Eels masquerading as snakes sounds interesting, and that is before they go hunting with their friends the groupers...

 

The moray eels in the  family Muraenidae are bony fish or teleosts that form a strikingly successful clade within the eels (Anguilliformes). Most of the approximately 200 species are marine, but some can be found in brackish water or, occasionally, freshwater. They are most abundant in tropical and subtropical regions, where they live in crevices of rocky areas or coral reefs. Despite their fierce reputation, these creatures are rather secretive and tend to avoid humans. The family is subdivided into two presumably monophyletic subfamilies, the true morays (Muraeninae) and the snakemorays (Uropterygiinae).

Body form

Moray eels are only one of many groups of fish that have independently adopted an eel-like or anguilliform shape; other examples include the African catfish and the jawless lampreys. Being anguilliform, they are slender and extremely elongate, with the longest species, the slender giant moray (Strophidon sathete), reaching a maximum length of almost 4 metres. Despite showing the general anguilliform body plan, moray eels have evolved a number of specialisations for crevice dwelling. These include reduction and posterior placement of gill arches, secretion of protective mucus and the lack of scales. With their reduced fins, small eyes (they rely mainly on smell) and patterned body, these fish are strongly reminiscent of snakes. And in more than a superficial fashion.

Feeding

Within the carnivorous Muraenidae, two morphotypes have been described that correspond to fish-eating (piscivorous) and shell-crushing (durophagous) jaws. The jaw structures are clearly different, and also the dentition is adapted to the different diets.

Piscivorous jaws

Many species of moray eels (e.g. in the genus Gymnothorax) are active predators that go in search of relatively large prey. How they achieve this had long been a mystery, as these fishes do not seem to employ the standard method of suction feeding that is used by most other teleosts. Probably due to their narrow mouth and elongate body, moray eels are unable to create the suction pressures needed for pulling large prey into the oesophagus. So, as an alternative to the usual hydraulic-based prey capture and transport, these fishes utilise their highly mobile pharyngeal jaws (pharyngeal jaws are a second set of jaws located in the throat of many fishes, in addition to the oral jaws in the mouth). Once they have captured prey with their elongate oral jaws, which are covered with long, sharp and recurved teeth, moray eels rapidly protract their raptorial pharyngeal jaws into the oral cavity to grasp the prey. The prey is then transported by alternating movements of oral and pharyngeal teeth, providing a constant and secure grip on the prey. Sounds familiar? Indeed it is, because this is strikingly convergent with the way snakes feed (although here, cranial kinesis is much more pronounced). And the convergences with snakes do not stop there, because moray eels also involve in their feeding a process known as “knotting” where they loop themselves around the prey.

Durophagous jaws

Not all moray eels are fearsome predators attacking large prey. Some species are in fact durophagous – they feed on hard-shelled animals such as crustaceans. Accordingly, their oral jaws are short and recurved and equipped with molariform teeth that are well suited for crushing. This evolutionarily derived feeding morphology is found in the genera Echidna and Gymnomuraena. A molecular phylogenetic analysis based on mitochondrial gene loci has provided evidence that these two genera have acquired their durophagous morphology independently and that there might be at least three separate evolutionary origins within Echidna. Several other groups of fish have also modified their oral jaws into crushers, such as pufferfish and members of the family Sparidae that includes sea breams and porgies.

Feeding associations

Following

Being slender and elongate, moray eels can probe narrow crevices and flush out prey that is inaccessible to most fishes. So a number of other fish species have been observed to follow hunting moray eels. For example, in Mexico, a grouper, the Pacific mutton hamlet (Alphestes immaculatus) has been observed to follow the jewel moray (Muraena lentiginosa), together with a seabass, the barred serrano (Serranus psittacinus). Both following species compete for prey plucked from crevices by the free-swimming moray eel. Such following associations are generally common among marine fish and thought to be beneficial to the following species but probably detrimental to the one being followed.

Cooperative hunting

Feeding associations can, however, benefit all parties involved, and this is demonstrated by the most remarkable example. In the Red Sea, the roving coral grouper (Plectropomus pessuliferus) and the giant moray eel (Gymnothorax javanicus) engage in highly coordinated and communicative cooperative hunting. They normally employ very different hunting techniques and are active at different times of the day – while the diurnal grouper searches for prey in the open water, the nocturnal moray eel sneaks through reef crevices. Hungry groupers have been observed to approach moray eels in their hiding place and initiate the hunt by giving off a visual signal that includes high-frequency head shaking. This is most extraordinary, as active signalling to kick off a joint hunt seems to be extremely rare. Rather than assuming a specialised role during the hunt, grouper and moray eel do what they always do, with their hunting skills complementing each other very well - potential prey is neither safe in the open water nor in between rocks. So both partners increase their hunting success compared to hunting on their own. Interspecific cooperative hunting had not been demonstrated in fish before and is rarely found in mammals (where, interestingly, it always involves humans). Not even within a species, cooperative hunting is particularly common. So why has it evolved in this case? The reason might be that prey is always swallowed whole by either the grouper or the moray eel. This prevents monopolisation of food by one of the partners, which would, for example, occur in the case of carnivores hunting down a large prey animal and then competing for the carcass.  

Distribution

Like most reef fishes, moray eels are mainly found in the Indo-Pacific, but about 50 species in several genera occur in the Atlantic. A molecular phylogenetic analysis provided some evidence for multiple invasions of the Atlantic from the Indo-Pacific, possibly one in the Pliocene about 2.8 MYA and one or two in the Miocene. Multiple invasion of the Atlantic has also been demonstrated in butterflyfishes (Chaetodontidae) and wrasses (Labridae).