Anthias: Wakatobi’s Living Colors

Published December 9, 2019 in MARINE LIFE

Seen from a distance, many of Wakatobi’s coral reefs look as if they are covered in swirling clouds of orange, pink, and yellow. A closer examination reveals what’s really going on as hundreds or sometimes thousands of brightly-hued anthias darting frenetically to and fro. These little fish are so common and so abundant that it’s easy to take them for granted and overlook their presence. But if you pause to take a closer look, these ubiquitous reef dwellers can become a fascinating subject worthy of any fish watcher’s attention.

If you pause to take a closer look, these ubiquitous reef dwellers can become a fascinating subject worthy of any fish watcher's attention.

If you pause to take a closer look, these ubiquitous reef dwellers can become a fascinating subject worthy of any fish watcher’s attention. Photo by Glen Cowans

Lots and lots of little fish

The more than 170 species of fish collectively known as anthias are among the most abundant and wide-spread species found on coral reefs. Though most varieties grow to lengths of only 10 to 15 cm/4 to 6 inches, they are actually members of the grouper family, which includes some of the world’s largest bony fish. Anthias feed on zooplankton, which is one reason why they are in constant motion as they nosh on small passing morsels. And that’s also why you are more likely to encounter large numbers of these fish on the outer edges of a reef or the top of a wall, as these are the places where moving water is more likely to bring dinner.

Anthias are often found where there's hard coral cover with nooks and crannies close to some type of protective cover. Photo by Walt Stearns

Anthias are often found close to some type of protective cover where there’s hard coral with nooks and crannies. Photo by Walt Stearns

You may also notice that anthias are most prevalent in places where there’s hard coral cover with plenty of nooks and crannies such as on the reefs of Wakatobi. These fish don’t like to stray far from protection and given their bright patterns, which makes them an easy target for predators to track, any anthias that ventures too far out into open water becomes the equivalent of a live fishing lure. Some species prefer the coral cover at top of the reef, while others can be found on the edges of slopes and walls at depths down to 100 meters/330 feet, but never far from protective cover of some type.

Perpetual motion

The constant, frenetic activity of anthias may seem random, but there’s actually a lot of individual drama taking place. Unlike schooling fish, which assemble in controlled, egalitarian formations for a common goal such as protection or movement. By contrast, anthias form shoals, which biologists define as a group of individual fish grouped together but pursuing disparate goals. One of those goals, of course, is feeding, but that only accounts for part of the comings and goings of the shoal.

Male anthias defend their territory against intruders through a series of threat displays, sometimes having to ward off potential rivals by nipping and swiping, which can escalate to a full lip-lock.

Male anthias defend their territory against intruders through a series of threat displays, sometimes having to ward off potential rivals by nipping and swiping, which can escalate to a full lip-lock. Photo by Wade and Robyn Hughes

Anthias have a complex social order based around small groups lead by a dominant alpha male and a harem of perhaps a dozen females and sometimes one or more subservient beta males. These alphas are jealous of their harems and highly territorial. They defend their turf and their ladies against intruders through a series of threat displays, and at times may have to ward off potential rivals in contests that may begin with nips and swipes, then escalate to a full lip-lock that may result in the brutal dismembering of the loser’s jaws.

At the same time that male infighting and turf wars are taking place, the females in the harem maintain a distinct social hierarchy, with the larger and older fish vying for dominance while subordinates accept their place in the pecking order. Add to this the mating rituals —more on that in a minute — that take place on a regular basis, and multiply it by the numerous individual harems in the larger shoal, and it’s easy to see why watching a collection of anthias can be both a challenge and a delight.

The female anthias in the harem will maintain a distinct social hierarchy, while the larger and older fish vie for dminance. Photo by Walt Stearns

The female anthias in the harem will maintain a distinct social hierarchy, while the larger and older fish vie for dominance. Photo by Walt Stearns

Life changes

Like a number of fish, anthias can change their sex. Technically, they are known as protogynous hermaphrodites. All are all born female, and when the harem’s dominant male dies, one of the larger females in the harem will make the gender change. As they await the chance to man up, these more mature females may become less interested in breeding, perhaps due to increasing levels of testosterone building in anticipation of their transition. The sex change process takes a couple of weeks, and in many species results in not only new genitalia but also an increase in size and fin length and a more dramatic color pattern that shifts away from pinks and yellows to more purple hues. And as an added quirk, these changes can be reversible. If a harem’s reordering produces too many males for a sustainable social order, some of the less dominant contenders may transition back to female status.

Anthias activity isn’t all about infighting and defending territory, there’s also some romance. When the dominant male feels the urge to merge, he engages in a courtship display that is often described as a “U swim.” Starting from a position above the harem, the amorous male will dive towards the reef, flick his ventral fins, then make a u-turn and ascend to his starting point. Eventually, these underwater acrobatics attract the attention of a partner, who joins the male at the bottom of his dive for the joint release of gametes. Anthias are pelagic spawners that consign their eggs to the currents. In less than a day, those eggs will hatch into tiny larvae that develop fins and mouths within two weeks, and became fully-formed miniature adults within a month. From that point on, anthias who avoid the reefs many predators may live from three to five years as they join one of the most colorful and energetic displays of life in the world’s oceans.

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