SEA ANEMONES ARE PROBABLY BETTER KNOWN FOR THE COMPANY THEY KEEP than for their unassuming, hard-working selves. On Caribbean dives, I rarely pass by one without checking it out for exotic little cleaner shrimps that might be in residence. If there aren’t any, I’m disappointed and move on.
In the Pacific, of course, you hardly have to check them out to be aware of their frenetic, constantly on-the-move, high-visibility companions – clowns and other anemonefishes.
Except, what about the sea anemones themselves? They’re low-keyhard-working animals, as different from our usual conception of animals as may be. Don’t they deserve attention, too? For that matter, what are sea anemones? Time for sea anemone facts!
FOR US, NOT SO MUCH STING
Sea anemones are members of Phylum Cnidaria (literally, “nettle,” as in “stinging animals”), along with corals, jellyfishes, hydrozoans and other stinging animals. Stinging-wise, they’re geared to taking on microscopic zooplankton and other small stuff, so few of them represent a stinging threat to us.
Stick your finger into the tentacles of a giant sea anemone (Condylactis gigantean) and it’ll feel more like sand paper than a sting. (Note: In the “Don’t Touch the Reef School,” you probably shouldn’t do this. There are some species of sea anemones that can hurt.
TENTACLES, LOTS OF TENTACLES
They’re in Class Anthozoa (“flower-like animals,” along with corals). But sea anemones occupy their own order, Actinaria (“rays”). They’re sort of coral polyps writ large.
They have much more prominent and numerous tentacles, ranging from dozens to the hundreds. They lack the hard calcium carbonate exoskeletons stony corals enjoy. And, they live solitary lives, scattered around the reef, often backed into protective crevices, not like corals as interconnected colonial communities.
Land-focused naturalists long ago gave them the name anemone because of a fanciful resemblance to the graceful garden flower of that name. For a while early naturalists did think they might be plants.
SEA ANEMONE FACTS
- There are 1,000 or so species of sea anemone species found in oceans worldwide, in habitats deep and shallow, tropical, temperate and cold.
- Most sea anemones earn their livings filtering microscopic plankton from the passing current. But many are also active carnivores that catch and devour prey as diverse as small fishes, crabs and loose mollusks that may be swept their way. Some larger species can feed on bigger game, like starfishes and sea jellies.
- Fair is fair: They in turn are subject to predation by starfishes, slugs and fishes like eels, flounders and cods.
- Sea anemones share the basic Cnidarian structure of a cylindrical, sac-like body topped with an oral disc – a surface with a single opening for both ingestion and expelling body wastes – surrounded by tentacles.
- With so many species and habitats, they vary widely in sizes, colors and anatomical specifics. They can range in size from less than an inch to five or more feet in diameter.
WHAT ARE SEA ANEMONES, AGAIN?
- Their bodies can be short and thick or long and narrow. Same with the tentacles – long, short, knobby, curly.
- Anemone’s columnar bodies are supported by pedal discs, bases stuck to hard surfaces like dead coral, rocks, sponges, pier pilings and sea shells. Sometimes the shells are the ones hermit crabs carry around with them as mobile homes. If the hermit changes shells, it transfers its anemone to go along with it.
- Sea anemones’ often feature tentacles arranged randomly. But they may be formed in discernible circles. Many species have tentacles laid out in multiples of six.
- Tentacle colors can vary from off white and pale yellow to blues, lavenders and greens. In the species Condylactis gigantean – “giant anemone” – it’s the tips that may be strikingly colored.
With a lot of sea anemone species, it’s unusual to see the body beneath the animal’s multitude of tentacles. You may catch glimpses of broad bodies in species like the Indo-Pacific’s Heteractis magnifica – “Magnificent Sea Anemone.” It has a predilection for locating itself in areas of strong currents or surges that rattle its broad, flat body.
But in a Caribbean species like C. gigantean – the giant anemone, although in this case “giant” is about 12 inches across – the tentacles are the whole show. These guys usually settle in crevices that give their bodies protection while they extend their tentacles into the current.
With tentacles armed with cinidarian nematocysts – tiny, stinging “harpoons” – they grab passing prey, paralyze it and forward it toward the mouth. During her Touch-the-Sea diving programs on Bonaire, the marine naturalist Dee Scarr used to feed them chunks of hot dog to demonstrate how they grasped onto stuff and ferried it toward digestive doom.
It’s impossible to talk about sea anemones without talking about all the creatures that live in symbiotic relationships with them – among them, cleaner shrimps, little crabs and anemonefishes. In the concept of symbiosis, these are commensal relationships. Both anemones and critters benefit from them without harming the other.
The faithful fish and critter companions benefit, of course, with protection from predators, gained from life within an anemone’s jungle of stinging tentacles.
The shrimps – spotted cleaner shrimps (Periclimenes yucatanicus), Pederson’s cleaning shrimps (Periclimenes pedersoni) and squat anemone shrimps (Thor amboinensis) – often require careful searching to spot. They feed on detritus and tiny planktonic organisms that get caught in the sea anemone’s mucus, contributing the host’s health.
While doing this, they carry on their main business of waving their own little tentacles to attract passing fish in search of cleaning services – the removal of their own parasites and dead and diseased scales.
The stars of the sea anemone show are the clownfishes, false clownfishes and other members of the anemonefish clan in the Indo/Pacific basin. Picking plankton from the passing currents, the little guys swim tirelessly back and forth in the tentacle sea like hyperactive kids in a play area ball pit, protected from their hosts’ stings with a thick coat of mucus.
In return, they provide tentacle-cleaning services of their own, plus a nutritional boost from nutrient-laden anemonefish poop. Once established in a sea anemone, an anemonefish will spend its entire life there. And, of the 1,000 sea anemone species, it should be mentioned that they do this in only 10 of them.
Which can mean that more than one species of anemonefish can be found in a single sea anemone. It’s likely to be territorial within the anemone, with each shrimp, crab and brittle star inhabiting its own part of the host.
MORE SEA ANEMONE FACTS
- Some shallow-water species augment their predatory diets with the nutritional glucose boost provided by zooxanthellae, the photosynthetic dinoflagellates that embed themselves in the anemones’ tissues just as they do with coral polyps. This can account for greenish or yellowish tints.
- Sea anemones can move, but not far, and very slowly. In fact, they creep so slowly their movement is hard to discern just by watching it. In times of stress, they can inflate themselves and float to a new location. But mostly, once located in a desirable spot, they stay there. They’re considered sessile, or animals attached to the sea bottom.
- When disturbed, they can circle the wagons by pulling their tentacles in and shaping their bodies into a tight ball.
- Sea anemones can be male or female, or male and female – that is, hermaphroditic, capable of producing both sperm and eggs.
- In any event, they make new sea anemones by broadcast spawning, releasing their sperm and eggs into the water column in a coordinated mass. Considering the relatively low density of anemone populations on the reef it does appear that the survival rate for anemone larvae is somewhat low.
- On the other hand, some of the larger species are believed to live for as long as 100 years.
- Recent research has determined that sea anemones and land-based plants do in fact share some genetic code, possibly a remnant of a far-distant common ancestor before plants moved onto dry land.
- All of this relates to sea anemones in Order Actiniaria. Tube-dwelling anemones (Order Ceriantharia, “wax flowers),” are a different group of animals that live not on the sea bottom but in it. They hide in tubes concealed in the sediment, extending their tentacles to feed only at night.
PRINCIPAL SOURCES: Marine Biology, Peter Castro, Michael Huber; Coral Reef Animals of the Indo-Pacific, Terrence Gosliner, David Behrens, Gary Williams; Reef Creature Identification, Florida, Caribbean, Bahamas, Paul Humann, Ned DeLoach; Marine Life of the North Atlantic, Andrew Martinez; Indo-Pacific Coral Reef Field Guide, Gerald Allen, Roger Steene; Anemones, National Aquarium; Giant Caribbean Sea anemone, Oceana; Fact Sheet: Sea Anemones, Marine Biological Association; Sea Anemones Are Half-Plant, Half-Animal, Gene Study Finds, Live Science; Sea Anemones, Encyclopaedia Britannica; Sea Anemone, Wikipedia.org.