Fencing Flatworms

Two Speckled Flatworms (Pseudobiceros bajae)

Speckled flatworms in the Sea of Cortez. Flatworms like to glide over the reef, searching out food and romantic partners, in tropical seas. Although well suited to that lifestyle, they lack all body cavities, nor respiratory or circulatory systems. They do posess bilateral symmetry, and head and tail ends. The Baja Peninsula gives Speckled Flatworms their species name: bajae, of "of Baja". I like the way their speckles twist and change color along their edges. This black species requires a lot of strobe power for illumination.

Flatworms are hermaphrodites: they all have both male and female reproductive organs. When two flatworms of the genus Pseudobiceros meet, they first engage in "penis fencing". While protecting themselves, they both try to pierce the other's skin with their own penis, which is sharp and hard (and known technically as a stylet). The "winner" of the fencing contest injects their opponent with sperm. The injected sperm makes its way from the wound through the "loser's" simple flatworm body to the ovaries, and fertilizes the eggs. The "father" then swims away and the "mother" is stuck with carrying the eggs until they are ready to lay. It certainly doesn't sound like a fun first date.