Stalking the crevices of coral reefs in the tropics around the world are the damselfish. They claim the nooks and crannies within the reefs, and then they defend those nooks and crannies with their lives.
Okay, so that’s a little dramatic. They probably aren’t willing to die for their homes, but they will fight tooth and nail to keep their homes from other damselfish in the housing market. These are some of the most territorial fish in the reefs, but their aggression is all based on the quality of their homes. The better quality the home, the more they fight to defend it. If their home isn’t as hidden and defendable as they’d like, then they aren’t that heartbroken when someone invades. That just gives them a reason to go seek a better home.
Aggression is a key trait to how the damselfish survive. The less aggressive among them are kicked out of optimal habitats, leaving them vulnerable to all the creatures that prey on them. And if they don’t have a territory, they are less likely to mate and therefore less likely to reproduce.
There are very few reasons damselfish will leave their homes. The main reason is food. Since prey won’t knock on their door, they go chase it down. The damselfish don’t go out that far though, staying within range of their habitats. These forays are also the shortest trips damselfish take.
When damselfish leave their homes for lengthier periods of time, it typically means they are in the process of courting and mating. In the same way a meal won’t come to them, neither will a mate. Just like the rest of us, the damselfish must seek out their partners.
The third reason a damselfish would leave the safety of their territory is to locate new territory. The best territory in the opinion of the damselfish would be whatever offers the best protection. And a decent view is always a bonus.
You know how in high school the big football quarterbacks always end up with the pretty cheerleaders? That isn’t how it works for the damselfish. Females won’t mate with males just because their big or even based on how many eggs they helped produce last time. Contrarily, males try and impress the bigger females, because bigger females mean bigger ovaries which mean more eggs.
So how do the males impress their ladies? They simply swim up a water column and then swim back down against the current as fast as they can, a process called signal jumping. Swimming against the water column creates a pulsing sound, which the females use in determining a mate. If a male can produce more pulses, it makes him the most vigorous, and lady damselfish choose mates based on how vigorously they are courted.
When a female is ready to lay eggs, she deposits them in a male’s territory, and not necessarily the male who helped produce the eggs. The males then spend the rest of their time guarding the eggs, swimming in a circular pattern around the nest to insure that nothing happens to the eggs.
What these fish crave are small creatures, like little crustaceans or plankton, and even algae. The strength of the current ultimately decides what the damselfish are likely to be eating. Slow current speeds means they can swim up higher in the water columns and feast on the plankton that live near the surface, or any other organic matter they can get their mouth around really. Stronger current speeds means the damselfish stays closer to the reefs and forages around the coral.
A fun fact about these fish is that males have smaller stomachs during reproduction. This is partially because of limited resources. Females feed more during pregnancy to sustain the little baby damselfish, and then once the eggs are laid the males spend their time guarding the eggs, not eating. Smaller stomachs means they don’t need to feed as often, meaning they don’t have to leave the eggs as often.
Some species have another type of food they like to eat: unhatched damselfish. This usually happens if it was a small clutch, and usually while the eggs are still in the early stages of development. The male guarding them may decide that the effort put into guarding them isn’t worth it and choose to eat them instead. In the species where this is common, a female will usually leave her eggs with a male who already has other eggs in later stages of development, as that means he is trustworthy and not as likely to eat the babies.