Damselfish: The Farmers of the Reef

Damselfish are small, vibrantly coloured fish that inhabit coral reefs globally. While most live in marine habitats, some species inhabit brackish and freshwater bodies. Clubbed under the family Pomacentridae along with clownfish (relatives of Nemo), there are around 250-300 known species of damselfish. Damselfish exhibit a diverse array of colours — from vibrant shades of orange, red, yellow, or blue like the peacock damsel (Pomacentrus pavo) to more subdued shades like the Indian or cloudy dascyllus (Dascyllus carneus) and Indo-Pacific sergeant (Abudefduf vaigiensis), or a relatively duller appearance like the dusky farmerfish (Stegastes nigricans). Like most other marine animals, damselfish too can change colour in response to threats, stress, mating season, and age.

Damselfish are known to be very territorial and aggressive, often chasing intruders away from their small reef territories. Among the defining characteristics of Pomacentridae (Greek: poma = cover or lid; kentron = spike) are the serrated edges found on the operculum (bony flap on each side of the fish’s head that protects the gills) of several members of this fish family.

While clownfish and damselfish belong to the same family, there are some key differences between them. Damselfish are slightly larger and can be more aggressive than clownfish. Clownfish, made famous by the “Finding Nemo” movies, form lifelong mutualistic associations with anemones (i.e., both gain from the relationship), earning them another name — anemonefish.

Adult damselfish are mostly found inhabiting areas and holding territories close to rocky outcrops, coral heads, and turf algae-rich areas. Turf algae are carpets or mats of small, filamentous algae, most of which are either in the juvenile growth stage or are species that do not grow taller than a few centimetres. Some damsels, like the whitetail damselfish (Dascyllus aruanus), camouflage themselves against staghorn coral, while juveniles of the threespot dascyllus (Dascyllus trimaculatus) are also found sheltering among the spines of sea urchins in addition to coral branches. 

Courtship

Damselfish have a unique courtship ritual, and males work very hard to woo females. Courtship includes a display of vibrant colours and patterns more pronounced than the male’s normal colouring, and a swimming display called the “signal jump”. The male swims up the water column near his territory and then rapidly swims downwards — a ritual he may perform up to several hundred times a day! The male produces sounds (specific mating calls or chirps) with each rapid downward swim — the more vigorous the jump and louder the sound, the more desirable the male.

Males keep and guard small territories where they try to impress and invite females to lay eggs. They clean an area and prepare a nest for the female to lay eggs in. Subsequently, they guard the eggs and aerate them periodically.

Damselfish can display a variety of mating strategies. Some, like the Indian or cloudy dascyllus (Dascyllus carneus) and black damsel (Neoglyphidodon melas), form distinct mating pairs for breeding. In contrast, others may exhibit polygyny (one male mates with several females). Like wrasses, groupers, and other marine fish, some damselfish species may exhibit protogynous hermaphroditism, i.e., they mature as females but transition to become dominant males based on social cues. 

Damselfish can be herbivorous, carnivorous, or omnivorous and feed on a range of items, including plankton, small crustaceans like shrimp, and algae. However, unlike most other herbivore fish, algae-eating damselfish do not graze on just about any algae. They have selective preferences, which they cultivate in their algal fields, periodically “weeding” out undesirable algae. Interestingly, while they may feed on these carefully cultivated algae, the tiny invertebrates and other plankton settling on the algae are of higher nutritional value and interest to the fish.

 

Algae farmers

Small damselfish are not as eye-catching as a reef’s larger, more vibrant denizens, but their role in the ecosystem contributes significantly towards coral-algae dynamics. Abundantly found in coral reefs, damselfish aggressively defend small patches of turf algae in their territories, farming and cultivating them meticulously.

In the bustling reef, it takes only a few minutes to spot damselfish hovering over their small territories, swimming back and forth as they weed out undesirable algae, discarding them outside their “gardens”. Observe them for a few more minutes, and you will see them intimidate or chase away other (sometimes larger) herbivores, and even attempt to intimidate the occasional scuba diver.

Damselfish territories have multiple ecological benefits in a reef. The algae provide refuge for benthic invertebrates (invertebrates living on or in the bottom surface) like smaller crustaceans, brittle stars, and polychaete worms. These “cultivated’ gardens” (or algal mats) also provide a habitat for other kinds of plankton, like amphipods and copepods, that play an important role in the diet and nutrition of fish. Studies have found that the algal mats within damselfish territories are home to a greater diversity and number of invertebrates than algae patches outside these territories. This is because the damselfish defend the algae (and thus, the invertebrates) within the territories and actively chase away intruders, thus protecting the invertebrates from predation.  

However, a single drawback might overshadow all these benefits, especially in the context of climate change, which poses a constant threat to corals. Damselfish tend to the algae in their territories, expanding the area covered by the turf algae. Unfortunately, this poses direct competition to corals, especially injured or young coral recruits. Damselfish territories are unsuitable for coral larvae and coralline algae, and some damselfish actively kill live corals by biting off the polyps, making way for more turf algae to take over. These algal mats grow and spread fast, often growing over coral patches within damselfish territories and spelling doom for injured or stressed corals that do not get the opportunity to recover. Other herbivore fish that generally keep algal growth in check outside damselfish territories by grazing on them are actively chased away from these defended patches.

While most damselfish species are not considered at risk, a handful are on the IUCN Red List because of declining populations. Regardless, all reef fish face the challenge that rapidly warming oceans and repeated bleaching events pose.

Damselfish are a colourful and charismatic part of the ocean’s tapestry. From their mating dances to their fierce territorial displays, these small fish pack a punch in the marine world, and it is worth slowing down to observe them tend to their small gardens of algae.