On coral reefs—where it’s not unusual to find tens of thousands of species and millions of individuals within a few square meters—many animals have co-evolved to live not only alongside each other, but often on top of or even inside one another. This image of a multitude of sponge-dwelling creatures illustrates how complex and multi-layered these reef relationships can become.

Much like the coral heads they grow on, sponges are sessile animals whose sprawling structures not only provide habitat for a variety of species, but can also give rise to complex relationships. Fish can often be found taking shelter in the wavy furrows of large sponges, where they’re protected from myriad predators on the reef. Over time, many of these fish have evolved characteristics that enable them to blend in with the walls of their porous protectors, making them even more difficult to spot. This slender sponge goby (Phyllogobius platycephalops), for example, is translucent, with just a tinge of pinkish-orange, a coloration that allows the fish to all but disappear as it traverses the ridges and canals of the sponge. Unfortunately for this goby, blending in with its surroundings provided little protection from the ghostly parasites clustered around its fins.

Fish and their uninvited passengers aren’t the only animals that take advantage of the reef’s cavernous sponges. Many crustaceans—such as the two crabs that can be seen clinging to the ceiling above the goby—also take refuge within them. Although this relationship between sponge and crustacean is often mutually beneficial, with the filter-feeding sponge taking in food particles sloughed off as crabs and shrimp eat, a recent study found that this is not always the case. Examining the stomach contents from one species of tropical shrimp (Typton carneus), researchers found that the crustacean’s diet largely consisted of sponge tissue from its host. They even suspect that the shrimp’s unique shear-like claws have evolved to aid its spongy appetite.

Whether or not the sponge will evolve a defense against the parasitic shrimp remains to be seen. Other studies have shown that some sponges emit chemical defenses to protect themselves from predation. Indeed, this type of evolutionary arms race is what has produced much of the biodiversity for which coral reefs are known. As a species evolves and adapts, its symbionts—both beneficial and harmful—do as well. Eventually this co-evolution can lead to entirely novel species branching off to fill a new ecological niche, adding another layer of complexity to life on the reef.

Balicasag Island, The Philippines

David Fleetham

David Fleetham is one of the most published underwater photographers in the world, with more than two hundred magazine covers to date. His award-winning work has been displayed in museums around the globe. Fleetham began diving and photographing underwater in 1976, first in the cold, rich waters of British Columbia, and since 1986, in Hawaii. He is a founding member of The Ocean Artists Society.

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