The oceans harbor a wide array of reproductive tactics, and some of them are pretty darn weird. This week we have already learned about parasitic males and some slightly unromantic mating strategies. We also learned that the movie “Finding Nemo” would be far different if screen writers accurately depicted the symbiotic relationship between Nemo, Marlin and their “amnemonemomne” home. But that isn’t the only thing Pixar got wrong. Anemone fish (also called “clown fish”) start their life off as males (like Nemo and Marlin) and live in a single anemone with several other males and a single, larger female (Figure 1). The movie is doing okay so far, but here’s the hitch- when that large female dies (o, like Nemo’s mom did!) the breeding group does not simply usher a new female into their ranks. The largest of the males actually becomes the new female. Yup, that’s right- the male changes sex.
Anemone fish are hermaphrodites (meaning a single individual has both male and female reproductive organs at some point in life) and they are by no means the only fish to utilize this interesting mating tactic. Hermaphroditism is fairly common in invertebrates (like the sea slug) but fishes are the only vertebrates known to be functional hermaphrodites (as opposed to cases of hermaphroditism which arise due to mutations and individuals are not functionally reproductive). In the over 33,000 species of fishes, hermaphroditism has independently evolved multiple times in different groups, and it takes on several different forms.
Types of Hermaphroditism:
- Simultaneous Hermaphroditism- In this form of hermaphroditism, a single individual possess functional male AND female gametes (reproductive cells- i.e. sperm or eggs) at the same time. This means that at any given point in time, the individual can reproduce as either a male or a female. I know what some of you are thinking- but no, these fish do not mate with themselves (or self-fertilizing) – with the exception of a single species (the killifish, Rivulus marmoratus). Other examples of simultaneous hermaphroditic fishes include some species of deep-sea lizard fishes , some coral reef fishes, like hamlets, and in many sea basses (Figure 2).
- Sequential Hermaphroditism– This is where an individual sexually matures as one sex, and then later in life changes to become another sex. There are even different forms of this type of hermaphroditism. An individual can either start out as a male and change into a female (termed protandry) or can start out as a female and change to a male (termed protogyny). Generally, after an individual switches sexes there is no going back, but in some species of gobies, individuals can flip-flop between sexes multiple times (called bi-directional hermaphroditism). As if entirely changing from one sex to another once wasn’t crazy enough!
Sequential hermaphroditism is more common in fishes than simultaneous hermaphroditism. If you have ever gone scuba diving or snorkeling on a coral reef, you have probably witnessed several sex-changing hermaphrodites. Wrasses, gobies, and some groupers are some of the more recognized fishes utilizing this reproductive tactic. While some of these groups bear no obvious sign of their gender (at least, none recognizable to us) other species undergo a more conspicuous sex change. Parrotfishes, for example, (Figure 3) are protogynous hermaphrodites that change body coloration at each stage of their life: Initial phase individuals are typically females while terminal phase individuals are usually sexually mature males.
Why be a hermaphrodite?
Really, this question centers on one important point: hermaphroditism (like any trait) will proliferate through a population if it increases an individual’s reproductive success (i.e. increases the number of offspring an individual produces which survive, thereby spreading the hermaphroditic genes to subsequent generations).
So how does hermaphroditism increase reproductive success? Short answer: it depends on the situation. But here are some clear-cut examples.
In extreme environments, like the deep sea, where there are very few individuals to mate with, it is advantageous to be able to mate with the first fish you can, regardless of gender. This is to say, if you don’t see many fishes in the sea, it would suck if you only had male gametes and the only other fish you came across was another male. It is much easier if you can swing either way and have both male and female gametes. This is the case for the lizard fish (Figure 1B).
For other fishes, one sex have a much higher reproductive success at a certain size. For example, males that are in strong competition with other males will have higher reproductive success if they are larger and can out-compete smaller males. This is the case for some protogynous fishes (like the parrotfish, Figure 3). The male will be most successful at mating when he becomes larger, but to keep from missing out on mating while the individual is small, it makes sense to take advantage of this time by continuing to mate as a female instead, where size doesn’t matter as much.
On the other hand, fishes produce exponentially more eggs the larger they get. In some cases, for protandrous fishes like the anemone fish (Figure 2), it makes more sense to use the time when the individual is small to mate as a male and then switch to mating as a female once the individual is large enough to make a LOT more eggs.
When you look at these examples, the better question becomes: Why wouldn’t you be a hermaphrodite?! Well, life is all about trade-offs. Although hermaphrodites are able to take advantage of certain situations in a way that maximizes their reproductive success, they put a lot of energy in to doing so. It is very energetically costly to produce both male and female gametes at the same time (and may result in producing fewer gametes overall) and to change sex halfway through life. I mean, think about it: These fish are entirely reworking their reproductive system- the hormones produced, the reproductive structures, and the types of gametes they make!
Why does hermaphroditism matter?
For one, we can learn a lot by studying how these fishes are able to entirely reorganize their reproductive systems and why hermaphroditism is advantageous in different social and environmental settings. But furthermore, hermaphroditism has some pretty large implications on how we manage our fisheries. For example, a large number of groupers are protogynous hermaphrodites (Figure 4). This means that as individuals get larger, they change sexes. Although seemingly innocuous, problems arrise when we start demanding larger groupers on our plates. By preferentially fishing the largest individuals in a population, we are selectively removing all of the males from the population. This skews the sex ratio in the population and, if not properly managed, could result in a female-heavy population that is not as reproductively successful.