Benthic Cnidaria by Sean Chamberlin
The Cnidaria, which include the familiar sea anemones, jellyfishes and corals, derive their name from a specialized cell called a cnidoblast or nematocyst, unique to all cnidarians. These cells act like miniature, hollow harpoons to inject poisons into their prey and enemies. Anyone who has been stung by a jellyfish can attest to the power of these cells. Beyond their sting, cnidarians play an enormously important role throughout the world ocean. Corals have been called the “canary in the coal mine” of global warming; along with jellyfish, corals may act as early warning systems for negative human alterations of marine ecosystems.
Because of their radial symmetry (having a symmetrical arrangement around a central point), the cnidarians have been called “the flowers of the animal kingdom.” Certainly, a tidepool of sea anemones with their tentacles waving give the impression of a flower garden and many cnidarians “farm” algal symbionts. Yet most of the 9000+ species of cnidarians are flesh-eating carnivores, feeding on a range of animals from small zooplankton to small fish.
We focus our attention here on two groups of cnidarians: 1) the Anthozoa, a class that includes sea anemones, solitary corals, hard corals, soft corals, sea fans and sea pens, among others; and 2) the hydrocorals, benthic members of the class Hydrozoa, who, like corals, are reef-builders.
The anthozoans (Class Anthozoa) evoke images of faraway tropical isles and crystal clear blue waters. That’s because many anthozoans—especially the reef-building stony corals—inhabit the tropics, where warm temperatures and oligotrophic waters favor their growth and reproduction. Yet equally stunning in their beauty are the deep-sea corals that inhabit the cold and deep waters of the world. These reef-building corals—the shallows and the deep—provide food, protection and shelter to tens, perhaps hundreds, of thousands of marine organisms. Their handiwork even dwarfs that of humans: the Great Barrier Reef in Australia is the most massive structure built by any organism and plainly visible from space.
Given their widespread extent, it should be no surprise that the anthozoans represent the largest class of cnidarians and host a diverse number of species, including sea anemones, tube anemones, solitary corals, stony corals, black corals, blue corals, soft corals, sea fans and sea pens, among others. Equally diverse are the kinds of habitats in which they are found: from the intertidal to the sea floor, from the tropics to the poles. While most attach to hard substrates, The soft-body forms, like the anemones, can burrow in sediments.
Despite the stony exterior of the reef-building corals with which those of us who have visited souvenir shops are most familiar, they are, in fact, built by soft-bodied anemone-like polyps that inhabit the pores and interstices of the calcium carbonate matrix. As such, the anemones provide a good model for the body plan of anthozoans. A typical sea anemone consists of a muscular cylinder with a foot (basal disk) at one end and a flattened mouth (oral disk) surrounded by tentacles at the other. The interior of the anemone may be divided into a series of partitions (septa) in which digestive and reproductive processes can tale place. Food is caught with the nematocyst-containing tentacles and carried to the oral groove where it is moved by a pharynx into the central gastric cavity. Reproduction may be asexual, in which the anemone simply splits in two (self-cloning) or sexual, in which gametes may released by males, females or hermaphrodites into the water (or held in brood pouches). Fertilized eggs develop into a planula which may drift and feed before settling to the bottom.
Reproduction in corals follows the pattern for anemones, except that on coral reefs, like the Great Barrier Reef (and other reefs), the timing and release of gametes is highly synchronized and coordinated among coral species, a phenomenon known as mass spawning or mass broadcasting. From October to December each year, a few to several days following a full moon, more than 140 coral species release their gametes into the water over a period of four to five days. Because the eggs and sperm of different species don’t cross-fertilize, this synchronized spawning is thought to offer “survival in numbers” and to take advantage of ideal current patters that insure the return of the planula to a suitable location. (e.g., Harrison et al., 1984; Wilson and Harrison, 2003).
In corals—hard or soft—the skeleton is excreted by the ectoderm (outer skin layer) in folds of tissue at the polyp’s base. The resultant coral cup exhibits a series of radial ridges called septa. Any number of new polyps may bud from the original, giving rise to a colony of polyps, each of whom build their own cup. In an almost fractal-like way, a colony of polyps creates the architecture of the coral colony type, ranging from the tree-like staghorn coral to the maze-like brain corals to the mushroom-like plate corals. Except for the solitary corals, all corals that you see are colonies of individuals.
While many types of corals are able to build skeletons, the most well-known and the most numerous are the reef-building or hermatypic corals in the order Scleractinia, commonly called the stony corals. These corals require an algal symbiont—a dinoflagellate—to carry out the metabolic processes for building their calcium carbonate skeleton, a symbiotic relationship known as mutualism, where both species benefit. The symbiont not only provides energy in the form of fixed, organic carbon but it also provides essential compounds in the biochemical pathways that enable calcium carbonate secretion. In turn, the coral animal provides essential nitrogen compounds to the symbiont. For these reasons, stony corals are confined to shallow waters where light is abundant and warm waters, typically where temperatures remain above 73° F (although cold-water hermatypic corals are known).
The nature of this symbiotic relationship between corals and algae has been the subject of intense research due to worldwide increases in the incidence of coral bleaching, a phenomenon whereby the coral expels its symbionts. Bleaching is presumably caused by prolonged elevated seawater temperatures but other factors may contribute as well. For reasons that are not entirely understood, higher-than-tolerable temperatures cause a breakdown in the relationship between a coral and a symbiont and the algae are expelled. If conditions improve, the coral may recover its symbionts and survive. If stressful conditions are prolonged, the coral may die. While coral bleaching occurs even under natural conditions, the increasing number of bleaching episodes has led to international efforts to monitor and report conditions where bleaching may occur. NOAAs Tropical Ocean Coral Bleaching Indices web site provides near real-time information and maps on thermal stresses for 24 selected reef “hot spots” around the globe.
While increasing ocean temperatures as a result of global warming certainly play a role in bleaching, a number of other factors also appear to be important, including disease, sedimentation, eutrophication, habitat destruction and species-shifts associated with fishing. The rate of coral bleaching appears to be severe and a number of scientists have predicted catastrophic decline and extinctions of coral reefs worldwide. Nonetheless, continued research will be needed to assess the long-term temporal and spatial effects and the degree to which these communities adapt or exploit new habitats to overcome anthropogenic disturbances.
While the degree to which humans are responsible coral bleaching and the worldwide decline in coral reefs may be debated, the rapid decline in populations of deep-sea corals has only humans to blame. Long-known but only recently appreciated, these cold-water corals, many of them hundreds of years old, have borne the effects of deep-sea trawling for more than a century. Increasing use of this type of fisheries in recent decades has exacerbated an already existing problem. The photographs of deep-sea coral communities before and after trawling provide striking evidence of the destruction these trawlers leave in their wake.
In response to concerns that these organisms were being eliminated before their ecological role or potential benefit to humans could be evaluated, more than 1,136 scientists from 69 countries called upon the United Nations to halt fishing techniques that lead to the destruction of deep-sea corals and sponges. In their statement to the UN, submitted on February 15, 2004, the scientists wrote: “As marine scientists and conservation biologists, we are profoundly concerned that human activities, particularly bottom trawling, are causing unprecedented damage to the deep-sea coral and sponge communities on continental plateaus and slopes, and on seamounts and mid-ocean ridges.” While some nations have already banned these methods in their waters, increased efforts will be needed to insure that deep-sea communities are managed responsibly.
Question ##: What is the role of scientists as advocates for issues of human concern? Can science and advocacy be kept separate?
In modern times, coral reefs have been called the “canary in the coal mine” as a way of expressing the concern that human-induced changes in our global environment threaten the survival of our species. While it is not the role of scientists to decide whether or not this statement is true, science can provide the kind of objective and self-consistent observations that enable citizens and policy makers to make informed decisions. Oftentimes, the answers are not clear cut and definitive conclusions are not possible. However, by educating yourself in the science behind these issues, you can, at least, present an opinion based on the best available facts. Such an approach is far preferable to making decisions based on hearsay or advocacy.
The hydrocorals, well-represented in Florida and California, feature polyp stages that produce hard skeletons of calcium carbonate. Like their cousins, the true corals (Class Anthozoa), these animals are important reef builders, particularly in temperate environments. Off Carmel, California, tree-like colonies more than 100-years old have been found. The hydrocorals also include the encrusting fire corals, some of which can be quite painful to the touch, as experienced by Chamberlin diving off Palm Beach, Florida, as a youth. Hydrocorals also appear to be sensitive to changes in seawater temperatures brought about by climatic events: in Panama, two species of hydrocorals were eliminated (presumably to extinction) by the 1982-83 El Niño. While some hydrocorals produce planula that swim for a few hours, most complete their entire life as benthic organisms.