Pseudocerotidae Family of Flatworms

Two Flatworms of the Pseudocerotidae Family can be found in this website:

Phylogeny: Flatworms of the Pseudocerotidae Family fall within the phylum Platyhelminthes. They are in the subphylum Rhabditophora, the order Polycladia, and the suborder Cotylea. The Pseudocerotidae Family is one of six families in this suborder. The Pseudocerotidae Family contains eleven genera and two hundred forty-seven species. The name Pseudocerotidae comes from the Greek words meaning “false horn”. Perhaps this refers to the soft anterior sensory organs.

Morphology: Pseudocerotid Flatworms are oval to elongate in shape, and flattened dorsally. They are unsegmented and have bilateral symmetry (equal on both sides of their centerline). They have prominent tentacles made from folds in the anterior margin. Their gut is incomplete, in that they lack an anus. They are characterized by lacking a sucker behind the female gonophore,  their eyespot arrangement, their pharynx (muscular ruffled mouth) and tentacles. They do not have respiratory or circulatory systems. Instead, they exchange gases by diffusion at the cellular level. They move via tiny bristles or cilia and two layers of muscles under the skin. In an emergency they are capable of swimming via rhythmic muscular contractions. They do not have formal eyes but have two eyespots, which allow them to sense light. They have the ability to quickly replace missing body parts. Some are brightly colored and contain toxic materials to help avoid predation. Flatworms of the Pseudocerotidae Family reach a maximum of 15 cm (5.9 inches) in length.

Habitat and Distribution: Pseudocerotid Flatworms are generally found in protected waters on hard substrates such as rocks, pilings, rubble, and reefs,  but they can also be found on soft substrates, mangrove roots, and stands of algae. They live from the intertidal zone to very deep water, including deep sea thermal vents.  They are active either diurnally or nocturnally. Peudocerotids are found worldwide, in tropical to temperate seas. At least six species from this family are found in Mexican waters.

Reproduction:  Pseudocerotid Flatworms are simultaneous hermaphrodites (having both male and female reproductive organs). They reproduce sexually through hypodermic insemination (stabbing the penis through the body tissue). The eggs are laid in masses on the bottom substrate. The eggs hatch into Muller’s larva, which eventually metamorphose into adult organisms.

Ecosystem Roles: Most Pseudocerotid Flatworms are predators that feed on ascidians, barnacles, bryozoans, coral polyps and mollusks.  A small percentage of the family are herbivores that feed on algae and diatoms. Once their food has been digested, they expel it out of their mouth. In turn they are preyed upon by crabs, fish and nudibranchs.