Carcinophyly (nonfiction)

In some deep-sea species, attach to other animals (e.g., crabs) in certain behaviors.

Yes — this refers to a specialized reproductive behavior (sometimes called carcinophyly or reproductive parasitism) seen in certain deep-sea snailfishes, primarily in the genus Careproctus.How it worksThe snailfish uses its ventral adhesive (suction) disc to attach to the body of a king crab (family Lithodidae, such as golden king crab Lithodes aequispinus, scarlet king crab L. couesi, or others).

The female then inserts an elongated ovipositor into the crab’s branchial (gill) chamber and deposits a mass of eggs there. The eggs adhere and develop inside the protected, aerated space of the crab’s gills. The crab’s normal respiratory water flow helps oxygenate the developing embryos.

Multiple species have been confirmed via DNA barcoding of the eggs, including Careproctus melanurus, C. colletti, C. furcellus, and C. simus. An individual crab can carry eggs from one or more snailfish species.

Benefits and costs

For the snailfish: The hard, spiny carapace of the crab provides excellent protection from predators. The mobile host also keeps the eggs well-oxygenated and may help disperse the young. Larval survival appears enhanced by this arrangement.

For the crab: The egg mass compresses the gills, can cause localized tissue death (necrosis), and in severe cases leads to significant gill damage or loss on one side. This impairs respiration and has been linked to higher mortality of crabs in commercial fishing holds.

The relationship is generally considered parasitic (or at best a highly one-sided commensalism), as the crab receives no clear benefit and can suffer harm.

Additional observations

Small snailfish (juveniles or adults) have also been photographed hitchhiking on the carapace or legs of stone crabs and king crabs in the deep sea (hundreds to over 1,500 m depth), again using the adhesive disc. This may facilitate finding hosts for egg-laying or provide transport and protection.

This is one of the more unusual reproductive strategies among fishes and is particularly associated with certain deep-water Careproctus species that still retain a functional suction disc.

• Grok @ X (28 October 2026)