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 It’s amazing to consider that these tiny gelatinous sea squirts could have given rise to all backboned life on Earth.
Bobbit worm waiting to ambush its prey. Anilao, Luzon Island, Philippines.
A playful Maori octopus, the world’s third largest species of octopus, attaining 22 lbs in weight. South Australia.
Unsexy Beasts
In addition to the many sessile invertebrates that are fundamental for coral reef ecosystems that remain rooted to the reef, there are of course many active and mobile invertebrates too. Mollusks, crustaceans (a group of arthropods that includes many familiar species such as lobsters, crabs, barnacles, and terrestrial pill bugs), echinoderms (an evolutionarily distinct group of animals that includes sea stars, urchins, and crinoids), and polychaete worms (a branch of worms with noticeable bristles) are some of the most easily spotted, but they rarely come up when people think about the biological diversity of coral reefs.
Nudibranch (Aegires villosus). Anilao, Luzon Island, Philippines.
Nudibranch (Favorinus mirabilis). Sangeang Island, Indonesia.
Nudibranch (Caloria indica). Raja Ampat, West Papua, Indonesia.
Nudibranch (Glossodoris stellatus). Raja Ampat, West Papua, Indonesia.
Nudibranch (Nembrotha kubaryana). Solomon Islands.
Nudibranch (Tambja tentaculata). Raja Ampat, West Papua, Indonesia.
Worms might be the least appreciated of reef animals, but their role in breaking down coral rock is important for the background functioning of the reef. Feather duster worms filter food from the water and tend not to leave their burrows, but other polychaete worms are much more active. Fireworms scurry about, confident that their irritating bristles will ward off predators. Another worm that appears fearless is the Bobbit worm. Named in honor of the infamous incident in which Lorena Bobbitt cut off her husband’s manhood as he slept and then threw it out of the car window into a field, these terrifying worms emerge from the sand during the night and stand upright with a pair of enormous pincers held open so they’re ready to pounce on any unsuspecting fish. They can reach ten feet in length and are a nightmarish predator for small reef fish. You certainly wouldn’t want to complete your one hundredth dive at night where these worms are found, since the one hundredth dive is traditionally done in the nude and the worm may get confused.
Nudibranch (Chromodoris sp.). Hachijō-jima, Japan.
Nudibranch (Miamira alleni). Anilao, Luzon Island, Philippines.
Nudibranch (Halgerda willeyi). Hachijō-jima, Japan.
Nudibranch (Sakuraeolis nungunoides). Sangeang Island, Indonesia.
Nudibranch (Okenia kendi). Lembeh Strait, Sulawesi, Indonesia.
Mollusks are another group that features prominently on coral reefs. There are many sessile forms, such as giant clams and oysters, but also many mobile groups including thousands of species of sea slugs, snails, little-known groups such as the many-plated chitons, and, of course, the cephalopods. Mollusks are an extremely species-rich group, with 80 percent of mollusks belonging to the class Gastropoda, which includes all snails and slugs. Clearly, the cephalopods, a group of around eight hundred species containing squids, octopuses, and nautiluses, have evolved in a different direction than the slugs. Nautiluses are a fascinating group of spiral-shell-dwelling free-swimming cephalopods. Of the several thousand known fossil nautiloid species, which were once one of the ocean’s dominant marine predators, there are now just a few marginal extant species. Cephalopods have evolved to be some of the most intelligent animals on the reef; certainly, the most intelligent invertebrates. They have highly developed nervous systems and brains as large as some equivalent-sized vertebrates, which have afforded them unparalleled learning and memory.
Peacock mantis. Dumaguete, Negros Island, Philippines.
Nectria
