This year marks the 30th anniversary of my Ph.D.-related publications about tiny, shrimp-like animals called Jassa. I set out to discover a way to tell the species apart based on their appearance. But soon I asked why they looked that way.
First, a little about Jassa. It’s a genus of 20 species of marine, colonial amphipods, in which each male or female lives in a self-made tube that it attaches to almost anything hard, from rocks and docks to boat hulls. Species of Jassa live on most rocky shores and harbours around the world from Newfoundland to the Antarctic (not yet in the Arctic, but see Cruising the Globe Undetected). Anchored in their tubes, they extend into the water to capture passing plants and animals or scrape up bits of dead organic material called detritus. Being without a tube could be a death sentence as Jassa is easy prey to fishes.
Yes, it’s mostly a sedentary life, but things get interesting when it comes time to mate.
Inside its tube, Jassa grows in stages, splitting its old outer “skin”, or cuticle, after it has grown a new, slightly larger one inside, a process called molting. For a mature female, this is the only time that her cuticle is flexible enough to allow the release of her unfertilized eggs into her external brood pouch. For a mature male, this is the only time that he can fertilize her eggs.
This opportunity to mate is very short, only a couple of hours long. So there is fierce competition among adult males to find a female that is receptive to mating. The females broadcast their receptiveness by releasing attractive chemicals. The adult males abandon their tubes, roving to find the receptive females. But the females are choosy. They will only accept males with the right look. These males have to have a thumb (an enlargement on a gnathopod, or pincer). The thumb signals an intent to mate. Thumbless males are intent on evicting the female from her tube, and so she drives them off.
The thumbed males fight to guard the receptive female until she is ready to molt and release her eggs. Once fertilized, the embryos grow inside the eggs until hatching as miniature Jassas.
In my doctoral research, I discovered that not all Jassa thumbs are alike. In every species, big males had big thumbs but some little males had smaller thumbs than would be expected for their body size. Some also had ornaments that the big males did not have. I thought this might indicate that the little males behaved differently and follow-up studies by other scientists have confirmed this for Jassa marmorata.
The little males appear when there has been not much protein from phytoplankton and zooplankton to feed on, so they had to depend on a less healthy diet of bits of detritus. But what little thumbed males lack in size, they sometimes make up for in behaviour.
Faced with a big male with an over-sized thumb, the little males seemingly don’t stand a chance. But in fact, they do. They are quick to mature and their small thumbs are big enough to signal the female not to reject them. When they outnumber the big males, they win the mating. And females will accept more than one male to mate with, increasing the odds that little guys can succeed too.
So, different strategies work for different situations. The fascinating Jassa mating ritual dances to the rhythm of how a male presents his thumb.