Parasitic copepods and isopods that sometimes attack marine aquarium fishes

By Jay F. Hemdal
Excerpt from the March/April 2023 CORAL Magazine

Some forty 40 years ago, I was watching a group of Purple Firefish (Nemateleotris decora) being held in a wholesale livestock facility. One fish had a yellowish tangle of “tubes” protruding from its abdomen. At the time, I had recently learned that some collectors in the Indo-Pacific were utilizing tiny harpoons to capture gobies for the aquarium trade. The idea was that, rather than using cyanide, the collector would spear the fish in a fin or other non-critical location in order to capture it. Indeed, Mandarin Dragonets would show up at wholesalers with tiny holes in their fins from being captured in this manner. Going back to the Firefish—I jumped to the presumption that this particular fish had been speared in the gut, and these were its intestines protruding out. Visually, that all made sense back then, but the reality was, as I later learned, that the “intestines” were just the egg case of a copepod parasite.

Two Lessons: On rare occasions, some very strange predators can appear, uninvited, in the marine aquarium. And: Things are not always what they seem to be!

From the crushing ocean depths to the driest deserts to the cold harsh alpine slopes, arthropods are ubiquitous in the world’s ecosystems. As a group, the phylum Arthropoda is considered a shining example of evolutionary diversity. It has been conservatively estimated that three out of every four animal species on this planet is an arthropod, and there are many factors are involved in arthropod classification. A gross generalization would consist of two groups: aquatic arthropods (crustaceans) and terrestrial arthropods (insects, spiders, millipedes). For the marine aquarist, the crustaceans hold special interest.

Crustaceans include the shrimps, crabs, mantis shrimp, and their kin; by some estimates, there are 30,000 or more different species living today. The vast majority of these are marine, though some have moved into freshwater areas. Like all arthropods, crustaceans possess a rigid exoskeleton made of chitin. This “armor” is one of the reasons for the arthropods’ success. It provides protection from predators and attachment sites for muscles. As the organism grows, the exoskeleton must be replaced. This is called molting and is one of the most dangerous times for the crustacean. Until the new exoskeleton has hardened, the crustacean is vulnerable to predation and injury. Some crustaceans, such cleaner shrimps and dwarf reef lobsters, are considered “ornamentals” and are obviously very popular inhabitants of home many home aquariums. Other types of aquarium crustaceans are not as welcome—parasites and “micro-predators.”. Some are obligate parasites with specific host species, while others are generalists and can live separate from a fish, returning periodically to feed on the fish’s skin. The smallest of these are disease-causing organisms, which are caused by a variety of very small crustaceans that can parasitize fish. The different species have a wide variety of body shapes, but because they are all arthropods, so they always have jointed legs and segmented body parts.

Non-parasitic amphipods, copepods, and isopods are very common cryptic inhabitants of aquariums. Seen as little white specks moving about the aquarium glass, these creatures (often called “pods” by aquarists) are harmless but sometimes indicate that the primary inhabitants (the fish) are being overfed.

“Pod” characteristics

Amphipods tend to be flattened side to side and have two different types of legs. None of these are directly parasitic to fish, but some can (rarely) harbor fish parasites during part of their life cycle, and others are micro-predators of sea jellies.

Copepods are flattened top to bottom, and non-parasitic ones often have large antennae. In parasitic species, the antennae are replaced with grasping hooks. So, if you see a large number of copepods, all with large antennae and not attached to a fish, they aren’t parasitic (though the parasitic species are free-swimming during the copepodite stage).

Isopods tend to have legs that are all the same (“iso” means “same,” and “pod” means “foot”). They are also either cylindrical or flattened top to bottom. The cirolanid isopods look a bit like pill bugs that you can find in your garden, but they are known micro-predators of fish.

The term “parasite” has some particular sub-categories that are important to understand. Parasitism is a close relationship between two species, where one organism, the parasite, lives on or inside another organism, the host, causing it some degree of harm, and which are adapted structurally to this way of life. These are “obligate parasites” that cannot survive without a host. There are also “facultative parasites” that can live as a true parasite, or can live freely in the environment, without a close association with a host species. The line becomes even more blurred with some copepods and isopods that are normally free-living, but can and will attack fish to feed on them. Cirolanid isopods are the most well-known organism in this category.

What is a “micro-predator?

Symptoms of copepod infestation can be a bit too general and vague for most aquarists to discern visually. The fish may flash (scratch), act nervous, or breathe heavily. In severe infestations, the fish’s skin will develop blood spots and the fins may become tattered.

In older aquariums that have a high bio-load (large number of fish), an aggressive type of copepod may take up residence. These micro-predators hide in the gravel and under rocks during the day and swim up into the water column at night to feed on the fish’s skin. Fish in aquariums with an infestation of this type of copepod will be seen hovering near the surface, as far away from the substrate as possible. They may also show other signs of stress, such as pale coloration, jerking movements, and rapid breathing.

In other instances, the female copepod’s twin egg sacs can be seen emerging from under the fish’s skin. Branchiurans (Argulus) can sometimes be seen by observant aquarists, but as they are clear, their 10- mm bodies don’t stand out very well. Aquarists may find these on wild-caught freshwater stingrays. In these fish, the Branchiurans cause what is termed , “Wandering Spot Disease.”. These copepods crawl over the body of the stingray, causing areas of pale skin discoloration that moves with them,; even to the point of leaving trails on the skin. Marine rays have been noted to contract a similar disease.

In other cases, such as with Ergasilus, (a gill parasite), they are too small to be seen directly, or are hidden in the gill chambers, so you need to look for secondary symptoms as mentioned above. A diagnostic dip will aid in the identification of these pests. The dip also serves as a temporary treatment, as it rids the fish of most of the parasites attached to it at that moment (but no dip is 100% effective, and reinfection is usual common once the dipped fish is returned to the infected aquarium).

The purple Purple firefisFirefishh, Nemateleotris magnifica, has a copepod parasite specific to that species— – Serpentisaccusus magnificae. Described by George Blasiola in 1979, this parasite is seen on newly imported firefish. The body of the parasite is buried in the flank of the fish, behind the gills, either on one or both sides. When the female copepod develops its egg sacs, these emerge from the fish’s skin as tiny coiled strands. Treatment should not be attempted for this parasite, as the adults are buried too deeply into the body of the fish for physical removal to be done safely, and no known chemotherapeutic will eliminate internal crustacean parasites without also harming the host fish. This copepod is thought to have indirect development with a series of planktonic larval stages, so there is no real concern of the eggs hatching and then fully developing and causing reinfection of the host.

Coprophagic copepods In a few instances, sedentary fish are seen with copepods living around their anal opening. This is most commonly encountered in scorpionfish;, it is unknown if these copepods are feeding on the fish’s tissue, or just waiting for feces to emerge to feed on those, or a combination of both.

Many of the parasitic isopods are large and easily seen with the naked eye. These include the infamous Cymothoa exigua “Tongue-Eating Isopod,” whose female seeks out, consumes, and replaces a fish’s tongue as it grows. Males are often found replacing a gill arch in the same fish. This parasite usually infects snappers and the like, but there is a record of this, or a similar species- infecting clownfish. While newly collected fish may arrive harboring these parasites, there are no records of this parasite reproducing in captive marine fish. It is presumed that they are unable to complete their larval life cycle in captivity.

Elthusa sp. is another large isopod found in the gill chambers of some fish, including Kole Tangs, Ctenochaetus strigosus, from Hawaii. Other large Cympthoid isopods are seen externally on reef fish. These are also not known to reproduce in aquariums, so once controlled by removal, they don’t return.
Parasitic isopods tend to favor larger fish, as well as those living in inshore, or degraded habitats.

Cirolanid isopods are the most common pest of this group seen in aquariums. These micro-predators often make their way into aquariums through the introduction of fresh live rock. Treating the affected fish themselves fails because these predators are capable of leaving the fish and living free in the aquarium until the treatment is concluded. In one European public aquarium, populations of these organisms had become established in their systems over many years. The symptoms of their presence was seen indirectly: each morning, the fish were noted to be hanging near the surface of the aquariums, exhibiting pale coloration and obvious discomfort. What was happening was that each night, these isopods would emerge from the rockwork and begin feeding on the skin of the fish. In this extreme case, the resolution was to use risky, very strong poisons to remove the isopods. Home aquarists typically find that setting nightly baited traps to remove the isopods, although time- consuming, is a safer means to control this issue.
Some species of shrimp (notably the skunk cleaner shrimps, Lysmata spp.) arrive with epicaridean isopod infections that look like white lumps on the side of the shrimp’s abdomen. Since the isopod is a crustacean itself, there is no chemical treatment that will affect the parasite without also harming the shrimp. Surgical removal is also not advised because crustaceans can only repair damage to their shells when they molt, so when the isopod is removed, a large wound will remain.

Control measures
Hyposalinity: This term refers to a long-term bath treatment for marine fishes in which the salt content of the water is lowered and held below the point that certain parasites can survive. The salt level reached, and the time of the treatment are the two variables. The level most-often suggested is 16 ppt (1.012 Specific Gravity, while fish-only systems can tolerate a salinity of 11 ppt (1.008 SG). Hyposalinity is sometimes called Osmotic Shock Therapy (OST) and can be used as a primary treatment against certain marine fish diseases or as a component of an active quarantine for newly acquired fish.

Insect Growth Regulators: Cyromazine and Diflubenzuron (Dimilin) are sometimes used for the control of crustacean parasites in fish. Never apply these compounds in aquariums that house arthropod invertebrates. They only affect these parasites when they molt, so terminal molt adult parasites are not harmed. This means that you must remove the adults through a dip process, while at the same time, dosing the exhibit to kill off all larval and sub-adult stages.

Pesticides: In the past, a frequently suggested treatment for crustacean parasites was the application of an organophosphate pesticide, such as Trichlorfon (Dylox or Metrifonate). These products are dangerous for aquarium use; they can be toxic to the aquarist and cause sensitivity reactions in many species of fish. Other treatments need to be considered first.

Chemical dips: One way to deal with these parasites in the aquarium is through a series of dips—formalin at 166 ppm for 45 minutes or freshwater for 5 to 7 minutes. Two difficulties are seen with this method, however. First, returning the fish directly to the infected tank allows for rapid reinfection. Second, these parasites have pinching mouthparts that make them difficult to dislodge during a dip treatment. One method used by public aquariums is to give the entire aquarium a high-dose (166 ppm) bath with formalin. After about 30 minutes, the tank is very quickly drained to 20 percent full. Previously mixed seawater is then added to the tank to refill it. This process exposes the fish to about 45 minutes of formalin above 150 ppm and then leaves a residual formalin dose in the tank of 33 ppm, which is generally tolerated as a constant bath by most fish.

Obviously, this cannot be done in aquariums where invertebrates are present, and it does require a substantial amount of saltwater used in the water changes. Furthermore, a second treatment is often required. For reef aquariums, removing the fish to a treatment tank and allowing the main aquarium to lie fallow for at least 45 days is the best treatment. Remember that formalin is very toxic and home use is restricted in many countries.

Biological control: Cleaner shrimp, gobies, and wrasses (e.g. the Bluestreak Cleaner Wrasse, Labroides dimidiatus) may help lower the number of crustacean parasites on fish in an aquarium. However, some cleaner animals are very specific in the parasites they are able and willing to clean. For any cleaner animal to be effective, the parasite must be the correct size and in a location that can be reached by the cleaner. The question also remains, “Who cleans the cleaner?”?
Cleaner shrimp such as the Scarlet Cleaner Shrimp. Lysmata amboinensis, and Banded Coral shrimp, Stenopus hispidus, have been shown to reduce parasite loads on fish, but only protozoans (Cryptocaryon) and worms.

There are two cleaner gobies commonly raised for the pet trade; the Neon Goby, Elacatinus oceanops, and the Sharknose goby, Elacatinus evelynae. Their wild diets are mostly unknown, but limited reports indicate their diets include larval Gnathiid isopods, as well as fish mucus and scales.

Cleaner wrasses feed on fish mucus, but also remove parasitic Gnathiid isopods, as well as monogenean flatworms from their client. The question also remains, “who cleans the cleaner”? It turns out that cleaner wrasse can serve as a vector for disease transmission as they cannot clean themselves, yet they travel from fish to fish potentially spreading disease (Narvaez et-al, 2022).

Physical removal: Because these parasites are often singular and easily seen, physical removal is frequently suggested as a means of control. Removal of these large parasites always carries with it the risk of extensive tissue damage to the fish, as well as secondary bacterial infection. Due to handling stress, any fish undergoing physical parasite removal should be sedated first.

Do nothing: In many cases, the best treatment is to do nothing. Most of the larger crustacean parasites have larval stages that cannot be completed in aquariums, so they will eventually die out on their own. In other cases, the removal process (physical or chemical) is riskier to the fish that just living with the parasite.

Jay F., Hemdal is a long-time Aquarium Curator for a public zoo aquarium in Ohio and author of Advanced Marine Aquarium Techniques (TFH, 2006).

Colin, Patrick 1975 Neon Gobies. TFH Publications, Neptune New Jersey.

Hemdal, J.F. 2023 Diseases of Aquarium Animals. In-press.

Wade Dugdale, W., Metzger, K. 2016 Health Impacts of Cymothoid Parasitic Isopods on Symphodus tinca and Boops boops Hosts in the Northwestern Mediterranean Sea. University of California, Santa Cruz

Grutter, A.S. and Bshary, R.I. 2003 Cleaner wrasse prefer client mucus: support for partner control mechanisms in cleaning interactions. Proc. Biol. Sci.

Hemdal, J.F. 2023 Diseases of Aquarium Animals. In-press.

Narvaez, P. Morais, R.S. Vaughan, D.B., Grutter A.S., Hutson K.S. 2022. Cleaner fish are potential super-spreaders. J. Exp. Biol.

Vaughan, D.B., Grutter, A.S., Hutson, K.S. 2018. Cleaner shrimp are a sustainable option to treat parasitic disease in farmed fish. Nature
2023 Diseases of Aquarium Animals. In-press.

Wade Dugdale, W., Metzger, K. 2016 Health Impacts of Cymothoid Parasitic Isopods on Symphodus tinca and Boops boops Hosts in the Northwestern Mediterranean Sea. University of California, Santa Cruz
Hemdal, J.F. 2023 Diseases of Aquarium Animals. In-press.

Wade Dugdale, W., Metzger, K. 2016 Health Impacts of Cymothoid Parasitic Isopods on Symphodus tinca and Boops boops Hosts in the Northwestern Mediterranean Sea. University of California, Santa Cruz

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