Speckled Flatworm, Pseudobiceros bajae

Speckled Flatworm, Pseudobiceros bajae. Flatworm collected from the coastal waters off Bahía Concepción, Baja California Sur, March 2020. Collection, photograph and identification courtesy of Barry Mastro, Escondido, California. Identification reconfirmed by Bob Hillis, Ivins, Utah.

Speckled Flatworm, Pseudobiceros bajae. Underwater photographs taken off Playa Contramar, in Zihuantanejo Bay, Guerrero, in January 2023. Photographs courtesy of Ron Woheau, Zihuantanejo.

Phylogeny:  The Speckled Flatworm, Pseudobiceros bajae (Hyman, 1954)  is a member of the Pseudocerotidae Family of Flatworms. The genus Pseudobiceros is one of ten genera in this family, and there are thirty-five species in this genus. They are known in Mexico as Gusano Plano Motcado.

Morphology:  Speckled Flat worms are oval in outline and flattened dorsally. They are unsegmented, soft-bodied invertebrates with bilateral symmetry, meaning they are equal on both sides of their centerline. Their gut is incomplete, in that they lack an anus. They are in the Order Polycladia. Polycladia means “many-branched”, and refers to the highly branch digestive sack that they possess. They do not have formal eyes but have two eyespots, which allow them to sense light. They move via tiny bristles called cilia and two layers of muscles under the skin. In an emergency they are capable of swimming via rhythmic muscular contractions. They have prominent tentacles made from folds in the anterior margin. Speckled Flatworms are generally black, but may also be tan to orange, in color. They have white and orange or yellow spots on their dorsal surface, which extend to the margin. In my observations, I have noticed that the white spots are smaller than the orange and yellow spots. In many individuals, the orange and yellow spots are absent from the midline of the body.  Speckled Flatworms reach 7.5 cm (2.9 inches) in length.

Habitat and Distribution:  Speckled Flatworms are found on the underside of rocks intertidally  to depths up to 15 m (50 feet). This species is generally considered a tropical Eastern Pacific species, though they appear to have a somewhat discontinuous distribution. This could be due to lack of data or taxonomic errors. In Mexican waters the Speckled Flatwork is found throughout the Sea of Cortez and the central west coast of mainland Mexico. There are no records from the west coast of the Baja Peninsula, southern Mexico, or Guatemala, although they are found in El Salvador and Southern California. The on-line photos from Southern California lack the orange/yellow spots. Records of this species from the Indo-Pacific are probably taxonomic errors regarding synonymy.

Diet:  Speckled Flatworms are predatory carnivores that feed on small invertebrates.

Predators: There is very limited documentation of predators for Speckled Flatworms. Other species in this family are preyed upon crustaceans and fish. The coloration of Speckled Flatworms very closely resembles that of the dark morph of the California Chromodorid, Felimare californiensis. The Speckled Flatworm secretes a foul-tasting chemical to deter predators. The color pattern on the nudibranch warns predators that it is not edible. It is thought that the Speckled Flatworm has evolved its appearance to mimic the nudibranch as a defense mechanism. If that is the case, this is an example of aposematic Müllerian mimicry (mimicking the warning coloration of a toxic or bad tasting animal in order to escape predation, though you are non-toxic and edible).

Reproduction:  Speckled Flatworms are simultaneous hermaphrodites (having both male and female reproductive organs).  They reproduce sexually by hypodermic insemination. This occurs when both mating partners dart their penis toward each other to induce one to act as a male and the other as the female. The victorious one to penetrate the body wall is the dominant male. Fertilization is internal and the resulting eggs are attached to the undersides of rocks. After several weeks the eggs hatch into planktonic larvae and quickly settle out benthically.

Ecosystem Interactions:  Information specific to the relationships of Speckled Flatworms with other species has not been documented.

Human Interactions: Speckled Flatworms have no direct impact on human activities. From a conservation perspective they have not been formally evaluated however they are fairly common with a wide distribution and should be consider to be of Least Concern.

Synonyms:  Cryptobiceros bajae and Pseudoceros bajae.