Helmiäiskaloissa

Helmiäiskaloissa, also known as pearlfish, refers to a family of fish, the Carapidae. These are small, eel-like fishes that are known for their unusual commensal or parasitic relationship with other marine invertebrates, particularly holothurians (sea cucumbers). They often live inside the body cavity of their host.

The name "pearlfish" comes from the fact that some species were historically found inside pearl oysters, though this is not their primary habitat. Instead, their association with sea cucumbers is more widespread. The fish typically enters the sea cucumber through its anus and resides within its body cavity. It is believed that the sea cucumber benefits by having the pearlfish consume parasites or waste products, though in some cases the fish can cause harm to its host by damaging internal organs.

Pearlfish are characterized by their slender, elongated bodies, often translucent skin, and reduced or absent pelvic fins. They typically lack scales and their eyes are often small or degenerate, reflecting their dimly lit or internal environment. Their diet varies depending on the species and host, but commonly includes small invertebrates or detritus. Reproduction and larval development are not well understood for many species, but it is known that they have a pelagic larval stage before seeking out a host. Their distribution is global, found in tropical and temperate waters worldwide.