What Are Sarcotaces Parasites Found in Fish?

Sarcotaces are a genus of parasitic copepods discovered in marine fish. These organisms are known for their unusual appearance and parasitic relationship with their hosts. They reside within the muscle tissue of fish, often creating noticeable cysts.

Understanding Sarcotaces Parasites

Sarcotaces parasites belong to the Phylum Arthropoda, within the Subphylum Crustacea and the Class Copepoda. They are classified under the family Philichthyidae. These organisms exhibit significant sexual dimorphism. The females are large and highly transformed, appearing as sac-like or drop-shaped blobs that become deeply embedded in the host’s tissues. This transformation is so extreme that they lose many of the features associated with copepods.

In contrast, the males are minute and dwarf-like, retaining a more recognizable copepod form. The large, often pear-shaped females can grow up to several centimeters long and their bodies may be covered with wart-like growths. Because of their sometimes colorful and gem-like appearance within the host’s flesh, they are occasionally referred to as “gem parasites.” The females are so similar in appearance across different species that the tiny males are often required for accurate species identification.

The parasite is found within a gall or cyst in the skeletal muscle of the fish. This gall contains the large female parasite and is often filled with a dark, inky fluid. This black liquid is a result of degraded fish blood that the parasite has consumed. The appearance of these galls makes them noticeable when a fish is being filleted.

Host Specificity and Geographic Range

Sarcotaces parasites primarily infect marine teleost, or bony, fishes. Rockfishes of the genus Sebastes are among the most common hosts, but the parasites have been identified in a wide range of fish families, including:

• Moridae (cod-like fishes)
• Macrouridae (grenadiers)
• Serranidae (sea basses and groupers)

The parasites demonstrate a high degree of host specificity, with some Sarcotaces species believed to infect only a single or a few closely related fish species.

These parasites have a global distribution and can be found in various marine environments, from deep-sea habitats to tropical reefs. They have been documented in the North Pacific Ocean, the North Atlantic, and off the coasts of Japan and southern Australia. In some rockfish, the cysts are noticeably located near the anal opening, forming a sac-like structure from intestinal tissue.

The location of the cyst can sometimes cause a visible swelling on the fish’s skin, particularly if it lies over a bony area. However, in many cases, the infection is not externally apparent and is only discovered when the fish is cut open.

Life Cycle of Sarcotaces

The life cycle of Sarcotaces begins with a free-swimming larval stage known as a copepodid. This larva seeks a suitable fish host. Once it finds a host, the juvenile female burrows into the fish’s flesh, where it undergoes a metamorphosis into the sedentary, sac-like adult form. The female becomes enveloped by the host’s tissue, forming a cyst or gall that encloses it.

A small opening, or portal, is maintained from the cyst to the outside seawater. This opening allows for fertilization by the much smaller, free-swimming dwarf males, which enter the cyst to find the embedded female. It is not uncommon to find one or more males residing within the gall alongside a single female.

After fertilization, the female produces eggs, which develop into nauplius larvae within the protective cyst. These larvae are then released through the cyst’s opening to seek new hosts. Much of the life cycle, particularly the stages between the nauplius larva and the mature adult, is still not fully understood by researchers.

Impact on Host Organisms

A Sarcotaces parasite induces a reaction from its host. The fish’s immune system responds by forming a cyst of connective tissue that encapsulates the parasite, isolating it from the rest of the body. While this can cause tissue displacement and inflammation, the infection is not considered lethal to the fish.

The prognosis for an infected fish is good, especially if the number of parasites is low. However, some studies suggest that infestations can be associated with lower reproductive output in certain fish, like rockfish. The primary concern is economic rather than ecological. The presence of the dark fluid-filled cysts can stain the fish’s flesh during filleting, making the fillets unappealing to consumers and reducing their market value.

Sarcotaces parasites pose no health risk to humans. They are not infectious to people, and the parasites are localized within their cysts. The affected tissue is removed during processing, and standard cooking makes the fish safe to eat.