Now that I’ve described male and female Daphnia separately, it is time to bring them together. Yes, I come to the topic of sex. It is not as simple as it might seem. Remember, the majority of invertebrates that live in vernal ponds have rudimentary eyes at best and finding a mate is potentially difficult. Also, vernal pools in many places are highly turbid (visibility measured in millimeters, almost liquid mud) so even with good eyes they wouldn’t be able to find their mates.
How do they find one another? Although there are few studies on this topic, the best explanation is that females produce chemical signals for males to taste in the water. These chemical signals are called pheromones. I haven’t specifically mentioned any particular invertebrate because this is likely true for many of them. Jeanette Yen at Georgia Tech has shown this to be the case for copepods and Terri Crease looked for such pheromones in Daphnia. I return yet again to the idea that the medium is the message. The consequence for conservationists (even sex has important consequences for them!) is that water carries many messages and this is another. I don’t know how many arguments one needs to establish that the watershed of vernal ponds need to be protected. Water quality is so important to so many aspects of the lives of inhabitants – detecting predators, the taste of food (haven’t written about this yet), finding mates – that a deterioration in water quality by any sort of contaminant has the potential to really screw things up. Even sex.
Once a male finds a female, then what? This is not so easy to answer and, as usual, it depends. Flatworms have one set of issues (I haven’t told many flatworm stories but I will soon) and rotifers and crustaceans have others. As usual the stories I know best are about cladocerans. Like other crustaceans they can’t see their mates so even after finding them there are issues. Copulation is not internal in cladocerans, coupled with the observation that sperm are amoeboid, means the male has to get his equipment really close to where it needs to be in the female. But he can’t really see where to go. The best answer to this dilemma comes from studies of copepods and rotifers conducted by Terry Snell at Georgia Tech. He detected the presence of a concentration gradient of surface glycoproteins on females. Once a male finds a female he grabs on and then tastes his way to the best spot to orient himself to optimize the chance of a successful copulation.
I worked with Terry for a short while when I lived in Georgia and we tried to find a similar factor in Daphnia with no luck. In all likelihood we were searching for the wrong kind of chemical signal. But it is interesting that in all that mating couples I have seen the males are oriented exactly the same way. There must be one best position that gives the highest degree of successful mating.
Tomorrow: kinky sex
