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Cnidarians and Ctenophores Aquarium.Net Dec 96

In this months column Rob introduces us to Cnidarians and Ctenophores, December 1996 Index for Aquarium Net, Aquarium Net has numerous articles written by the leading authors for the advanced aquarist

A Reefkeeper's Guide to Introductory Invertebrate Zoology

Part 2: Cnidarians and Ctenophores (the gelatinous zooplankton)

by Rob Toonen

After discussing the Porifera (sponges) last month, we're on to the phylum with which most reefkeepers are primarily concerned: Phylum Cnidaria, the hydras, jellyfish, corals, and anemones. Some texts (e.g., Haywood and Wells, 1989) still try to lump Phylum Cnidaria together with Phylum Ctenophora ("comb jellies" or "sea walnuts") in the artificial category Phylum Coelenterata. Coelenterates were once thought related simply because jellyfish and ctenophores were both gelatinous plankton, and being a Jello-like predator was deemed unique enough to warrant placing them into the same taxonomic group. However the similarity between jellyfishes and ctenophores ends there. The body symmetry is different, the number of embryonic dermal layers is different, the specialized cells used to catch prey differ, the gut is far more complex in ctenophores, and the list goes on. Even after it became obvious that jellyfishes and ctenophores were unlikely relatives, many researchers were convinced that ctenophores were simply a specialized form of jellyfish by the observation that a couple of ctenophores were found to possess nematocysts (specialized stinging cells of cnidarians). However, careful work with one of these species ( Haeckelia ) revealed that the ctenophore was incapable of producing the cells, but rather took unfired cells from their prey – the tentacles of a jellyfish ( Aegina ) – and used them for it's own defense (Mills & Miller 1984). This phenomenon is known as kleptocnidae, and occurs in several unrelated groups of predators on cnidarians. The exact connection between jellyfish and ctenophores is still unclear, but it is obvious that they are not closely related, and the term coelenterate is now used only in casual discussion of the gelatinous predators of the sea rather than as a taxonomic designation.

Ctenophores ("comb-jellies" or "sea walnuts", pictured right ) are a group about which relatively little is known, both in terms of biology and the hobby. Although they, like most jellyfishes, are almost never imported for the hobby, they are seen in some displays of public aquaria (the Monterey Bay Aquarium, for example), and I have decided to include them because I have received several requests on the rec.aquaria.* newsgroups for information on the care of these unusual animals. Ctenophores are entirely marine, and with a couple of exceptions (e.g., Coeloplana which glides across the sea floor looking for prey), are all planktonic. They are basically transparent, and like jellyfishes, appear to be basically small, gelatinous blobs of snot that float around in the ocean currents. Also like the jellies, they are predatory, and require a significant effort to keep successfully (you need a tank specifically dedicated to these animals and a ready supply of live food).

The two main features which link all ctenophores are the possession of specialized prey-capture cells called colloblasts and they all possess eight rows of ciliary plates at some point in their life (the movement of which is controlled by a unique apical sense organ ). Colloblasts serve much the same function as the stinging cells of cnidarians (I'll discuss these in detail next month), but have a very different structure. Each colloblast has a coiled filament attached to a head covered with secretory granules. When triggered these granules secrete a very sticky mucus-like substance which adheres to the prey. Entangled prey is then "reeled in" by muscular contraction of the tentacles, and transferred to the mouth. This method of prey capture works best for small zooplankton, on which most ctenophores specialize, but many ctenophores are also capable of capturing larger prey, such as jellyfish. Some ctenophores occur in coastal waters and are relatively hardy (e.g., Mnemiopsis , Pleurobrachia , and Beröe ), while others are "blue-water" (only occur in the open ocean, far from shore) species which rarely survive contact with other solid objects (e.g., Cestum often explodes on contact with the inside of a Ziplock bag during collection attempts). Obviously, such fragile forms are not well-suited to aquarium culture attempts.

The most remarkable thing about ctenophores is that they are the largest animal on the planet capable of moving by ciliary action. Cilia are tiny hairs which stick out of individual cells, and (like our skipping-rope analogy of last month) beat together in waves to push the animal around. The reason that these animals get the common name "comb jellies" is that dozens to hundreds of these cilia are arranged into transverse bands which may or may not be partially fused to form eight paddle-like "comb" rows ( ctenes ) running around the ctenophore's body. The transverse bands in these rows beat synchronously, under the nervous control of the apical sense organ (which is only found in the ctenophores). The light reflecting off the tiny hairs forms a kaleidoscope of color any time the animal moves; the tiny rainbows created by the motion of the ctenes is what most fascinates many people with these animals and makes them a desirable tank occupant.

There are four classes of Cnidaria: the Hydrozoa (hydroids and hydromedusae), the Cubozoa (box jellies or sea wasps), the Scyphozoa (true jellyfish) and the Anthozoa (sea anemones, corals and sea pens). There are about 10,000 living species of Cnidarians. I was originally planning on discussing the characteristics which define cnidarians and ctenophores in this article, and then concentrating specifically on the biology and maintenance of gelatinous zooplankton (both ctenophores and jellyfish), but quickly found that the article was running long. Therefore, I have decided to split this article in half, and will instead describe the characteristics of ctenophores in this month's article along with information on the care of gelatinous plankters (including jellyfishes which are all cnidarians); next month I will discuss the characteristics of the cnidarians, but will concentrate on peculiarities of the Anthozoa (sea pens, corals and anemones).

The hydrozoans are largely an unappreciated group by most reefkeepers. They possess a number of specialized polyp types. Most notable are the gastrozoids for nutrition, the gonozoids for reproduction, and the dactylozoids for defense and prey capture. The fine "hairs" sticking from the surface of a fire coral are the dactylozoids, the same specialized polyps as compose the "tentacles" of the man-of-war (see below). These defensive polyps are equipped with unusually powerful nematocysts, and anyone who has been stung by either of these animals will not forget the sensation of the nematocyst penetration and the lasting effect of the toxin after the sting (I know I won't!). The polypoid stage of most species are small and nondescript. The hydrozoans that are large and obvious (like the Milleporina, better known as "fire corals," and the "Portuguese Man-of-War") are typically undesirable for most aquaria, but are still interesting (at least to me). They may be simple polyps attached by a common base, similar to a number of tiny anemones attached by the coenosarc (basically a set of pipes running between the guts of each polyp), or they may be complex fleshy colonies. They may also form massive calcareous colonies to rival the true corals, in which the polyps are still attached by fleshy tubes, but they run through tiny pits in the skeleton (e.g., Millepora ). The calcareous skeleton of the fire "corals" is practically external, with only a thin layer of epidermis covering the skeletal matrix, but like the true corals, milleporines rely on a commensal relationship with zooxanthellae. Finally, they may be large floating colonies in which different polyps are specialized for various functions; Physalia (the "man-of-war") is a colony of specialized polyps often mistaken for a jellyfish by inexpert observers. One polyp type forms the float, another the defensive/prey capture "tentacles" and yet others perform the digestive and reproductive functions.

Despite a few infamous members, most of these animals are small and nondescript animals (often referred to by such flattering names as "snail fur"). However, many of these animals also have the potential to be an interesting and attractive addition to reef or specialty display aquaria. For example, the stylasterine "corals" (e.g., Allopora , and Stylaster ) are often brightly colored (purple, red and yellow) with a thick epidermis covering the calcareous skeleton, superficially resembling an encrusting gorgonian. These are also the animals that produce many of the jellyfish aquarists would like to maintain in their homes. I wish that I had kept all the E-mail replies I have sent over the past couple of years explaining to people how to keep jellyfish, because there have been some very good questions, and it would have made writing this next section very easy.

Contrary to popular belief, jellyfish do not simply extend their tentacles and hope that some unlucky plankton swim into them. Medusae have species-specific shapes and "fishing" behaviors that lead to them specializing on one or a few prey types (Mills 1981a). Although these fishing behaviors lead to nearly exclusive capture of certain prey in the wild, jellyfish are highly prone to tank size and shape altering their ability to capture certain prey. The tiny (»5mm) hydromedusa Proboscidactyla flavicirrata feeds almost exclusively on the veliger larvae of gastropods in the wild (Mills 1981b, Larson 1987), but when placed in different sized containers with a variety of prey in the lab, the jellyfish preferentially captured nauplii (which were never found in the guts of animals collected from the field) in containers smaller than 10-15,000 times their body size, but returned to preying on gastropod veligers in larger containers (Toonen & Chia 1993). Although you may not really care, or need to reproduce natural feeding behaviors in the home, you should be aware that many species of jelly do have specific prey and some may not reach breeding condition unless provided with the appropriate diet. Thus far, however, I have not had problems keeping alive any of the jellyfishes I have caught by feeding them on live newly-hatched and adult brine shrimp ( Artemia ), at least for short periods of time.

It is difficult to say what the appropriate diet is, however, because relatively little work has been done on jellyfish. This is not surprising, because they are not very abundant, and there are many logistical difficulties associated with collecting and studying them in the open ocean. However, some common species are easy to keep and there is something known about their feeding requirements. I have personally kept a number of species, but Aurelia aurita , the moon jelly, and Cassiopeia andromeda the upside-down jellyfish were the easiest species to maintain that produce large attractive medusae. Most large shipments of live rock will probably have a number of hydroids included, and I have isolated a couple small and hardy species of jelly from Caribbean rock. When I searched carefully, I have found one of these little jellies (I have not been able to get a positive identification of these jellies, but they may be Eleutheria ) in all my reef tanks. Although these jellies are by far the easiest to raise, they are very small (less than 1 cm bell diameter when full grown), and the medusae have branched tentacles alternatively ending with suckered and "normal" tips. They swim only rarely in low current, and not at all in the high flow typical of a reef; they use their suckered tentacles to crawl over the tank and capture copepods and other small prey. This behavior is not exactly what most people have in mind when they say "I want to set up a jellyfish tank."

Aurelia , on the other hand, is one of the large, graceful jellies that inspire people to consider setting up a medusa tank in the first place. Jellies are obviously popular, because the Monterey Bay Aquarium added their "Planet of the Jellies" exhibit to the new permanent expansion. Aurelia is a temperate (cold-water) species, but seems to survive just fine at the low end of room temperature. If your home proves too warm for these animals you could set up a small chiller unit on the aquarium, but I would first try a cheaper alternative: evaporative cooling. A couple of computer `muffin' fans blowing across the aquarium do a fairly good job of keeping the aquarium temperature below ambient, and may cool the tank enough for your purposes. Cassiopeia is a tropical species which is by far the most common, if not the only, jellyfish sold in the pet trade. These medusae can be easily maintained in an aquarium, because they gain much of their nutrition from photosynthetic zooxanthellae. These scyphozoan jellyfish actually lack tentacles, and instead have highly modified "lips" on the manubrium (extendible mouth). The frilly lips are loaded with nematocysts which capture small prey ( Artemia make a reasonable choice), or can be released into the water column if the animal is disturbed (when swimming through large swarms of these animals divers often experience a sensation likened to swimming through soda water – the `bubble-popping' sensation derives from the freely-floating nematocysts firing when contacting the skin). Although it is unlikely that anyone will experience an adverse reaction to the sting of the upside-down jellyfish, some people have reported painful swelling in response to contact.

The frills on the manubrium are interspersed with bladder-like growths (which can range from dull brown to bright purple in color, and millimeters to inches in length) which house concentrated zooxanthellae. Having said that however, I have found that they need an inordinate amount of light, and have only flourished in a tank with a very high output of metal halide lighting in my experience (something on the order of 750-1000 µE/m²/s – this is higher than most SPS reef tanks!). I have also found that they would only reproduce sexually when there were detectable levels of ammonia in the system. I thought that it may have been a coincidence that I could measure ammonia in the tank (I fed heavily with Artemia – both for the adult jellies and the polyps in the tank – and did not use a filter) when I noticed larval production, but after speaking with keepers at the Monterey Bay Aquarium, I was told they found the same thing with their display (they didn't notice any reproduction until they doubled the halide lighting on the tank and took out all filtration). I have only found these animals in eel-grass beds and mangrove swamps in the Caribbean, where there is relatively high decomposition and low water exchange relative to the reef environment, and think that the zooxanthellae may need the supplement to meet the energetic requirements of the jellies.

The polyp stage of both these jellies is far from spectacular, but the small (about 1-2 mm high by ¾ -1mm in diameter) nondescript polyps undergo a process called strobilization (the typical developmental pattern of the schyphozoans) in which the anemone-like polyp divides itself into a series of flat disks (the polyp starts to look like a stack of tiny dinner plates with tentacles), and releases them one by one as ephyra (baby jellyfish). The ephyra do not yet look like a complete jellyfish, but rather have 8 arms each ending in a pair of finger-like lobes. As the ephyra grows, the arms grow together, attach and form the swimming bell of the medusa. The medusae are the sexually mature stage of the life cycle, and they produce eggs and sperm which develop into nonfeeding planula larvae. The larva swims around for a variable, but typically short period of time before settling and attaching to the bottom. The settled juvenile develops quickly into a polyp which is capable of both repeated asexual reproduction to form large clonal aggregations of the polyps, and strobilating to produce more ephyra. There are many jellies that deviate from this typical pattern of reproduction, but it is still the basic theme of cnidarian reproduction (Anthozoans, of course, do not follow this developmental pattern because anemones, corals and sea pens all lack medusae).

The medusae of Aurelia are large and graceful attractions, but like all jellies, are built for the open sea, and are therefor poorly adapted to life in an aquarium. There are many gelatinous species that are too fragile to be practical occupants in home aquaria, but many species are hardy enough that they can be easily maintained in an aquarium if it is specifically set up for them. There are basically 3 designs that you can try depending on how much time and money you have to dedicate to your setup, how large the animals are, and how sensitive are the species you plan to culture. I will discuss all three of them in only brief detail here.

The first two are simple and cheap, but also pretty hard on the animals -- if you intend to keep sensitive species, you should skip directly to the third option. The first method is to use a simple hexagonal tank in which you set a large sponge filter (the larger the filter, the more diffuse the suction of flow through it) with an undergravel uplift tube covering the airline and the rising bubbles (if bubbles escape they can easily get into the swimming bell of the medusae and damage them or trap them on the surface). The plastic uplift tube should end slightly above the water surface so that any water moved up the tube by the bubbles is dropped back into the tank, breaking the surface tension and aerating the tank. This will also help to push any jellies on the surface back down into the water column. By running bubbles up the tube at the appropriate speed (a few per second) you should get very gentle turnover in the tank, but not run the risk of sucking all your jellies into the filter. I have maintained a tank like this with a healthy tropical hydromedusa culture for almost a year now.

In the past, when I set up a jelly tank, I would simply use a very diffuse flow-through system to keep the medusae in suspension. One simple way to do this is to use a small canister filter to move water from one end of the aquarium to the other. To do this you would set a spray bar across one edge of a large aquarium (say the left side) aimed toward the glass, and the intake at the other end of the tank (in this case, the right). Then both sides of the tank would be sealed off from the central portion by hot-gluing a section of fiberglass mosquito mesh across the aquarium about 6-10 inches (depending on the strength of your filter) from each end of the tank (the more powerful filter you get, the more space you'll have to cut off each end of the aquarium. Obviously, if you're going to "waste" 12-20 inches of your aquarium length, it had better be a long tank, so keep that in mind when the urge to put a large filter on your tank strikes you. The purpose of the mosquito mesh is to allow for undisturbed flow from the intake to the spray bar, but simultaneously prevent areas of high flow or suction that are likely to trap or damage the medusae. The jellies can then be kept in the central portion of the tank because the flow through the mosquito netting is so diffuse that the medusae which contact it can easily swim away of their own accord. There is no reason that you couldn't keep a hardier animal that is a stronger swimmer (seahorses, for example) on the ends of the tank, because any Artemia which escaped through the mesh would simply become food for the jellies in the central portion of the tank. In that way you could set up a very interesting display with smaller fishes on the ends of the aquarium and the medusae in the center (eventually this organization would break down if the jellies started to reproduce, but any medusae in the ends of the tanks would likely be killed by the filter in short order so the polyps may spread throughout the tank but wouldn't really change the overall effect of the tank). Anyhow, this is basically the technique we used to keep our jellyfish alive for research purposes, but because our tanks were flow-through, there was no need for the recirculating filter (natural sea water was simply pumped into one end of the tank, and drained back into the ocean at the other).

The final method is the most complicated and expensive, but also the most likely to succeed of all. You could build a special tank specifically to house your jellies. These large circular tanks, which institutions like the Monterey Bay Aquarium use to keep their gelatinous zooplankton, are basically a large ring sealed between 2 flat panes of glass. The ring has an opening along one side where water is piped through a thin tube slowly back into the tank, and a small overflow that allows excess water to dribble over the edge to a reservoir at the bottom. A small pump, basically similar to a dosing pump, is used to deliver water from the reservoir back into the tank. This constant flow of water, directed along the wall of the ring, causes a swirling current in the tank which forces the animals towards the center of the tank, and prevents them from either settling out of the water column or being trapped by the water movement. This is a serious undertaking (you'll have to specially order or design and build your own tank if you intend to try this), and will probably not be a common sight in home displays.

Despite the rarity of these beautiful animals in the hobby at this time, I think that it is possible to keep these animals successfully (I and many others have done so for research purposes). I hope that with a bit more time and information others may start to keep these animals for home displays.


Haywood, M. & S. Wells. 1989. The Manual of Marine Invertebrates. Tetra Press, Salamander Books Ltd. Blacksburg, VA. 208 pp.

Larson, R.J. 1987. Trophic ecology of planktonic gelatinous predators in Saanich Inlet, British Columbia: diets and prey selection. J. Plankton Res. 9:811-820.

Mills, C.E. 1981a. Diversity of swimming behaviors in hydromedusae as related to feeding and utilization of space. Mar. Biol. 64:185-189.

Mills, C.E. 1981b. Seasonal occurrence of planktonic medusae and ctenophores in the San Juan Archipelago (NE Pacific). Wassmann J. Biol. 39:6-29.

Mills, C.E. & R.L. Miller. 1984. Ingestion of a medusa ( Aegina citrea ) by the nematocyst-containing ctenophore Haecklia rubra (formerly Euchlora rubra ): phylogentic implications. Mar. Biol. 78:215-221.

Toonen, R.J. & F.-S. Chia. 1993. Limitations of laboratory assessments of coelenterate predation: container effects on the prey selection of the Limnomedusa, Proboscidactyla flavicirrata (Brandt). J. Exp. Mar. Biol. Ecol. 167:215-235.

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Last modified 2006-11-23 01:37