Symbiosis, sex change and olfactory imprinting: Clownfishes are more than just cute

After coming up to the boat from a dive on the Great Barrier Reef my dive companion (a somewhat macho Australian) said to me "Did you see that big coral trout?"  I said "no I totally missed it" to which he replied "you,re too busy staring at the dopey little damselfishes."  And it's true I love the little fishes.  I find it endlessly fascinating to watch them going about their business.  Damselfishes, for example, tend to be site attached and very aggressive.  They will attack much larger intruders (including divers) that stray into their territory by chasing and nipping them.  Because they have a tendency to make a home of a particular coral head, sea anemone, or other substrate you can easily observe their behavior and I have the distinct impression of a coral reef as a large and bustling metropolis.

Amphiprion melanopus on the Great Barrier Reef, Australia

One subfamily of damselfish are the clownfish or anemonefish (Amphiprion); the cute orange and white striped fish that gained fame as the star of Finding Nemo.  These fish have many of the types life history traits and behaviors that I think make damselfish so interesting.  As their name suggests anemonefishes have a special coating on their scales that allows them live and hide in the tentacles of sea anemones.  Sea anemones, like other species of cnidarians (a phylum including jellyfish) have stinging nematocysts which can be fatal to most small fish, but sea anemones and clownfish have a symbiotic relationship.  The clownfish are protected from predators and provided with food by the sea anemone and the anemone receives housecleaning, oxidation and nutrients from the anemonefish.  Different species of fish are adapted to live in different species of anemones; some clownfish can choose from several species of anemones while others can only live in one specific anemone species. 

In a clownfish colony the breeding female is the big boss.  All clownfish start out as male. The biggest, most dominant fish within a group will turn into a female and the second biggest fish becomes the mature breeding male while the rest of the fish remain in an immature juvenile male state and are chased and harassed by the breeding pair.  If the female fish dies the breeding male will grow larger and become a female and the most dominant juvenile will become a mature male.  The female lays the eggs on a hard substrate near the sea anemone and the male fish fertilizes the eggs and is responsible for fanning the eggs to oxidize them and removing dead or unfertilized eggs.  The female fish makes sure the male does his job by chasing him toward the eggs, she is quite the task master and bully.  About two weeks later the eggs hatch and the little larvae are carried out to sea where they spend about two weeks in the plankton developing further.  

After this time the little larvae start to develop orange coloration and deeper body, the tiny juvenile fish return to a reef.  Once they are on the reef finding the correct species of sea anemone to settle in as quickly as possible is essential for survival.  Anemonefishes recognize their host sea anemone by smell and they learn the odor of their host sea anemone when they hatch next to it, before they leave the reef. Anemonefish larvae that hatch in the absence of a sea anemone cannot recognize the smell of their host anemone when they return and settle on the reef.  This process of learning an odor during a specific period during development and then using that odor later in life is called olfactory imprinting.

Symbiosis, sex change and olfactory imprinting are three fascinating aspects of biology each of which could inspire a lifetime's worth of study and they're all found in one ridiculously cute, orange and white stripey fish.