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Underwater urbanites: sponge-dwelling snapping shrimps are the only known marine animals to live in colonies that resemble the societies of bees and wasps

Natural History, Dec, 2003 by J. Emmett Duffy

Diving the pellucid waters of the Caribbean Sea off the coast of central Belize, down past jewel-like transparent plankton, I see the ridge of the Belize Barrier Reef materializing out of the turquoise depths. Even before the reef becomes visible, I sense its proximity from the muffled crackling that issues from the submarine landscape. In places the crackling is so vigorous it sounds like frying bacon. The noise is the clamor or countless little "snapping shrimps" (also known as "pistol shrimps"), so named for the report each one produces by swiftly closing its disproportionately large and powerful fighting claw. The chorus of snapping and crackling is the sound of homeland defense.

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Although rarely seen, snapping shrimps are one of the great success stories of the Earth's tropical seas. Hundreds of species--all members of the family Alpheidae--and millions of individuals pack into the reef's innumerable nooks and crannies, even lodging in the delicate arms of feather stars [see photograpgh on page 43]. Where snapping shrimps truly flourish, though, is within the internal canals of the living sponges that pepper the reefs. Sponges often exceed even corals in both abundance and species diversity. And throughout the Caribbean, sponge canals--akin to the interior of a Swiss cheese--are teeming with snapping shrimps of the genus Synalpheus.

More than a hundred species of synalpheids have been described worldwide, ranging from the size of a rice grain to the size of a baby carrot. Some forty of the species are native to the Caribbean; most of them are ecological specialists that inhabit the passage-ways of one or, at most, a few sponge species. Their apparently unrequited dependency makes the shrimps parasites: they scrape their sustenance from the linings of their host sponges' canals with a small, specialized claw. (In fact, as the synalpheids live out their lives within a given host, their stomachs become packed with the delicate spicules that from the sponge's skeleton.) Because the inner architecture of their hosts provides hot only safe shelter but also a permanent food supply--and because the shrimps are undeterred by the sponges' formidable defensive chemistry, which foils most other predators and invaders--synalpheid populations have expanded to fill nearly every cubic centimeter of the sponge canals.

Having successfully escaped the reef's ubiquitous predators by occupying its living fortresses, sponge-dwelling snapping shrimps face a challenge Familiar to crowded urbanites everywhere: stiff competition for space. Several years ago, in an effort to understand why many reef animals adopt symbiotic lifestyles. I conducted a census of Caribbean sponge inhabitants. And I was puzzled, as several workers had been before me, by the paucity of female shrimps. I well remember the night back in 1988 in Panama when, bent over a microscope, I suddenly realized that the shrimp aggregations I was studying were not merely deficient in females. They comprised exactly one breeding female per sponge, even though the aggregations were made up of as many as sixty individuals. That was my first tantalizing clue that a few species of shrimps are social creatures, living in organized colonies--one colony to a sponge. This way of life--organized, cooperative defense of the community's turf--had previously been unknown among marine animals. Snapping shrimps, it turns out, are living the lives of social insects; they are the ants, bees, termites, and wasps of the deep.

Until quite recently, the retiring lifestyles and puzzling taxonomy of Synalpheus made the fascinating biology of snapping shrimps largely inscrutable, despite their abundance. Even my recognition that they live remarkable social lives came about entirely by accident. In retrospect, one can see why evolution might favor such close social relationships. By working together, a group of diminutive shrimps transcends the limitations of size and, with the aid of the single massive fighting claw sported by each individual, musters a formidable resistance against would-be intruders.

In our next several years of fieldwork my colleagues and I showed that at least rive Synalpheus species live in tightly packed colonies. Most colonies have just one breeding female each, though in a few colonies we identified more than one. All other members are most likely either males or sexually undifferentiated juveniles. But in S. regalis--a species found so far only on the Belize Barrier Reef--the analogy with the social insects is particularly close. A colony always includes just one breeding female and (judging by genetic evidence) one dominant breeding male, even though it tan have as many as 350 members. That genetic structure makes most of the members of the colony into full siblings: offspring of a single breeding pair that reigns as "queen" and "king" for most of the colony's life.

Such collections of kinfolk arise because the social species of synalpheid shrimps exhibit "direct development": their eggs hatch directly into crawlers rather than into the planktonic swimmers that typically hatch from the eggs of their close crustacean relatives. Born into a suitable sponge, juvenile synalpheids needn't travel far to fulfill their needs. Both observational and genetic evidence indicates that the juveniles of direct-developing Synalpheus species typically remain for an extended time--perhaps for life--in the sponge of-their birth. Periodically a few young adult snapping shrimps probably strike out in search of a new domicile, but in general, a colony of S. regalis is essentially a two-parent family with a whole lot of grown male children hanging around.

Underwater urbanites: sponge-dwelling snapping shrimps are the only known marine animals to live in colonies that resemble the societies of bees and wasps

Natural History, Dec, 2003 by J. Emmett Duffy

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