The elephant shark (Callorhinchus milii) is a fascinating fish that is not actually a shark, although it is fairly closely related. They are cartilaginous fish but belong to the subclass Holocephali, rather than Elasmobranchii which includes sharks and rays. The common name for this subclass is chimera, appropriately enough since it includes some very odd fishes.
In appearance elephant sharks are bizarre looking. Their body is similar to that of a dogfish but they have a fleshy extension on their faces that gave them their name. The function of this trunk like snout is still unknown but it is probably for digging up or detecting food items in the sea bed.
They are not very large, reaching a maximum length of about forty inches although most are much shorter. The average length of mature elephant sharks is a modest two and a half feet. They are a subdued mottled brown colour, probably for camouflage purposes.
The elephant shark diet appears to mainly consist of molluscs, in particular clams. This would explain their trunk, used to get at burrowing food items. They have also been known to eat other invertebrates such as jellyfish. Since they are not adapted for fast swimming it is unlike they eat many, if any, bony fish.
In turn elephant sharks feature in the diets of larger predators. These include ourselves and elephants sharks are fairly heavily fished, both commercially and recreationally. In spite of this they do not appear to be in any danger. The IUCN has classed this species as of ‘least concern’ and populations appear to be stable.
One of the reasons that, unlike many cartilaginous fish, elephants sharks are not endangered is because they are fairly fast breeders. Females lay eggs in shallow waters each year over the course of a few weeks. It does take the juveniles a few years to reach maturity but this is compensated for by reasonably long life spans. Tagging has revealed the maximum age reached to be at least fifteen years.
Little is known about their social behaviour but elephant sharks appear to segregate themselves by gender for most of the time. Fisheries report catching mainly either males or females each time. This is a behaviour shared with many sharks, and the reason for it is uncertain.
It has recently been discovered that elephant sharks have two unusual features that are not immediately apparent. They have eyes similar to ours, with the same cones used to see colour. Also their genome is an interesting one. It is the shortest of the cartilaginous fishes and, strangely, is closer to our own than a bony fish’s genome. This is unusual because humans are in fact more closely related to bony fishes than cartilaginous ones.
With stable populations and part of their range extending into marine reserves, elephant sharks are in no great danger. Solving the many mysteries about their behaviour and physiology is going to be much easier with plentiful stocks of a fish not seriously affected by human behaviour.