WITHOUT A BACKBONE		BY RONALD SHIMEK Ph.D.

RON SHIMEK

Another planktonic tentaculate ctenophore, Bolinopsis. This animal’s tentacles are small and impossible to see in the photograph. The animal is about 3 centimeters (1.2 inches) long.

Comb Jellies

They have a gelatinous body and because of this, they were initially taxonomically placed within the phylum Cnidaria (the corals and their many relatives). However, they were placed in their own unique and separate phylum by Hatschek in 1889. By about 1975, this change had made it into most invertebrate zoology texts, but one still finds an occasional basic biology text and a lot of the popular literature lumping them with the cnidarians as “coelenterates.” Most zoologists now consider the similarities to cnidarians to be the result of convergent evolution and basically adaptations for a planktonic life-style.

Although appearing superficially like cnidarians, they differ in several critical and important ways. Recall that one of the characteristics of the cnidarians is the presence of stinging capsules they use to kill and attach to prey. Ctenophores lack nematocysts and capture their prey by means of specialized glue cells called “colloblasts.” Unlike nematocysts, which are non-living cellular products, the colloblast is an actual living cell that disintegrates upon contact with a prey item to produce a sticky filament.

Interestingly, while no ctenophore secretes nematocysts, one species, Haeckelia rubra, uses them. It preys upon hydroid medusae and obtains nematocysts from its food. The ctenophore then uses the nematocysts from one prey individual to capture and eat others. Now, how does it catch that first medusa?

They move by paddling rows of fused ciliary plates called “ctenes” (or combs, as they look rather like combs), which are arranged in eight rows in adult animals. These large, fused cilia are secreted by the underlying epithelial cells and function like the banks of oars in an ancient galley ship. This type of locomotion is found only in the Ctenophora and is in decided contrast to the locomotion of cnidarians, which swim by muscular action.

Most cnidarians form a skeleton; if not the massive calcareous skeleton of corals, at least they form a proteinaceous skeleton. Not only do ctenophores lack any hard skeleton, they don’t even possess the cells that could make skeletons.

Additionally, while cnidarians only have a gut with a single opening (which may be called either the mouth or the anus, I suppose, depending on which way materials are moving), ctenophores have a complete gut with separate regions, not just a sac with a single opening. In fact, they go most animals several better, having a gut with several anal pores, not just a single anus. If a ctenophore could be anally-retentive, it could take that condition to new “heights.”

The gut has a throat-like region called the pharynx, which opens into a wider area where it gives rise to, typically, eight gut pouches that are generally arranged in symmetric groups around the perimeter. Generally, one gut pouch is found under each row of the locomotory structures called “ctenes” (presumably so digested nutrients can easily diffuse to the cells, directly above them, which need the energy to move the ctenes). The elongate, slit-like mouth is oriented at 90 degrees to the tentacle axis. Digestion starts in the pharynx; however, the food slurry is transported to the gut pouches and most digestion is intracellular in the gastrodermal cells lining the pouches.

Although superficially appearing to be spherically or radially symmetrical, most ctenophores have a bilateral symmetry, with two distinct sides. Others have a symmetry that is in some regards similar to that of anthozoans, where they are basically radially symmetrical, but with a few minor alterations of the body plan that give the appearance of dividing the animal into two halves.

Most ctenophores have two, generally branched, often long and exceptionally contractile tentacles. These tentacles have been called the most contractile structures in the animal kingdom. A common ctenophore in many oceans is the “sea gooseberry” (Pleurobrachia bachei). This animal seldom exceeds 2 cenitimeters (3/4 inch) in diameter, but it can have tentacles that may reach lengths exceeding 60 centimeters (2 feet), and that can contract to about 5 millimeters in length. Unlike the tentacles of jellyfishes, these tentacles retract into sheaths and are not hollow, but contain a muscle cell core with colloblast-laden epidermis. A few species of ctenophores, mostly in the genus Beröe, lack tentacles. Interestingly enough, all of these species eat only other ctenophores.

All ctenophores are carnivorous or parasitic, and range in size from about 5e millimeters in diameter to about 1 meter in length for the body of Cestum, the “Venus’ girdle.” Tentacles may extend to 30 meters in large individuals.

Most planktonic forms are clear and colorless, although some deep sea forms may be brightly colored. I once was fortunate enough to see a deep sea ctenophore with a brilliant orange football-shaped body that was over 30 centimeters (1 foot) long. The tentacles were easily 20 meters (66 feet) long.

RON SHIMEK

The common “sea gooseberry” (Pleurobrachia bachei), a planktonic tentaculate ctenophore. The tentacles are branched, but the branches have not extended. Note the food in the gut (colored dots = planktonic crustaceans) in the whitish area (pharynx).

Reproduction and development

The development can be very rapid. In Pleurobrachia bachei development from a fertilized egg to a feeding larva can occur in about 24 hours. In this species, sexual maturity in well-fed animals can occur within a week or two. Pleurobrachia appears to be able to live about two years, but some species have longer life spans.

There are two distinctly different types of ctenophores, each given its own taxonomic class. The class Tentaculata, characterized by having two tentacles, contains several orders, each with a specific body shape and life-style. One of these, the order Platyctenida, contains the benthic-dwelling ctenophores. These are the only ctenophores likely to be seen in aquaria, and, in fact, they are not uncommonly reported by aquarists. Most of the ones reported from aquaria are sometimes referred to as Coeloplana, but this identification has not been confirmed.

The platyctenes are suspension-feeding predators that extend their tentacles into the current and catch small prey or particulate organic matter. In aquaria, they undoubtedly feed on bacterial aggregates and small larvae released from the sand bed fauna. The ctene rows are found under the stalk or along its edges and allow the animals to move by slowly gliding over the substrate.

Tentaculate ctenophores generally eat small zooplankton, such as copepods, small fish or invertebrate larvae, and they may eat a lot! In the North Sea, man competes with cnidarians (the scyphomedusan Aurelia aurita) and ctenophores (Pleurobrachia bachei) for herring. The medusae and the ctenophores get about 98 percent of the herring, mostly by eating the fish larvae.

The other class is the Class Nuda, which, as the name implies, lack tentacles. There is only one genus in this class, Beröe. This genus, which has several species, is specialized to eat other ctenophores. Beröe looks sort of like a mobile plastic sack about the size of a cigar. It has a wide mouth at one end and a bag-like gut. It uses very large stiff cilia arranged around the edge of the mouth to catch prey. These act almost like teeth and allow the Beröe to catch and ingest its favored prey, other ctenophores, such as Pleurobrachia.

RON SHIMEK

Drawings of the two kinds of benthic ctenophores likely to be found in marine reef aquaria. A common generic name is given for each type. The erect forms are often found on gorgonians or live rock. The flatworm forms are often found living symbiotically on soft corals and echinoderms, such as sea stars or sea cucumbers. Both of these forms are small, with the body in the longest dimension seldom exceeding 5 millimeters (0.2 inch) long. Branched tentacles are characteristic of ctenophores.

Although ctenophores have been kept for long times in research aquaria (I have kept a Beröe for several months — but I had ready access to Pleurobrachia to feed it), they generally require the same special conditions that medusae need. So, large aquaria, such as the Monterey Bay Aquarium, which can afford to produce the large circular laminar flow tanks so the animals do not continually bounce themselves off the walls, can maintain them easily. I don’t know of any hobbyist who has successfully kept any of the swimming forms. However, I would expect that sooner or later some will be kept.

These are strikingly beautiful animals. The beating ctene rows are almost always iridescent, and under normal lighting the animals give the appearance of having flickering rainbows on each ctene row, and the body shapes are intricate and strange, which adds to their fascination.

Exotic introduced species have become problems all over the globe. Sometimes these introductions result in wholesale faunal changes, such as the devastation of the native fauna of San Francisco Bay. One such change has been recently documented concerning a ctenophore, native to the Atlantic, which has been introduced into the Black Sea.

Probably transported in a ship’s ballast water, this ctenophore, Mnemiopsis leidyi, is a common component of the North Atlantic. It was first noted in the Black Sea about 1986 and has had a devastating effect on the native fauna and human fisheries. Check out the link to the information about this environmental disaster for some interesting data on a very ugly event.

Links

Many good basic data can be found by following this link.

For many excellent photos (follow links to species names) and good information follow this link.

For data on the exotic and introduced species, Mnemiopsis, in the Black Sea follow this link (it has good photos).

Here is a photo of a specimen of Beröe, the only genus in the class Nuda.

For some good drawings of various ctenophores follow this link.

References