Damselfish

Damselfish are a diverse group of small fish within the family Pomacentridae. They belong to several different subfamilies, including Abudefdufinae, Chrominae, Lepidozyginae, Pomacentrinae, and Stegastinae. While most species in this family are relatively small, four specific species can reach lengths of 30cm (12 in). These larger species are Hypsypops rubicundus, Microspathodon bairdii, M. dorsalis, and Nexilosus latifrons.

Most damselfish live in marine environments, but some inhabit brackish or fresh water. They are found globally in tropical, subtropical, and temperate waters. Many species live in coral or rocky reefs, and some are popular as marine aquarium pets.

Foraging behaviors in damselfish often depend on environmental factors like water currents. Damselfish of all sizes primarily feed on caridea and copepods. The domino damselfish, D. albisella, is particularly active, spending more than 85% of its daytime hours foraging. The position of a fish in the water column changes based on current speeds. When currents are low, damselfish forage higher in the water column to access a larger flux of plankton. As current speeds increase, they move closer to the bottom. Feeding rates tend to be higher when currents are faster.

Additionally, smaller fish tend to forage closer to their substrates than larger fish, likely to avoid predation.

Many damselfish species exhibit intense territoriality and resource partitioning. This means they use aggressive behaviors to defend specific areas or food sources. For example, the threespot damselfish, S. planifrons, is a classic example of extreme territoriality. The dusky damselfish, S. adustus, spends the majority of its life within its own territory. Some species even practice a form of animal husbandry through reef farming. Near Carrie Bow Cay, Belize, longfin damselfish (Stegastes diencaeus) protect planktonic mysids (Mysidium integrum). These mysid shrimp fertilize the algae in the reef farms with their excretes. This process helps the algae grow, which in turn helps the damselfish stay healthy. The damselfish aggressively defend these farms against other fish that might prey on the mysids.

Courtship and mating strategies vary significantly across different damselfish species. In S. partitus, females do not choose mates based on male size or the size of the male's brood. Instead, female choice depends on the male's courtship rate. Males perform a behavior called a signal jump, where they rise in the water and swim rapidly downward. This rapid swimming creates a pulsed sound that females can hear. Females use these sounds to judge the vigor of the male's courtship. In the beaugregory damselfish, S. leucostictus, males intensify their courtship for larger females. This is because female size is correlated with ovary weight, meaning larger females are often more fecund, or able to produce more eggs.

Some species, such as the bicolor damselfish, E. partitus, exhibit polygamy. This means males may court multiple females at the same time. In this species, evolutionary selection favors males that begin mating as soon as the spawning season starts. Females also play a role in selection by evaluating the quality of a male's territory. Because shelter sites are essential for avoiding predators, females may check these sites before depositing their eggs. This ensures that their offspring have a safe environment from the start.

Spawning distance also affects the behavior of the species S. nigricans. During spawning seasons, females visit the territories of males at dawn. The distance to a male's territory influences how often a female visits. If the territory is close, the female may make many repeated visits. If the territory is far away, she might deposit her entire clutch in just one visit. This behavior is a way to manage risks. Frequent visits to a nearby territory help the female defend her food resources from other fish. However, traveling long distances increases the risk of being attacked by predators. Females change their visit patterns to minimize these overall costs.

Parental care in damselfish can sometimes involve complex or harsh survival strategies. The male cortez damselfish, S. rectifraenum, practices filial cannibalism. This is when a male consumes some of his own offspring. Studies show these males typically consume over 25% of their clutches. They usually target clutches that are smaller than average or in the early stages of development. Females often deposit eggs with males who are already guarding early-stage eggs. This is especially true for females with smaller clutches, which are more vulnerable. For the male, eating these eggs is an adaptive response. It allows him to save energy when a clutch does not provide enough benefit to justify the cost of guarding it.