The Coral Continuum

Stony corals, like this green star, can take many shapes -- knobby, mountinous, encrusting.
Stony corals, like this green star, can take many shapes — knobby, mountinous, encrusting.

WHEN PEOPLE SEE THE WORD “CORAL,” IT VERY LIKELY BRINGS TO MIND THE GREAT MOUNDS OF STAR AND BRAIN CORALS that dominate the reefs. In fact, “corals” include many organisms beyond the familiar stony formations.

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And “coral” itself is a flexible word. It applies to the coral exoskeletons that we see as the visible shells of hard corals, to the polyp animals that live within those exoskeletons – and to the similar polyp animals that make up other colonial polyp organisms.

Soft Coral Phil 2047
Delicate soft coral from the southern Philippines

THE “SOFT CORAL” CONUNDRUM

“Soft coral” is also a somewhat loose term. It’s used for the specific animals in the Order Alcyonacea. But it’s also applied broadly to all coral polyp animals that aren’t stony – alcyonaceates, gorgonians, leather corals, others. Whereas hard corals build reefs, soft corals are colonies of animals that live on top of the reef structures. While they only marginally help build reefs up, the broad range of soft corals, gorgonians and others help give reefs their diversity, are frequently quite beautiful and often adorn them exquisitely.

THE POLYP STRUCTURE

NOAA drawing

Even though they measure just a few millimeters across, corals are animals: they’re multicellular organisms that have nervous systems and digestive tracts, and sustain themselves by feeding on other organisms — overwhelmingly the plankton grabbed from the passing soup of algae, eggs and other tiny stuff. They’re colonial organisms that live in more-or-less organized groups of hundreds to thousands of individuals. They’re sessile in that once they’ve passed through the larva stage they’re fixed firmly to the bottom, or something that passes for the bottom, like the built-up skeletons of dead corals or the sides of piers or ship’s hulls, or other solid pieces of debris.

 

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In terms of body shape, coral polyps are symmetrically radial – a central body sac containing a mouth and gut, surrounded by tentacles to capture food. Polyps are round and symmetrical horizontally but vertically they have an oral surface, where the mouth is, and an aboral surface on the other side.

The mouth is the opening to a coral polyp’s gastrovascular cavity – essentially, its stomach – in which food is digested. The mouth is its only opening; waste is expelled through the same channel.

STRATEGIES FOR LIFE, PART I: THE STING

All coral polyps are members of Phylum Cnidaria, a term from the Greek word for “stinging,” which is what cnidarians do. They sting with structures on their tentacles called nematocysts. Sea jellies are cnidarians as well, with the basic polyp body structure – but they’re not corals. Jellies and fire coral stings might hurt you, but most coral nematocysts are scaled for microscopic prey and wouldn’t be a problem for you if you touched some. Which you shouldn’t do. Ever. The touch of  sea anemones (larger cnidarian polyp animals but also not corals), feels like being rubbed by very tiny velcro. Well, most of them. Since some may hurt, you probably shouldn’t touch them, either.

STRATEGIES FOR LIFE, PART II:  STONY CORALS AND ZOOXANTHELLAE

While corals use their tentacles to snatch plankton from the current for nourishment, most stony coral polyps also get help from zooxanthellae, symbiotic algae that live within their tissues. Stony corals are stony because they secrete calcium carbonate; zooxanthellae supply about 80 percent of their hosts’ nutrients and ramp up this process – and reef-building – immensely. Corals that can do this – build reefs – are called hermatypic. The small proportion of stony corals that don’t secrete calcium are called ahermatypic.

STRATEGIES FOR LIFE, PART III:  Calcium-free Filtering

A sea fan at Bonaire
A sea fan at Bonaire

Soft corals (the alcyonaceates), gorgonians and other variants may also host zooxanthellae, but they don’t secrete calcium and they don’t contribute significantly to the reef mass. Instead, their strategies involve different structures that maximize filtering opportunities by being stationed prominently in the water column, anchored to hard substrates by structures called holdfasts.

Soft corals – that is, alcyonaceates — create branching forms of fleshy stems supported by tiny calcareous spicules. Gorgonians are often built on internal rods of a hard protein material called gorgonin, reinforced by spicules. They include long, thin sea whips, bushy sea plumes, sea rods and flat sea fans, all supporting colonies of polyps that filter feed in the current.

Some gorgonians contain zooxanthellae that support their nutrition, but many varieties don’t. It’s actually the spicules that give many soft corals and gorgonians their colors. As for zooxanthellae, the zoox in sea plumes, for example, render them brown.

These leather coral polyps from the Great Barrier Reef clearly show an octocoral's eight tentacles.
These leather coral polyps from the Great Barrier Reef clearly show an octocoral’s eight tentacles.

ONE MORE THING: HEX’S AND OCTO’S

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Stony corals are hexacorals (Subclass Hexacorallia) – they have six symmetrically arranged tentacles. The polyps of nearly all non-stony corals have eight tentacles, which makes them octocorals (Subclass Octocorallia).

Nobody really goes  around calling stony corals hexacorals but since the octocoral grouping is so broad, the term “octocorals” is used a lot to describe all of them. On this blog, “soft corals” refers specifically to the alcynaceates. I use octocorals to refer to the group as a whole. When I talk about coral polyps, I generally make sure to use the word polyp somewhere.

Principal Sources: National Oceanographic and Atmospheric Administration; Life and Death of Coral Reefs, Charles Birkeland, Editor; Marine Biology, by Peter Caruso and Michael Huber; Touch the Sea website, Dee Scarr’s Always Be Careful campaign; Coral Reef Alliance; Other Sources.

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