Let’s talk about (fish) sex
When I hear the term “parental care” in the animal kingdom my mind jumps to wolf cubs in a den guarded by their aunts and uncles or fuzzy Emperor penguin babies sitting on their dad’s feet like in March of the Penguins (Fig 1). Fish, on the other hand, are usually considered the deadbeat parents of the animal world. Females lay eggs, males fertilize them, and the resulting brood of hundreds or even thousands of babies is pretty much on their own from there. Some fish are broadcast spawners; that is, the parents don’t even meet up to reproduce, they just send out their eggs or sperm into the environment and hope that those gametes will meet their counterparts out in the watery netherworld. The vast majority will never reach adulthood, but they produce so many even a tiny fraction of survivors is enough to maintain their population. And because the parents don’t have to spend energy taking care of the youngsters they can go on producing even more eggs and sperm! Since procreation is the ultimate goal of the evolutionary game, this strategy is passed on to future generations.
Some fish, however, prefer quality over quantity and invest a lot of time and effort into rearing their young. Here are a few of their stories:
Threespine stickleback are a group of small fish that lives almost everywhere around the Northern hemisphere, including both oceanic and freshwater populations. Males build elaborate nests where females will lay eggs (Fig 2). Males guard the nests while the babies develop until they are ready to leave the nest and survive on their own. Nest-building and –guarding comes with a host of risks, though. Stickleback males can be challenged by other males who are looking to build their own nests in the best territories (that is, places where resources like food and cover are abundant and predators are scarce). Roving packs of stickleback females will actually attack and cannibalize nests. And of course, those nests serve as easy targets for other fish predators like trout. So why do stickleback males build and guard nests? It’s a difficult question to answer, since we can’t just ask the fish. It’s thought that nests serve a role during female mate choice – females inspect them in much the same way they inspect other elaborate, ornamental traits(1). It’s also believed that males use their fins to aerate the eggs, pumping vital oxygen into the nest and thus ensuring proper development of his offspring (2). And given the predator-filled landscape of stickleback habitats, it’s highly likely that stickleback babies guarded by a male will be more likely to survive and procreate than stickleback babies left unattended.
Mouthbrooding: cardinalfish and cichlids
Mouthbrooding is another interesting strategy for fish parental care behavior (Fig 3). Mouthbrooding is where one of the parents holds the developing embryos in its mouth until they are old enough to survive on their own. Which parent cares for the eggs depends on the species – paternal mouthbrooding occurs in some beta species, cardinalfish, and the black-chin tilapia. Maternal mouthbrooding, on the other hand, is common among African cichlids. Xenotilapia, a group of cichlids, show biparental mouthbrooding, meaning both parents take some of the eggs
In paternal or biparental mouthbrooding, the mother lays her eggs, the father fertilizes them, and then he picks them up in his mouth (or both parents do, in the case of biparental mouthbrooding). In maternal mouthbrooding, it’s quite common for fertilization to occur in the mother’s mouth. Cichlid males actually have these distinctive pale spots on their anal fins called egg spots (Fig 4). A male will wave his anal fin gently in front of the female so that she will open her mouth to pick up the “eggs” on the fin; he uses this as an opportunity for fertilization. Females depend on the number, position, and coloration of these egg spots to determine whether a male is of the same species as she is.
Mouthbrooding, like nest-guarding, is actually quite costly for the parent, who doesn’t usually eat for weeks as the babies grow up (3). Females will not lay another batch of eggs until they spit out the eggs they’re currently tending. The advantage of mouthbrooding is that these fish can actually aerate and incubate their own babies, thereby ensuring proper development. Of course, these offspring are much better protected inside their parents’ mouths than out swimming by themselves in the cold ocean or lake. Cichlids can actually selectively swallow embryos that have died, recycling those essential proteins and other resources for her next brood.
Finding Nemo’s Dad
So, this Mother’s Day, when you’re thinking about all of the caring and loving parents in your life – remember, their love for you actually provides an evolutionary benefit to their genes! These benefits are not exclusive to humans or cuddly mammals or birds, either. Even certain fish will make extraordinary sacrifices for their babies.
Questions? Comments? Please sound off below! I’d love to hear from you :)
- Barber, Nairn, & Huntingford. 2000. “Nests as ornaments: revealing construction by male sticklebacks”. Behavioral Ecology 12(4): 390-396. doi: 1093/beheco/12.4.390
- Seventer, P. 1961. “A causal study of a displacement activity (fanning in Gasterosteus aculeatus L.)”. Behaviour Supplement 9: 1–170.
- Loiselle, P.: The Cichlid Aquarium, Tetra Press, 1985. ISBN 3-923880-20-0
I am a doctoral candidate in the Organismic and Evolutionary Biology program at the University of Massachusetts Amherst. I’m interested in how an individual’s genes and the environment in which it grows come together to determine its physical traits. I study a group of closely related freshwater fish called cichlids which live in the African rift lakes like Victoria, Malawi, and Tanganyika.