The World Beneath. Richard Smith
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 saoria sea star. Tasmania, Australia.
Blue sea star in the shallows. Raja Ampat, West Papua, Indonesia.
Pink-eared mantis. Dumaguete, Negros Island, Philippines.
Also among the reef’s active invertebrates are the arthropods, segmented animals whose more familiar members include insects, spiders, and centipedes. In the oceans, arthropods are equally diverse and include crustaceans such as crabs and lobsters, as well as the little-known pycnogonids, known as “sea spiders,” and horseshoe crabs. The sea spiders and horseshoe crabs are unusual and ancient branches of the group, rarely seen and minimal actors in coral reef ecology. The crustaceans, however, are a very important group, both in their free-living forms and as parasites, as we’ll learn in a later chapter. Barnacles, the familiar white mottling seen within the splash zone of the rocky shore or on the chins of whales, are nonetheless unexpected members of the crustaceans, and more recognizable as crustaceans in their larval form; as adults they remain cemented to the rock and rely on filter feeding. Shrimps, lobsters, mantis shrimps, and many types of crabs are very common on coral reefs and range in size from tiny species almost invisible to the naked eye to huge lobsters.
Pearlfish. Lembeh Strait, Sulawesi, Indonesia.
Echinoderms are the final group of dominant invertebrates and are entirely marine. They are simple and ancient animals with five main classifications: the crinoids, sea stars, brittle stars, urchins, and sea cucumbers. Among these various groups, echinoderms play a very significant role on coral reefs. Urchins furiously graze away algae; crown-of-thorns sea stars form plague proportions that can wipe out entire coral reefs; and sea cucumbers feed on the bacteria that cover sand grains. Echinoderms also provide homes for a huge array of other organisms. One of the most shocking examples is the pearlfish that lives inside the body cavity of sea cucumbers. They are a rod-like transparent fish that emerge from the cucumber’s anus at night to feed, and they pop back in during the day to hide inside the rarely predated echinoderm.
Coral reefs are composed of organisms from some of the most disparate branches from across the tree of life, all wrapped