by
Dr. Melinda Coleman
One of the most memorable (but somewhat useless) bits of information I ever learnt in a biology lecture at university was about the sex-life of barnacles. Those little critters that are stuck on the rocks, enduring pounding waves and searing heat just so happen to have the longest penis (relative to body size) in the world!! Think about it? How on earth are barnacles supposed to copulate when they cannot move and their nearest mate might be a couple of centimeters away? The answer is that they have incredibly long penises that are able to reach over to a neighboring mate and deposit sperm into its reproductive tract. This is but one of many fascinating ways in which marine creatures have evolved to overcome the problems associated with reproducing in the sea.
Sessile marine organisms (animals that are not able to move because they grow attached to the rocks or some other fixed substrate) have perhaps some of the most interesting sex lives because they must overcome the problem of finding, seducing and copulating with a mate, all without moving and often without ever touching their partners. A classic example is tropical corals on the Great Barrier Reef which spawn during a few nights each November. When corals spawn together, a thick “soup” of eggs and sperm is created, ensuring that each egg has the highest chance of being fertilized. Corals that ejaculate their eggs or sperm prematurely (or too late) will be unsuccessful because the chance of fertilization is low. So how do corals and other marine organisms know when the time if right for sex? The mechanisms that allow animals to know when the time is vary from critter to critter, but they all have one thing in common. All are able to “sense” some environmental condition such as the time of the tide, phase of the moon, amount of water motion, temperature or nutrients that signals them to synchronously reproduce.
What about animals that do not reproduce en masse? How do these critters overcome the problem of reproducing in the ocean when the size of eggs and even more so, sperm, are minute relative to the volume of the ocean? The answer is pheromones: the smell or chemical that a male or a female emits that the opposite sex finds irresistible. The eggs of most marine organisms have such a “smell” that sperm cannot resist. For example, abalone eggs emit a pheromone that attract sperm and causes them to rapidly swim towards the egg. Marine biologists have learnt this by entering the “bedroom” of abalone and watching what sperm do when eggs are stripped of their nice smell versus those that still have their natural perfume. Except by random chance, sperm simply cannot find eggs that do not emit these attractive pheromones. So next time you swim, dive or snorkel in the “bedroom” that is the ocean take a minute to remember the lengths to which marine creatures have to go to for a bit of hanky-panky.
Stay tuned for Part II: Seaweed sex!