A Spineless Column by Ronald L. Shimek, Ph.D.

10-oh-4, Good Buddy!!

Regular readers of this column, provided there are regular readers of this column, will realize that I like animals that tend to be odd, unusual, or simply bizarre. The animals in the group I will write about this month, the ctenophores, (pronounced "teen'-oh-fores") really satisfy all of those criteria. Interestingly enough, it is also a group that I thought I would never be writing about when I started commenting on the invertebrates found in reef aquaria. The reason for that viewpoint is pretty obvious, the vast majority of ctenophores are wholly planktonic and, generally, as hobbyists, we simply can't maintain truly planktonic invertebrate animals for any length of time in any sort of normal condition. However, in my blithe ignorance of what might turn up in tanks from time-to-time, I neglected to realize that representatives from several species of the relatively uncommon benthic ctenophores would hitchhike their way into our systems with some regularity. Over the past couple of years, it has become apparent that not only are the crawling ctenophores present in some systems, they are reproducing and living quite successfully in them. Additionally, they are quite widespread through the hobby.

Ctenophores are some of the most common planktonic animals in many oceanic realms, and in a discussion of their natural history it is easier to work, at least initially, with some of the common planktonic forms rather than the sessile ones. This is because the planktonic forms are largely transparent, and it is relatively easy to see most anatomical structures just by looking through them. The crawling forms are often opaque and quite highly colored. Additionally, while the sessile forms are very highly modified and changed from their planktonic ancestors, they do retain many basic features of those ancestors so that a knowledge of the basic planktonic form helps significantly in understanding the bottom crawlers.

Figure 1. A pelagic ctenophore, Pleurobrachia bachei, photographed in nature showing some of the morphology. Image copyright © Dr. Ronald L. Shimek, 2004.

These animals are placed by taxonomists in the Phylum Ctenophora. This is one of the smaller phyla, as there are only an estimated eighty to one hundred species. They are all marine; there are no freshwater or terrestrial species. They bear a superficial resemblance to some of the various jellyfishes which are life stages of some of the cnidarians, but they were recognized as being in a separate phylum by Hatschek in 1889. Nonetheless, some textbooks continued to treat them as part of a larger grouping called the coelenterates until the late twentieth century. The so-called coelenterata was an inappropriate combination of animal groups, containing all of the cnidaria (animals such as hydroids, corals, and jellyfishes) as well as the ctenophorans. This grouping was based on a superficial similarity of form between ctenophorans and some jellyfishes, coupled with the observation that one ctenophoran species, Haeckelia rubra, uses nematocysts to catch its prey. As nematocysts are really a unique property of the cnidaria (no other group has anything like them), this usage by the one ctenophoran was deemed sufficient to allow the combining of the two groups. Unfortunately for this view of the gelatinous zooplankton, we now know that no ctenophore secretes nematocysts. Haeckelia rubra, however, does use them. It eats medusae, and obtains the nematocysts from one prey animal to assist in capturing others (See Mills and Miller, 1984).

Structural Properties:

These are moderately-sized animals; the body ranges from 5mm to about 1 m in length, depending on the species. The tentacles of some of the larger species can easily extend about 20 m or more. They are rather simply organized; most of their structures are comprised of tissues; they have relatively few structures that could be called organs. Primitive forms have what may be called biradial symmetry. In other words, while most structures are arranged in a radial pattern around the axis of symmetry, many of these structures are arranged in a mirror image pattern on either side of the central plane of symmetry. Most pelagic ctenophores are roughly cylindrical with discrete "lateral" sides. However, they don't have a top or bottom, and may rotate around the center axis as the animal swims. Consequently, unlike a fish, pelagic ctenophores are not considered to be bilaterally symmetrical. The tendency toward becoming a fully bilateral animal with a front and back end as well as left and right sides reaches its acme in this group in the benthic, crawling species. These animals are quite "flatwormy" in appearance and are really bilaterally symmetrical.

As with most radially, and biradially symmetrical animals, they have an oral-aboral axis of symmetry. In other words, the gut runs right down the centerline of the more primitive forms; but they lack a brain and anything that might be considered to be a head. They do have a sensory region that surrounds, or is associated with, one end of the gut tube. This is pretty promising for armchair classifiers. It seems to indicate that ctenophores are really related to the mainstream of invertebrate evolution. This is because a neural aggregation with a sensory region surrounding the gut is really "THE" invertebrate way to make an animal. In annelids, mollusks, arthropods and an impressive array of smaller critters, the brain or its components surround the gut in the region of the esophagus or throat. In a snail, an octopus, or a shrimp, it is no exaggeration to say that the gut passes right through the brain. It would be tempting to say that ctenophores have something similar, except that their sensory nervous aggregation, called the polar field, is at the posterior end of the animal near the anal pores (they don't have just one anus either, but we will get to that momentarily).

Figure 2. This image, taken from Figure 209, Hyman, L. H. 1940, The Invertebrates, Protozoa through Ctenophora, Volume 1. McGraw-Hill Book Company. New York, shows the typical illustration of an inverted ctenophore. Hyman wrote a six volume series considered to be the classic English-language invertebrate reference and certainly knew the proper orientation of the animals. Note the inverted body and trailing tentacular fringe of the ctenophore implying that the animal moved upward.

Biologists are a rather conservative breed of human, and as a group, they generally consider it to be dogma that an animal's brain must be near its front end. Simple animals often lack a well-defined brain, but they often have sensory structures near the front end of the body. These are generally considered to be "evolutionary" precursors of brains. Well, this neural and sensory aggregation of the ctenophoran rear-end has really caused some of these armchair biologist theoreticians to have an indigestible case of circular reasoning.

It goes like this…

  • Brains are found near the front end of animals. 
  • The ctenophoran sensory field and neural aggregation is a primitive brain. 
  • Thus, the sensory field and neural aggregation is at the front end of the animal.
    • If you look through old editions of many textbooks, and a few new editions as well, you will find that the diagrams of the “typical” ctenophores show the sensory region at the top or at the presumed front of the animal where a “normal” animal would have its head.  The mouth is drawn pointing down or to the rear. 


These animals move mouth first, just like most good bilateral animals do through the rest of the animal kingdom. It would be ridiculous to say they move backwards through the world, but that is just exactly what has been assumed. Such a simple and "tidy" assumption about these beautiful animals; it makes everything so nice, keeping all presumptive brains up front. Well, that is all well and good, but simple observation shows that the animals move mouth first, and if the anally located polar sensory field helps the animal orient (which it seems to), then we can truly say that these animals are being given a bum steer… or, at least, steering directions from their bum.

With few exceptions, ctenophores have a pair of very extensible tentacles. The position of the tentacles on each side of the body is what gives most ctenophores their basic appearance of having two sides rather than being completely radially symmetrical. Internally, the gut also is divided into two halves as well, but that is not seen without magnification. The tentacles are branched, and the branches extend from the main tentacle axis like a fringe on only one side. They are made of a muscular central core that is surrounded by a layer of epidermis containing colloblast cells. Colloblasts are "glue" cells and there are several different types of them. When the tentacle contacts a prey item, the colloblasts explode releasing adhesive strands and granules and these adhere the tentacle to the prey. The prey, typically a small crustacean, struggles when the tentacles start to stick to them and this results in more of the tentacle getting wrapped around the prey item. The ctenophore then swims so that the mouth contacts the tentacle and eats the prey. Unlike the discharging of cnidarian nematocysts which are non-living, the act of capturing food by a ctenophore results from the destruction of many cells in the tentacular epidermis.

Figure 3. A smear preparation of a ctenophore tentacle stained with Methylene Blue, photographed using a microscope at 100x. There are many colloblasts in the field of view; one is labeled to show some of the parts. Image copyright © Dr. Ronald L. Shimek, 2004.

Unlike the bodies of jellyfish or most other gelatinous planktonic animals, ctenophore bodies are relatively rigid. They do not move by muscular means, which is yet another difference between ctenophores and medusa. Instead of propulsion by muscular contraction, they move by paddling their way through the world. They have eight rows of small paddles running along the body. In the primitive forms these rows are evenly spaced, but in some others, such as the benthic ones found in aquaria, the rows have been displaced. The paddles are called "ctenes," a word meaning "combs." They look quite like a miniature version of a styling comb used to keep a hairdo in place. Each of these combs is formed by several hundred large cilia which have been fused together in a common sheath. They are arranged in rows, called, not surprisingly, ctene rows, and the beat of these is controlled by the sensory polar field.

The aboral sensory region or polar field contains numerous sensory components such as statocysts which give information about orientation and which are connected directly to nerves that coordinate the locomotory beat of the ctenes. Additionally, the sensory region also contains photoreceptors or eyespots. Ctenophores do not respond directly to shadows and the eyespots can't form an image. However, they probably serve to keep track of changing day length and may indicate to the animal if it has descended too far down into the darkened depths. The sensory area also contains cells that have stiff cilia that project into the water. These cilia bend under the force of water moving past them, and probably indicate to the animal how fast it is moving. Ctenophores don't have nerves as we know them from vertebrates. In animals such as us, a nerve is a collection of nerve fibers or nerve cell processes which can extend some relatively great distance from the cell body. The nerve cell bodies are not in the nerves proper, but reside in the brain or in specialized aggregations of nerve cell bodies called ganglia. In ctenophores, the nerves are comprised of a mixture of cell bodies and relatively short fibrillar processes.

The mouth is slitlike and found on the end of the animal away from the sensory field. Typically, the mouth is relatively small and oriented perpendicular to the axis of the tentacles. Inside the animal is a rather capacious gut region referred to as the stomodeum. At about the midpoint of the gut, one branch arises from either side of the stomodeum. These branches each subdivide two mores times to form eight elongate gut pouches which are positioned under and parallel to the ctene rows. Digestion starts in the stomodeum, but most digestion is in the epithelia lining the gut directly under the ctene rows. Nutrients are easily and rapidly transferred to the ctene rows by diffusion, providing the animal with food and energy to move about. Indigestible food remains are passed out the anal pores at the aboral pole.

Reproduction and Development

Ctenophores are hermaphroditic, and the gonads typically develop at separate times. The gonads exist in discrete rows in the gut pouches under the ctene rows and develop from the gut epithelium. Gametes escape via the anal pore or through special gonoducts (found in only a few species). Fertilization occurs in the sea and embryonic development is very rapid. Generally, a characteristic larva called a "cydippid" is found by the end of the second day after spawning. Cydippids are small ctenophores which have only four rather than eight ctene rows. If food is abundant, these larvae will grow rapidly and may be reproductive within a few more days. The most abundant ctenophore, the sea gooseberry, Pleurobrachia bachei, may live a couple of years. The potential life spans of most ctenophores are unknown.

Figure 4. Left. A cydippid larva of a pelagic ctenophore, probably Pleurobrachia bachei.
Right. A cydippid larva of a platyctene ctenophore found in an aquarium. Note the similarities
of structure. Images copyright © Dr. Ronald L. Shimek, 2004.

There are several different taxonomic orders of planktonic ctenophores, while only one order contains species adapted to live on substrates. This latter group, called the "Order Platyctenida," contains those ctenophores found in marine reef aquaria. These animals have a flattened body that looks very "flatwormish," but differs from the flat worms in the presence of the ctene rows on the ventral surface and a pair of perfectly normal ctenophore tentacles that arise from pouches on the dorsal surface. Platyctenes may be visualized as a "deflated" example of a more typical pelagic ctenophore. The gut pouches lie internally in groups of four on either side of the animal, and differential growth has resulted in the ctene rows all being on the bottom and the tentacles being on the top. Additionally, they are colored and often have relatively ornate patterns on their upper surfaces. This "ornamentation" often matches the surfaces of the animals, such as the specific species of soft corals, or sea stars(1) (2), they live on. The color pattern probably provides them with some degree of protection from visual predators such as "nipping" fishes. Other sessile ctenophores, such as this Antarctic Lyrocteis, are neutrally colored.

Figure 5. An unidentified Platyctene ctenophore found in an aquarium. The tentacles are extruded but
not extended due to the lack of current in the bowl where the photography was done.
Image copyright © Dr. Ronald L. Shimek, 2004.

Figure 6. An unidentified Platyctene ctenophore found in an aquarium; possibly the same species as in
Figure 5, but collected and sent to me several months earlier. The tentacles are retracted; note the
flatworm-like appearance. Image copyright © Dr. Ronald L. Shimek, 2004.

Platyctenes are often found living on particular soft corals or corals, and may also absorb or eat mucus from their "host." It is unlikely that they actually eat the host's tissues. Platyctenes, generally, do not seem to specifically harm animals in reef aquaria, provided that there are only a few of the ctenophores. In a few instances, however, they have achieved plague proportions and may be so abundant that they actually smother their hosts or other animals. This later result is quite uncommon. In most tanks, they enter as hitchhikers, persist for a few weeks and then vanish.

Figure 7. Diagrams of benthic ctenophores showing basic shapes. So far, only the "flatworm"
type has been found in aquaria.

Although both pelagic and benthic ctenophores have been kept for long times in research aquaria (I have kept an individual of one pelagic species, Beroe, for several months - but I had ready access to its prey, another species of ctenophore, Pleurobrachia, to feed it), pelagic species generally require the same special conditions that medusae need. So, typically only large commercial aquaria, such as the Monterey Bay Aquarium, can afford to produce and maintain the large circular laminar flow tanks necessary so that the animals do not continually bounce themselves off the aquarium walls. 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.

Figure 8. Note the iridescence of the ctene rows on this ctenophore,
Bolinopsis. Image copyright © Dr. Ronald L. Shimek, 2004.

Incidentally, exotic or introduced ctenophore species have become problems in several regions of the world. Although simple and relatively small, ctenophores are voracious predators, and this fact, coupled with their astronomical reproductive rate can result in wholesale changes to the organism arrays if they are inadvertently introduced into environments where they were previously not found. Sometimes these introductions result in wholesale faunal changes. 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. It has recently been found in the Caspian Sea, and will undoubtedly destroy the native ecosystems in that region as well. Check out the link to the information about this environmental disaster for some interesting data on a very ugly event.

For a further detailed, but very readable, discussion of Ctenophores, I recommend Dr. Claudia Mills' website as a place to begin.

If you have any questions about this article, please visit my author forum on Reef Central.

References with interesting or background data:

Arai, M. N., G. A. McFarlane, M. W. Saunders and G. M. Mapstone. 1993. Spring abundance of medusae, ctenophores, and siphonophores off southwest Vancouver Island: Possible competition or predation on sablefish larvae. Canadian Technical Report of Fisheries and Aquatic Sciences. 1939,I-IV:I-IV,1-37.

Barlow, D., M. A. Sleigh and R. J. White. 1993. Water flows around the comb plates of the ctenophore Pleurobrachia plotted by computer: A model system for studying propulsion by antiplectic metachronism. Journal of Experimental Biology. 177:113-128.

Carre, C. and D. Carre. 1993. Minictena luteola, new genus and new species of Mediterranean Cydippida ctenophore with five types of colloblasts. Beaufortia. 43:168-175.

Cowan, J. H., Jr. and E. D. Houde. 1993. Relative predation potentials of scyphomedusae, ctenophores and planktivorous fish on ichthyoplankton in Chesapeake Bay. Marine Ecology Progress Series. 95:55-65.

Kozloff, E. N. 1990. Invertebrates. Saunders College Publishing. Philadelphia. 866 pp.

Matsumoto, G. I. and G. R. Harbison. 1993. In situ observations of foraging, feeding, and escape behavior in three orders of oceanic ctenophores: Lobata, Cestida, and Beroida. Marine Biology. 117:279-287.

Mills, C.E. and R.L. Miller, 1984. Ingestion of a medusa (Aegina citrea) by the nematocyst-containing ctenophore (Haeckelia rubra, formerly Euchlora rubra): phylogenetic implications. Marine Biology, 78: 215-221.

Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. Saunders College Publishing. Philadelphia. 1056 pp.

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10-oh-4, Good Buddy!! by Ronald L. Shimek, Ph.D. - Reefkeeping.com