True soft corals are among the most beautiful gems of the reef, yet they’re difficult to get a handle on etymologically. People insist on calling other stuff, like sea fans and mushroom corals, “soft corals.” Species in Family Nephtheidae deserve to be appreciated and understood by themselves.
TRUE SOFT CORALS ARE BEAUTEOUS TO BEHOLD AND DIFFICULT TO DEFINE. They’re beautiful in their delicate multi-branched structures, feathery clusters of polyps and striking pastel reds, yellows, blues, snowy whites and other colors. Sometimes called the “wildflowers of the sea,” they don’t build reefs so much as adorn them.
They’re challenging to explain because the term “soft coral” is so broadly used for everything not a stony coral that these “undersea wildflowers” can get lost in the discussion. They’re too-often stuffed into an all-inclusive soft-coral catchphrase with other non-stony corals like leather corals, blue corals, sea pens and sea fans, sea plumes and sea whips.
NEPHTHEIDATES – THE “LADIES OF THE HOUSE”
The true soft corals, the “wildflowers of the sea” ones, are just one category in this spectrum. Specific species have street names like “cauliflower coral” and “umbellate tree coral,” a takeoff on their umbrella shapes. But there are so many species that most live with just scientific names, like Lemnalia cervicornis.
Often, they’re so hard to tell apart that on the reef they can only be identified by genus, not species. They’re some of the loveliest creatures there, yet, maddeningly, there isn’t a widely accepted common-name for them other than “soft corals,” sometimes “tree corals.”
They’re located in Family Nephtheidae (from the Greek for “lady of the house”). For the group as a whole, we’ll just go with “true soft corals,” “tree corals” and Nephtheidates, a sort of made-up word.
MANY SPECIES, HARD TO IDENTIFY
There are hundreds of soft tree coral species in some 90 genera, found worldwide from the Arctic to the Antarctic. Most that divers are likely to see are denizens of the Indo-Pacific tropics. Only a few species are found in the Atlantic/Caribbean basin, and those are generally found at depths of 130 feet or more.
The dramatic Dendronephthia genus contains 250 species alone, often variations of reds, yellows and blues. Other prominent groups include the Lemnalia, Nephthea and Litophyton genera, (including the aforementioned cauliflower variety). Their species are often characterized by thinner stalks and creamy whites, tans and pink colors.
Identifying them by species is difficult, so much so that even reef-guide authors are sometimes hard put to be definitive on the reef. Positive identification often requires taking specimens to a laboratory and examining them under a microscope.
TRADITIONAL POLYPS, DIFFERENT APPROACH
In individual structure, true soft coral polyps mirror all other coral types – asymmetrically radial central body sacs measured in millimeters that contain a mouth and gut surrounded by stinging tentacles that capture food from the passing currents. In contrast to six-tentacled stony corals, they’re octocorals, with eight tentacles.
They’re colonial animals that collectively create complex living structures. However, instead of securing themselves inside protective corallite cups like stony corals, soft corals form thick trunks and slender branches of fleshy tissues with polyps clustered on the surface, often at the branch tips. In contrast to the smooth tentacles of stony corals, flowery soft coral tentacles feature tiny pinnates, or side branches. These contribute to Nephtheidates’ feathery, flower-like appearances.
While they lack hard exoskeletons, their structure is reinforced by skeletal spicules, small slender pointed calcareous or siliceous structures embedded throughout their tissues (similar spicules are found in sponges). The density and arrangement of spicules within a specimen is often a key factor in identifying species in the lab.
THE ZOOX BOOST – NUTRITION & COLORS
Like other coral polyps, many Nephtheidates live with the single-celled symbiotic algae called zooxanthellae embedded within their tissues. Like other corals, they filter microscopic zooplankton from the current for food. But the photosynthetic zoox give them both a nutrient boost and their striking colors. Still, true soft tree corals do better in nutrient-rich murky waters than do stony corals.
Perhaps because it’s easier to create calcium carbonate spicules than calcium carbonate corallites, soft tree tissues grow much more quickly than stony corals. They’re able to double and even triple the size of their colonies over a the course of a year.
They reproduce in the standard coral way: the release of eggs and sperm into the water column and eventual settlement of a single larva on a suitable substrate. Then that single polyp begins a cloning cascade, reproducing itself into a complete colony of cloned, identical animals.
DON’T MESS WITH NEPHTHEIDAE
Since a key fact of life on the reefs is that there’s always something willing to eat something else, you might suppose that extending tentacles into the surrounding waters would be an invitation to grazing by fish and other critters. Soft corals don’t have that problem.
Basically, while some nudibranchs and other snails may dine on true soft coral tissues, our Nephtheidate friends have developed chemical defenses that make them toxic or bad-tasting to predators.
DON’T CROWD, EITHER
And, unfriendly to neighbors and fellow travelers. To safeguard their space, they can release these defensive chemicals into the water to prevent other corals from getting too close.
And while you may see shrimps, decorator crabs and brittle stars establishing residence on soft coral trees, these chemicals keep algae, sponges or tunicates from gaining footholds on their surfaces.
ALSO, TRUE SOFT CORALS ARE MADE FOR WALKING
One thing that sets Nephtheidates apart from other corals is that they’re not totally sessile. By extending the tissues at their bases, they can move, albeit quite slowly. Thus they can shift into new spaces on the reef – killing stony coral polyps in the way as they go.
NOTE TO HOME AQUARIUM ENTHUSIASTS: BEWARE!
All of this makes them a questionable choice to place in aquarium tanks with other marine life. Keeping them happy and in peaceful coexistence with other animals can be challenging.
On the other hand, these chemical dynamics have a plus side: Medical researchers have looked hard at thems for applications in human medicine.
PRINCIPAL SOURCES: Marine Biology, Peter Castro, Michael Huber; Indo-Pacific Coral Reef Field Guide, Gerald R. Allen and Roger Steene; Coral Reef Animals of the Indo-Pacific, Terrence M. Gosliner, David W. Behrens, Gary C. Williams; Reef Coral Identification, Paul Human, Ned DeLoach; Fact Sheet 16 – Soft Coral, Marine Education Society of Australia; Spot the Difference Between Hard Coral and Soft Coral, reefdivers.io; Soft Corals, Wild Singapore; Corals Without Skeletons, Living Oceans Foundation; Sea Raspberry, National Oceanic and Atmospheric Administration; Dendronephthya, Wikipedia.org; Lemnalia Soft Coral, SaltWaterFish.