News / Species Spotlight / Ceriantharia: More Than Just A Tube (03/13/19)

Ceriantharia: More Than Just A Tube

Ceriantharia: More Than Just A Tube
The humble tube anemone is a deceptive creature. Given its overall appearance, one could easily mistake a specimen as being just another sort of sea anemone, but the two groups could hardly be any more different from one another. The morphological similarities are only skin-deep, and, in fact, tube anemones have no immediate relatives.

In most recent classifications, tube anemones are classified in their own subclass, Ceriantharia, alongside (but distinct from) the octocorals and hexacorals. To understand what makes them so unique, we only need look to their namesake tube, constructed from layers of mucus and sediment. This is held together by the discharged threads of a specialized organelle known as the ptychocyst, which is more or less like a non-stinging version of a corals nematocysts.

If we take a peak at what lies under the proverbial hood, well find no shortage of unique features. The body of a tube anemone (the part hidden within the tube) is elongate and ends in a small pore, the function of which has seemingly never been investigated. Contrast this with a sea anemone (Subclass Hexacorallia, Order Actiniaria), which typically ends in a muscular foot (or, in some of the burrowing varieties, a bulbous swelling). In either case, there is no pore.

Internally, things get weirder. A cross-section of a sea anemones body will reveal a number of thin partitions (mesenteries), radiating inwards like the spokes of a wheel. As an anemone grows, new mesenteries are added in the interspaces of old ones, and this holds true for groups like stony corals, corallimorphs, black corals, and zoantharians. But not with tube anemones! These instead have whats called a multiplication chamber on one side of the body, and its here and only here that growth occurs. As a tube anemone grows, two new mesenteries, corresponding to two new tentacles, emerge at once.

Speaking of tentacles, a tube anemone is easily recognized by the unique arrangement of these. Along the perimeter of the oral disc we find the marginal tentacles, and towards the center, surrounding the mouth, we find the labial or oral tentacles. Both of these are arranged into pseudocycles that correspond to the relative length of the mesenteries and typically follow a repeated pattern. For example, a species might have mesenteries that are arranged LONG-SHORT-long-short, with the first couple being longer than the second. And the effect of this can be seen externally, as the tentacles themselves project at different angles. The common reef species in the family Cerianthidae always have a large number of tentacles, generally arranged into four pseudocycles, while the less commonly seen (and mostly nocturnal) species in Arachnactidae only have two pseudocycles and thus appear relatively flat.

There are major developmental differences as well. In anemones and corals, the larvae take the form of a ciliated ball known as a planula, which after a short period of time settles onto the bottom to begin life as a polyp. But tube anemones offer a different approach entirely. The larvae from this group are instead quite long-lived, lasting for several months, and early on in their development both marginal and oral tentacles appear, which are used to capture prey while floating about in the plankton. In effect, they are functioning as a type of jellyfish in their youth, only later transitioning to the benthic lifestyle of a coral.

These planktonic juveniles have often been scientifically described as distinct species; however, its never been shown that any ceriantharian species can actually complete its full life cycle entirely within the plankton. This likely means that we have separate scientific names for the juveniles and adults for many tube anemones, and, in fact, there are more known species of planktonic juveniles than there are adults. This surplus of juvenile species may correspond to a number of undescribed species that have been documented, some from coral reefs and others from the deep sea. The true diversity in this group can only be guessed at, but, when compared to other coral groups, there simply arent that many tube anemones out there, perhaps around 100 in total.

Identifying the species in this group is notoriously difficult owing to the general similarity in form that most of the larger ceriantharians possess. This is particularly true for those found on Indo-Pacific reefs, as theres been remarkably little study done in the past century or so. Youll find names like Cerianthus and Pachycerianthus used with wild abandon within the aquarium trade and even in some field guides, but these are more often than not simply guesses. Even the experts cant identify these without dissection.

Only one species seems to be common on Indo-Pacific reefs (and likewise in aquariums), identified as Pachycerianthus magnus in the scientific literature. It is most often found with bright neon colors, but its variability can cause confusion. The marginal tentacles seem to come mostly in one of three different colors (red, clear, dark purple), while the labial tentacles usually offer a stark contrast, being either a light yellowish-green or a dark purple. Also of note is the faint patterning seen on the marginal tentacles, present as two parallel stripes interspersed with widely spaces spots. These spots are fine pores called cinclides that are thought to allow water to quickly escape during retraction into the tube. This is yet another unique feature in this group.

In intertidal locations and other sandy habitats away from reefs, Cerianthus filiformis is often the dominant tube anemone. This species tends to grow a bit larger (though with a similar number of tentacles, roughly 150). Its color scheme is more muted, coming in various pastel shades and seldom with much contrast between the marginals and labials. There are often transverse bands near the base of the marginal tentacles, and in some specimens there are purple tips. Another possible Cerianthus from the Indo-Pacific has a much lower tentacle count (around 80) and occurs in heavily silted habitats.

The small family Arachnactidae is a common but little-known group with a very different look. Unlike the moplike tentacles of a cerianthid, this family has far fewer (usually less than 50) aligned around the perimeter of the oral disc. In Arachnactus, the oral disc is held elevated off the substrate, and, in life, these look similar to a deep sea crinoid. One of the most spectacular of all tube anemones is an undescribed purple and white species from this genus known only from the Philippines. The last genus to discuss is Isarachnanthus, which lays flat along the bottom, emerging only at night. Two species exist in the Indo-Pacific, I. bandanensis (with a brown ring on the oral disc) and an undescribed species that is mostly clear.

In aquariums, the only species that ever seem to be available are the showy, dramatic cerianthids, in particular, the brightly colored Pachycerianthus. Arachnanctids are virtually unheard of in captivity, owing to their reclusive nocturnal habits. Isarachnathus has been documented to feed on fishes, so it may be for the best that this genus is unavailable, but other tube anemones appear to mostly entrap passing zooplankton and pose little risk. Unfortunately, misinformation abounds among aquarists, with many professing that tube anemone are murderous stinging beasts that will kill any and all fish. They are not.

Ceriantharians are, as far as known, all azooxanthellate, necessitating that they be fed semi-regularly in captivity. A diet of frozen shrimp or chopped fish or any similar meaty food will suffice. Water flow should be relatively gentle, as brisk currents can prevent the proper extension of the tentacles. The tube can be either buried in sand or positioned among an aquariums hardscape or even propped up with some PVC tubing. Specimens do sometimes evict themselves from their tube, but this is little cause for concern, as a new tube can quickly be produced.