Persian Carpet Flatworm

• It is hermaphroditic and practises penis fencing: two individuals duel, each trying to inject its sperm into the other's skin and to avoid receiving it, because whichever is fertilised takes on the cost of producing the eggs.
• Its colouring is aposematic: it accumulates toxins and advertises to predators that it is not worth eating.
• It has no anus: waste leaves through the same pharynx by which food comes in.
• Its mating is one of the most cited behaviours in biology: it is a hermaphrodite, and both individuals try to inseminate the other while avoiding being inseminated. The result is a duel known as penis fencing.
• The two face off, deploy their paired penises and thrust at each other, trying to inject sperm into any part of the opponent's body by piercing the skin. This is hypodermic insemination.
• And the reason for the duel is economic: producing eggs costs far more than producing sperm, so it pays each one to be the father and not the mother. The loser bears the cost of the clutch.
• The injected sperm migrates through the body until it reaches the ova.
• It swims by undulating the edges of its body in a motion reminiscent of a manta ray or a nudibranch, and its colours are aposematic.
• It is constantly confused with nudibranchs, but it is a flatworm: it has no shell, no external gills and no rhinophores, and its body is a sheet with no visible organs.

Skeleton type

Invertebrate - no rigid skeleton (flatworm)

Habitat and environment

Coral reefs and bottoms with sea squirts of the Indo-Pacific

Sources consulted: Michiels y Newman (1998), Nature; Rudman, Sea Slug Forum; Newman y Cannon, revisiones de Pseudocerotidae