The Little-Known Lives of Fish Larvae

Did you know that the young of many fishes might look completely different to the adults? During reproduction, most fish produce eggs which hatch into microscopic larvae. Some are as small as a pencil point (2 mm). These larvae often spend much of their lives floating in the water column and are thus capable of dispersing very far distances, in some cases, from Trinidad and Tobago to the Gulf of Mexico! As they develop, they learn to swim, hunt and avoid predators.  In many cases, these larvae do not resemble their adults as they often have strange temporary adaptations that allow them to survive in the open ocean. Because many fish larvae are not easily seen by the naked eye, they are often overlooked and are poorly known. In this series of blogs, we highlight the interesting larval forms of some of the fish species present in Caribbean waters.

Moray Eels:

Today we start with a common Caribbean reef inhabitant, the moray eel (Family: Muraenidae), of which 15 species are known from the region. You may not think of eels as fish but they most certainly are!

Morays are often seen with their heads extended from holes or crevices in reefs. As adults, these fish have long snake-like scaleless bodies that are coated with a mucous layer which are sometimes responsible for giving them their vibrant colors and patterns e.g. the green moray.  Morays have NO pectoral and pelvic fins; their dorsal, anal and caudal fins form a single continuous fin which surrounds most of the body.

In contrast, the larvae of moray eels, known as LEPTOCEPHALI (Singular = leptocephalus), are rarely sighted. The external appearance of leptocephali and adults are so different that for about a century they were thought to be a special type of marine fish and were placed into their own Genus Leptocephalus.

Direct observations of the change from the leptocephali to the adult-like form in an aquarium eventually disproved this assumption. These larvae are transparent and ribbon-like (laterally flattened). They have no red blood cells, no spine, no bones, and only a very small layer of muscle tissue, thus, the body is supported by a gelatinous matrix. The head of the leptocephalus larvae is very small relative to the rest of the body, hence the name (leptocephalus = “small head”). The internal organs are small and the gut is a simple tube. The types of material observed in the gut indicate that leptocephali do NOT feed on phytoplankton and zooplankton like most other fish larvae, but on marine snow like material. The time spent in this larval stage is species specific varying from three months to over a year. This is much longer than the larval stage of most other fish species. As the larva changes into the adult-like juvenile forms (elver), body length and depth is reduced, red blood cells are formed and the body moves from transparent to opaque due to the thickening of the skin and increased skin pigmentation.

Apart from eels, this leptocephalus larval stage is also seen in other Caribbean fishes such as tarpon and bonefish. These fishes all belong to the Superorder: Elopomorpha.  In Trinidad and Tobago, Grand ecaille (Megalops atlanticus) and Banane (Albula vulpes) belong to this group.

Click here for a link to video footage of large leptocephali filmed by divers off Indonesia. They are most likely ribbon eel (Rhinomuraena quaesita) larvae, a species of moray eel that we do not have here in the Caribbean. Nevertheless, they give a good idea of what leptocephali look like. Quite Amazing! Enjoy!

Blog Author — Dr. Farahnaz Solomon