Few humans have gone through a sex change. Changing sex in humans is a major effort, involving long-term counselling, surgery and medical treatment. Very few other terrestrial vertebrates, if any, change sex during their life cycle. In contrast, in the underwater world, sex change is very common. And this sex change does not involve any surgery, any hospital stay and any medical treatment. How do marine animals change sex so easily, and why do they change sex in the first hand would be reasonable questions to ask.
First of all, in many animals sex is not determined by chromosomal factors. Thus in many reptiles such as sea turtles the temperature during egg development determines what sex the offspring will have.
In marine systems, non-chromosomal sex determination is mostly a consequence of social rank or, in some cases size, which in itself may be correlated to social rank. Thus most wrasses, basletts and groupers start out as females while large individuals turn into males, while anemone fish start out as males, and, typically, only the largest individual in an anemone changes to being female. In both sex-changing groups, all individuals have both testis and ovaries, but at the most one of those are active at a given time.
Why change sex then. The sex change from female to male is more or less exclusively seen in species where males keep harems or gather in communal mating areas and fight for dominance over females. In such species, it is utterly pointless (with one exception that I get back to in a later blog) to be a small male. As small males do not get any matings, and few individuals reach sized large enough to dominate other males, it makes a lot of sense to start out life as female to ensure participation in the reproduction. Only if an individual for some reason has a better than average life will it reach sizes large enough to change sexes. Such a sex change, when large, will give access to many females and will be very successful in terms of offspring production.
The sex change from male to female does, to my knowledge, only occur in species with strict monogamy. In fish, as in most animals, large individuals will have more excess energy to put into sex cells. As the miniscule sperm cells are cheap to produce, and eggs are many times larger and much more energetically expensive to make, it makes a lot of sense for a monogamous pair to let the larger individual be a female and the smaller a male. Obvious examples of this are the anemone fish, where typically only one individual is a female and at the same time the largest individual in the anemone, while the rest of the resident fish are males. Only when the female dies will the former second largest change sex and become a female.
Finally, some species have no sex determination, but lives life as both sexes. Such animals are called hermaphrodites. A well-known example from land is the common earthworm. The classic example from marine systems is the nudibranchs, where every individual is both male and female. When nudibranchs mate, both partners transfer sperm and both have their eggs fertilized.
A few week ago i posted about a white Ghost Pipefish, which i believed was a Halimeda Ghost Pipefish. I was already not sure at the time, but when revisiting the spot yesterday, i saw that i was wrong. It turned out, that it is a Velvet Ghost Pipefish – which is also a very rare fish. But even cooler is, that there are now two of them. The bigger one – the one that we have seen previously –was a male before. But when coming back this time it already turned into a female. Probably the arrival of the smaller male made it change sex. Ghost Pipefishes can change sex and turn into females. They can be recognizes by the pouch shaped pectoral fins in which they then keep their eggs. Unlike Seahorses and Pipefishes, where the male incubates the eggs, it is always the female Ghost Pipefish, that takes care of the unhatched eggs.
To follow a guest request we went to see a sea fan with Denise Pygmy Seahorses on yesterday’s morning dives. This particular fan had 3 individuals living in it. As the seafan is easiely accessable from both sides, they are very cool to watch. Two of them had big blown up bellies and were obviously pregnant. Most probably these two indiviuals are gonna be proud fathers within the next week …
As all seahorses the male denise pygmy seahorse is the one that incubates the eggs in his belly. Pygmy seahorses mostly live in Pairs (sometimes several in one fan) and release their hatchlings usually at night. The babies then drift off in to a pelagic state and hopefully will settle down on another seafan. In Lembeh at least 4 species of Pygmy Seahorse can be seen: Bargibanti, Pontohi, Severn’s Pygmy Seahorse and Denise.