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.
Prey are well adapted to evade predators, and predators are correspondingly well adapted to catch prey. For most of us it is pretty reasonable to accept that such adaptation happens by natural selection, leading to long-term evolution of animals, making them better to either catch prey or evade predators, whatever end of the food chain you happen to be on. Thus natural selection affects traits such as foraging efficiency or anti-predator behaviours that lead to longer lives, quicker growth rates and, both directly and indirectly, higher reproduction rates. Most of my earlier blogs have more or less built on the assumption of natural selection affecting adaptations of animals.
There is another kind of selection, sexual selection, that is a little bit harder to understand. Sexual selection is the process where traits that directly affect the likelihood of securing a mate is changed over time, leading to the evolution of traits that sometimes seem to act contrary to natural selection in that sexually selected traits rather decrease life expectancy and growth rates. There are many examples of traits governed by sexual selection on land. Bird song, brightly colored males in many birds and lizards, antlers on deer and males adapted for fighting other males for access to females are examples that we all can relate to. It is thought that sexual selection in terrestrial systems are well as important as natural selection in shaping many aspects of populations and also a major force in driving speciation.
Are there examples of sexual selection in marine animals? Well, such examples are without doubt not as obvious as on land. The most obvious is dimorphism between sexes, that is that the two sexes differ in size. Many fishes, such as many species of wrasses and groupers, have males that are much larger than females. Males of such species secure their mating by either fighting with other males for mating rights or showing of to females in order to make the female choose the performer. This is certainly a sexually selected character. Some crabs seem to have very large males as compared to females, and that could be related to mating coinciding with female molting. Males can detect this molting some days before it actually happens, and try to protect “their” female from other males. Thus large males will be able to fend of smaller males, thus monopolizing pre-molting females.
When females are lager than males, it is very seldom a sexually selected character. In most marine monogamous species with a size difference between the sexes, the female will be the larger. This is not due to the female competing for mates, as the pair is monogamous, but rather that size affects egg production positively much more than size affects sperm production. Thus, in many cases, it makes sense for a monogamous pair to consist of a large female and a small male.
Another possible example of a sexually selected trait could be the extraordinarily long “nose” that some xeno crabs have. I have no idea if this is correct, or even if “nose” length of xeno crabs are related to sex, but is could be.
Otherwise, I find it surprisingly rare with clear sexually selected traits in marine animals. It could be related to the mating methods many marine animals use, where sex cells are released into the water and is more or less anonymously left by themselves to find a suitable cell to fuse with. This method of mating somewhat precludes mate choice or mate competition, thus making the force of sexual selection very weak compared to that of natural selection. I will get back to mating methods on reefs and reef-near areas in a later blog.
Last night I was feeling a little bored and felt that I needed to relieve some urges from deep in my soul. The only cure to that was to go and watch some sex!
Before you all start thinking im losing my mind, I am, of course, talking about Mandarinfish (Synchiropus splendidus). These beautiful little fish are nymphomaniacs, and get it on every-night. They are one of the most frustrating things to photograph and i havent tried to take pictures of Mandarinfish for about 4 years, so i thought it was about time for some stress!
I headed out with Johan and his two guests (Alex and Barbara). I took my own guide, Marnes, so that Alex and Barbara didn’t have to hear my foul language when i missed the shots that i wanted :).
In the end it was quite a nice dive with some success. Only one other boat arrived and they graciously put their guests a little further down the patch from us and i think everyone had a good time 🙂 By the time the other boat left we were just finding some frogfish, Bobtail Squid and cool nudis mating. For more details on the Mandarinfish dive i did a creature feature post on the NAD site. For those of you too lazy to click there, here are some photos below, but there are more on the website:
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.