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      Dot and dash butterflyfish. Fiji.

      Corallivores, those organisms that feed on corals, are easily spotted on healthy coral reefs. I became fixated with one corallivore in particular: a damselfish called the big-lipped damsel. I had only ever seen pictures of the fish and was having a hard time finding them in the wild. Finally, while diving a small marine reserve in the southern Philippines, I spotted one of these superficially unspectacular fish by chance in a rich coral garden. I identified it by the quick glimpse I caught of its preposterously large lips as it fed on a branching coral. These lips are so large that they’re surely the true inspiration for the inflated Hollywood “trout pout.” However, these lips serve a purpose, as does everything in nature. After finding more of these fish a year later in West Papua, I spent an hour or so watching their antics and confirmed the function of the lips. These fish are obligate corallivores, and rather than utilize the approach of the dainty butterflyfish that pluck one polyp at a time, the damselfish instead completely denudes the surface of the coral of tissue, their sensitive flesh meeting the sharp coral skeleton beneath. Their voluptuous lips protect them from cutting and tearing their skin against these edges.

The most well-known of specialist corallivorous fishes must be the butterflyfish, which are ubiquitous and conspicuous members of coral reef communities around the world. Because most butterflyfish completely rely on healthy and diverse coral assemblages, they quickly respond to changes in coral health and abundance by moving location; in extreme cases of ecosystem degradation, they can die. In the early 1980s it was suggested that butterflyfish could act as indicators of coral health, as they are easy to survey and changes in their populations are easy to document.21 It would be much more difficult to survey corals directly as they are so numerous and can be very hard to identify. Butterflyfish can be quite picky about their favored food sources, so their population fluctuations also help to highlight changes within specific coral groups. They are also territorial, and the density at which they will tolerate other butterflyfish is directly related to local food supplies. Therefore, these coral reef staples can tell us how many species of coral are present in an area, as well as their density.

      Spawning

      One evening in mid-November, after spending the twilight hour making observations on the social behavior vswam up into a cut in the reef full of corals, sponges, and crinoids. There was an excitable energy in the water, and before long I started to notice a few little white specks lifting off a coral. Very quickly, several corals and even a crinoid began spawning, releasing great clouds of sperm and eggs. It was magical to witness this event firsthand. The crinoid appeared almost on fire as a smoke of gametes were released. The coral took a subtler approach, releasing visible individual large gametes into the water.

      Spawning barrel sponge. Tubbataha Reef, Sulu Sea, Philippines.

This type of reproduction is known as “broadcast spawning,” or “mass spawning.” Most corals and other animals that are permanently stuck to the reef release their sperm and eggs into the water in this way, where fertilization and development then occur externally.22 Mass broadcast spawning is one of the greatest natural spectacles on Earth since most of a given reef’s organisms are synchronized to spawn on one or a few nights per year, turning the water surface into a noticeable slick due to the density of the spawn. Most species in a given area tend to spawn on the same evening, although there may be other evenings throughout the year where conditions suit the spawning needs of other species. Mass spawning of different species may be an adaptation to overwhelm predators and prevent them from taking too much of a toll on the reproductive potential of a single species. Interestingly, researchers have found that spawning just several hours apart on the same day is enough to prevent coral colonies from cross-fertilizing, and this may lead to reproductive isolation and the evolution of different species.

      Spawning Acropora coral. Wakatobi, Sulawesi, Indonesia.

Mass coral spawning typically occurs a few nights after a full moon. Environmental cues remain vital in enabling one hundred-plus species of corals and other organisms to orchestrate their spawning on the exact same evening of the year, with each species typically utilizing just a four-hour window in which to spawn. Various environmental cues such as day length and luminance from the moon appear to allow the creatures to synchronize with other members of their own species.23 Other organisms plan their schedules around the same yearly cycle to exploit the coral spawning. Whale sharks make great migrations from across the Indian Ocean to coincide with the spawning at Ningaloo Reef in Western Australia, as the spawn significantly contributes to the sharks’ annual dietary intake. The tourism industry has also developed around the regularity of the spawning and the shark’s arrival.

      Sessile Invertebrates

      A huge variety of sessile organisms, in addition to corals, occupy the hard substrates of reefs, including sponges, bryozoans, tunicates, gorgonians, sea anemones, and corallimorphs (large disc-shaped animals, superficially similar to sea anemones). Some of these organisms are so ancient that they haven’t shared a common ancestor since the times of the very earliest animal life on Earth.

Sponges, one example, are very simple animals that draw water in through their sides using tiny whip-like cells to create a current. Recent research on sponges has found that they may be the missing link that helps to transfer organic matter in the water column into a form that the reef’s inhabitants are able to consume, filling in another missing part of Darwin’s Paradox.24 The cells in a sponge’s filtering system have a very high turnover rate, producing a consumable detritus for reef creatures. In the Caribbean, sponges are more abundant and species-rich than corals and have a huge variety of important roles on the reef, even physically cementing the whole structure together. Some sponges have zooxanthellae, like corals, whilst others have bacteria that help to fix nitrogen into a form that they can use in growth.

      Many other organisms attached to the reef contribute to the diversity of coral reefs. The Anthozoa, which includes not just the corals, but also a wide variety of their relatives such as gorgonians, sea anemones, and corallimorphs are very diverse. Some of these are solitary and consist of a single polyp, such as sea anemones and corallimorphs, which are both known for their noxious stinging tentacles. Gorgonians are considered to be a type of soft coral, and like their closest relatives, do not produce a calcium carbonate skeleton. Gorgonians are largely defined by their fan-like shape, which gives them their common name, the sea fan. Some gorgonians grow to be the size of a car; others form bush-like shapes. Often, the only way to identify them at a species level is to view them under microscopic magnification.

      Sea anemone tentacle detail. Wakatobi, Sulawesi, Indonesia.

Tunicates, sometimes also known as “ascidians” but more commonly known as “sea squirts,” are prevalent on most coral reefs though they are generally small and largely ignored. Sea squirts are simple animals that have a single inhalant and single exhalent opening, where water flows in and out to be filtered of nutriment. One of the most interesting aspects of their biology is that they are the most foundational living relative of the vertebrates. Vertebrates are one of the great branches in the tree of life and include those animals—humans, fish, amphibians, birds, and other mammals—that have a backbone. The link between tunicates and vertebrates has been shown through genetic analysis as well as identification of some very early key features of backboned animals seen in tunicate’s tadpole-like larval forms.25

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