They are all Transsexuals

The popular anemone fishes are mostly known for their symbiosis with giant sea anemones, their interesting behaviour, and beautiful colours. But they also have another lesser known but interesting side to their lives: Their life cycles includes transsexual ‘stunts’

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In the early part of their life cycles, after some days out in the blue as pelagic larvae, anemone fishes settle on the coral reef once they find a suitable host sea anemone. Usually, there are a small group of anemone fishes in one large sea anemone.

The first one in the pecking order in a sea anemone turns into a female and mates with number two, which will remain a male. The rest of the fishes in the pecking order also remain males. They are not allowed to mate with the female. Only the number one male in the pecking order, the alpha male, can mate with the female.

The phenomenon of sex reversal is a fascinating part of anemone fish life history. Sex change occurs in many fishes. For example, it is now well established that most wrasses (Labridae) and parrotfishes (Scaridae) begin adult life as females and later assume the more colourful male phase. Similar changes are widespread among groupers (Serranidae).

A leader with no balls

What sets the anemone fishes apart in this respect, however, is that the sex change goes from male to female. As mentioned above, the largest and most socially dominant fish in a particular anemone is generally the female whose gonads are functioning ovaries with remnants of degenerated testicular tissue.

The smaller male, which in species such as A. frenatus and P. biaculeatus may be less than half the size of the female, has gonads that are functioning testes but also possess non-functioning or latent ovarian cells. If the dominant female dies or is experimentally removed, the male’s gonads cease to function as testes, and the egg-producing cells become active. Simultaneously, the largest of the non-breeding individuals becomes the functioning male.

This adaptation allows continuous reproduction; without it, an adult would have to await the arrival of a fish of the appropriate sex (which it would be only 50 percent of the time), thereby losing valuable breeding time, or it would have to seek out a mate, leaving its anemone and thereby risking predation both on itself and on its symbiont.

Within the tropics, spawning occurs throughout most of the year, although there may be seasonal peaks of activity. In subtropical or warm temperate seas, as, for example, in southern Japan, reproductive activity is generally restricted to spring and summer when water temperatures are at their highest.

At Enewetak Atoll (located at about 11°N in the central Pacific), spawning is strongly correlated with the lunar cycle: most nesting occurs when the moon is full or nearly so. Moonlight may serve to maintain a high level of alertness in the male, which assumes most of the nest guarding duties. Moreover, because newly hatched larvae are attracted to light, moonlight may draw them towards the surface, thereby facilitating their subsequent dispersal by waves and currents.

Anemone fishes are unique among damselfishes in forming permanent pair bonds that sometimes last for years. In other damsels, one male may mate with several females during a single spawning episode, and different sets of females are often involved in subsequent spawnings. However, pair-bonding in most species of anemone fishes is very strong and is correlated by the small size of their territories (centered on their sea anemone) which is, in turn, correlated with the unusual social hierarchy that exists in each “family” group.

On edge

Several days prior to spawning, there is increased social interaction, as expressed by chasing, fin-erection, and nest preparation. The male becomes particularly bold and aggressive, chasing and nipping his mate. He also displays by fully extending his dorsal, anal, and pelvic fins, while remaining stationary in front of or beside her. During the nuptial period, he selects a nest site, usually on bare rock adjacent to the anemone. Initially, the male spends considerable time clearing algae and debris from the site with his mouth; he is eventually joined in these activities by his mate.

Spawning, which occurs most often during morning hours, generally lasts from about 30 minutes to more than two hours. Once it commences, the tiny, conical ovipositor of the female is clearly visible. A number of eggs are extruded through this structure on each spawning pass, when the female swims slowly and deliberately in a zig-zag path with her belly just brushing the nest surface.

She is followed closely by her mate, who fertilizes the eggs as they are laid. Numerous passes occur during each spawning session. The number of eggs deposited ranges from about 100 to over 1000, depending on the size of the fish and on previous experience. In general, older, more experienced pairs produce more eggs than do recently formed pairs.

Anemonefish eggs are elliptical or capsule-shaped, are about 3-4 mm in length, and adhere to the nest surface by a tuft of short filaments. They incubate six to seven days. Just prior to hatching, the embryo, which has undergone rapid development, is clearly visible through the transparent egg membrane: the most noticeable features are the large eyes with their silvery pupils, and the red-orange yolk sac that is responsible for the general colour of the entire egg mass when viewed from a short distance.

Throughout incubation, the nest is guarded and cared for by the male. He chases other fishes from its neighbourhood, especially potential egg-eaters (e.g. wrasses). The male frequently visits the nest to fan the eggs with his pectoral fins and to remove dead eggs and debris with his mouth. The female is mainly occupied with feeding during this time, but occasionally assists the male with his duties.

The embryos hatch one night after about a week (dependent on the species) and the tiny embryos swim to the surface guided by the moonlight and out on the open ocean, away from predators, and to a life as pelagic larvae for one or two weeks (also dependent on the species). Out on the blue the larvae grows by feeding on zooplankton, and returns another night to settle on the reef, detecting a suitable host sea anemone, and the life cycle has once again made on full round. ■


Press releases from Divers Alert Network (DAN)