all conspecific fights between moray eels as rivalry motivated by competition over a resource. In one video the occurrence of a third, passive individual suggested a fight over a mate, and the winner went to this passive eel in a non-aggressive way at the end of the video (SuiTube5 2012). The video by Deep Sea Fishing (2021) contained a sequence in which two eels both tried to establish ownership over a squid, and the fight was thus over food. Where fights occurred near the hiding spot of one of the opponents, we assumed territorial reasons, and this seemed to be the most common reason for conspecific fights (Table 1). Moray eels do not stay in the same hole their entire lives, but they can display temporal den fidelity and can be found in the same place during consecutive surveys (Kendall 2014), which makes territoriality a possible scenario. Kendall further found that E. nebulosa, juveniles more often remained in their dens than adults, but den fidelity of E. nebulosa was overall slightly lower than in other species that were studied. From the evidence available on the screened videos, same-species fights were rivalrous, and appeared to be mostly motivated by territoriality.
Overall, fights within and between moral eel species appear to be commonplace in different species, but this behaviour is not well documented in scholarly publications. The deeper, V-shaped wounds from bites leave scars, and fish surveys could assess how frequently morays are marked in such way, which would give us a better understanding of how this behaviour may shape populations and their distributions. Same-species experiments near dens and away from dens could confirm the territorial motivation for fights. Further observations and videos could be made at places where this behaviour has commonly been observed, e.g. in the Thai Andaman Sea, and in the Maldives (Table 1). Any additional research in this context would be beneficial.