Q. I have, or should I say had, a 75-gallon reef tank that includes a variety of soft and hard corals, several species of hermit crabs, six small burrowing sea cucumbers, two species of shrimp, a blue linkia starfish, an Australian sea apple and 12 different fish, including two fairy wrasses, a mandarinfish, three anthias, a flame angelfish, a bicolor blenny, a longnose hawkfish and several small gobies. All was going well. My soft corals were growing, my hard corals were flourishing and the fish were all fat and healthy, except for one of my anthias. Then one day my sea apple committed the ultimate sin. It apparently got caught in a water pump in the tank, was torn apart by the impeller and its bits and pieces were spewed all over the tank.
When I returned home from work that day all the fish in the aquarium were dead! Ugh! Thankfully, my wife had already hidden the razor blades and I made it through the next few days, even though I was very depressed because of the demise of my fish. I was never warned by the shop owner that this could possibly happen! Is it common? Do all sea cucumbers do this? How about the cucumbers I have in my tank for sifting the live sand. Are they going to kill my fish? Help!
A. I am sorry to hear of your losses. I have seen, or heard, of this happening on a number of occasions. Unfortunately, there are some shop owners or employees at tropical fish stores who know about the risk in housing certain sea cucumbers in an aquarium but say nothing. There are also some aquarists who are told about the risk but decide to add these animals anyway.
Although they look quite benign, there are a number of sea cucumber species whose body walls and internal organs contain a toxin known as holothurin that can rapidly deplete your fish population. This is such an effective ichthyotoxin that natives in some parts of the Indo-Pacific will use the macerated bodies of certain sea cucumbers to incapacitate fish.
For example, Frey (1951) wrote how natives of Guam introduced sea cucumber tissue fluids into tide pools to kill the fish they contained. It’s interesting to note that in most cases, sea cucumber toxins do not kill other invertebrates in the tank, only fish. However, studies have shown that the toxin actually will kill smaller invertebrates, including worms, cnidarians (i.e., Hydra) and mollusks.
Holothurin can be released into the tank in several different ways. Two of these are defensive strategies, known as evisceration and Cuvierian tubule expulsion. Evisceration, which is the more common of the two strategies, can take two forms, depending on the species of sea cucumber in question. In one form the cloaca ruptures and the respiratory trees, the digestive tract and the gonads are expelled, while in the other form the anterior end of the body ruptures and the tentacles, pharynx and part of the intestine are discharged. This behavior does not kill the sea cucumber, which simply regenerates the organs. In some sea cucumbers these organs contain holothurin.
Expulsion of the Cuvierian tubules is a behavior used by far fewer sea cucumbers. The Cuvierian tubules are long white, pink or red structures that are attached to one, or both, respiratory trees. If the cucumber is attacked by a predator it will expel this sticky, stringy mass of tubules — which can also contain holothurin — out of its anus. In cases where the sea cucumber is attacked by a crab or lobster, they may become entangled in the sticky tubules and slowly die. The tubules, like the organs expelled during evisceration, can then be regenerated.
Another way certain sea cucumbers could be lethal to your fish is if they die in your tank, or if they are damaged by a piece of mechanical equipment (like powerhead impellers), as happened in your situation. If this should happen, the toxin could be released from the body wall or internal organs. Finally, the gametes of some sea cucumbers are toxic and if they are consumed by your fish, the fish will perish.
What will cause a sea cucumber to eviscerate or expel its tubules? Well, as mentioned earlier, if a sea cucumber is attacked by a predator or is damaged by a filter or pump, it may engage in one of these two behaviors. But, it may also do so if it is handled in or out of the water, or if it is exposed to sudden changes in water temperature, pH or oxygen levels. The latter can occur if a pump should fail, if the sea cucumbers are exposed to crowded conditions, if a tank is overfed, or if organisms should die in a smaller tank, a new tank or a tank with inadequate biological filtration.
So what species of sea cucumbers are potential fish killers? According to Nigrelli and Jakowska (1960), at least 60 species of sea cucumber are toxic. One of the most deadly members of this group is the spotted sea cucumber (Bohadschia argus). This species is commonly encountered on coastal reefs and reef flats in the Indo-Pacific, but fortunately, it rarely makes it into the marine aquarium trade. When upset, it expels Cuvier’s tubules, which will wipe a tank of fish out in minutes. The toxin is so deadly that on one occasion I saw it kill all the fish in an open system (that is, a pond that had fresh seawater running through it constantly).
Another species known to expel tubules and holothurin is the five-toothed sea cucumber (Actinopyga agassizii), which is found in the Bahamas and parts of the Caribbean. The impatient sea cucumber (Holothuria impatiens), which is almost cicumtropical in distribution, and the golden sea cucumber (H. parvula), which occurs off Bermuda and the West Indies, also expel tubules that may or may not be toxic.
Two of the more popular cucumbers are the Australian sea apple (Pseudocolochirus violaceus) and the tricolor sea apple (Pseudocolochirus tricolor, also known as P. axiologus). They are also toxic. These strikingly beautiful members of the family feed by flailing their colorful tentacles in the water column, which capture small pieces of suspended debris. But, if they are upset by drastic changes in their environment or suffer mechanical damage, a sea apple will eviscerate. The internal organs of this species do contain holothurin and will quickly decimate a tank full of fish. The gametes of the sea apple are also toxic.
Other cucumbers that have been reported to kill fish if injured or stressed include the pink cucumber (Holothuria edulis) and the medusa worms (genus Synapta or Euapta). The latter two genera include a number of species that are very elongate, measuring to more than 3 feet in length, and can range in color from brown or tan to pink.
In the mid-’80s these were very popular with those marine hobbyists who wanted something unusual in their tanks, and they were thought to be of benefit to the home aquarium because they acted as efficient scavengers. Little did we know at that time that these creatures could “nuke” all the fish in your tank when they were shredded by a powerhead or damaged when they were sucked up in the siphon of an external filter.
Fortunately, there are many species of sea cucumbers that are not deadly to fish. These include those species that are regularly collected from the Florida Keys and Gulf coast of Florida. They are used by reef aquarists to stir up live sand in a tank containing this substrate. Most of these belong to the genus Holothuria.