This “Mystery Mass” is Pretty Fishy

October 24, 2019

Question: I speared a 15” blue rockfish at Ocean Cove over Labor Day weekend. It had a large belly but no eggs or anything extraordinary within its stomach or fillets. Within the gut cavity, attached to the end of the intestine (near the anus), there was a 3.5 cm white mass with a black tip at the free end. It was very hard and calcified. I cut it in half longways and found that the translucent white skin came off easily, but the black mass inside was very dense. The black mass also seems to rub off and stain anything it touched, like a gummy charcoal. I took pictures, with pink arrow on Gyotaku print of this fish marking the approximate location and orientation within gut cavity. Is this some kind of parasite, or just a calcified gonad/kidney stones? Would this mass give any reason to avoid eating the fillets? (TL)

Answer: What a vivid description you provided! And the photos are very helpful for reference. The condition you’ve described is caused by a parasitic copepod called Sarcotaces. Copepods are arthropods and are in the same taxonomic group of animals as crabs and insects.

The juvenile Sarcotaces can be found in the water column in hopes of finding a host. In this case, the host was the rockfish you caught. Sarcotaces inserts its head into the skin of a fish and is eventually encapsulated by the fish’s flesh, except for the last body-segment which maintains connection with the outside seawater. This parasite is commonly found in rockfish, usually located near the vent where the surrounding intestinal tissue forms a sac-like protuberance inside the abdominal cavity. Sarcotaces feed on blood from the fish, and the digested blood becomes a dark fluid or paste inside the sac. When the parasite dies, the tissue forms a closed cyst, which is what you found during filleting.

There are no human health concerns with a fish infected with Sarcotaces, although if the fillets become tainted with the dark fluid from a ruptured sac they may become visually unappealing to eat. Careful removal of the parasitic sac and handling of the fish as usual is recommended.

That is the pod from Invasion of the Body Snatchers

Interesting.

and gross!