Fascinating Animals: Nudibranchs

I’ve never really understood my friend Kenneth‘s fascination with gastropods.  Sure, some snails and slugs have very interesting mating rituals, and watching snails crawl around their environment eating leaves can be very meditative.  But in the end, they’re slow and slimy and not particularly aesthetically pleasing.

All of that changes, though, when you go underwater.  Sea slugs, or nudibranchs to use the more proper name, are brilliantly coloured, beautiful, and really just downright weird (which, of course, makes them fascinating).

***Check out this photo gallery for some really awesome images

Beyond just being pretty and not quite as slow as their landbound counterparts, nudibranchs have developed some really neat feeding and hunting behaviours.  Many of them feed on stinging creatures such as Man O’War jellyfish and sea anemones.  But not only have they developed an immunity to the stinging cells of their prey — nudibranchs actually recycle these stinging cells.  When a nudibranch consumes a prey item with stinging nematocysts, it will not digest and destroy these cells, but will instead pass them from its gut up to the surface of its skin.  How exactly the nudibranchs avoid being stung by the venomous cells is not yet totally understood, but once the cells are positioned correctly within the nudibranch’s body, it can use them to sting and deter potential predators.

Most nudibranchs have very rapid life-cycles, living for less than a year (even giant species like Hexabranchus sanguineus, which can reach 40cm in length, do so in a single year).  Some species live for only a month or so, and time their hatching to match up with a particular glut of a favourite food: Fiona pinnata stays in its larval state, delaying maturity, until it finds a suitable piece of floating debris on which it can live out its life, eating goose barnacles.  Once it finds a barnacle-encrusted piece of stuff, though, it rapidly grows up, taking only a few days to grow into an adult.  Other nudibranchs may time their maturity to match up with seasonal jellyfish blooms.

Despite being commonly referred to as “sea slugs”, nudibranchs are actually shell-less sea snails.  As larva, many nudibranchs still possess a shell, only losing it when they metamorphose into adults.

Like landbound slugs and snails, nudibranchs are hermaphrodites, possessing both male and female genitals.  Self-fertilization, though, does not occur — a nudibranch must find another of its species in order to mate.  Both individuals will come away from such a meeting having been fertilized, having performed both the male (depositing sperm) and female (accepting sperm and being fertilized by it) parts of the mating ritual.

Nudibranchs are, unfortunately, something that you’re unlikely to see in an aquarium.  Their short life cycles and very specific food requirements make them a hard animal to keep in captivity.  Fortunately, though, they are common throughout many of the world’s oceans, and so scuba-diving trips in many parts of the world are likely to yield amazing glimpses of these really awesome critters.

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