California Flatworm, Hylocelis californica

California Flatworm, Hylocelis californica. Flatworm collected from a tidal pool in the greater Los Cabos area, Baja California Sur, January 2012. Size: 2.4 cm (1.0 inch) x 1.4 cm (0.6 inches).

Phylogeny:  The California Flatworm, Hylocelis californica (Heath and McGregor, 1912), is a member of the Cryptocelidae Family of Flatworms. The Cryptocelidae Family contains one subfamily and eight genera. The genus Hylocelis contains three species. Both the family name Cryptocelidae and the genus name Hylocelis come from the Greek words that mean something like “hidden spot, stain, or cavity”. This could refer to the cryptic eye spots on this species or that it hides in hidden cavities.

Morphology:  California Flatworms have unsegmented, flattened, soft-bodies. The bodies are elliptically shaped and are a tan olive color. They possess highly branched digestive tracts of chocolate brown or olive green that form highly visible zigzag lines radiating from the central main intestine to the frilled margin. They have a duo-glandular adhesive system. This allows them to secrete an adhesive that attaches anchor cells to the substrate, allowing the animal to cling to the substrate. They also possess a substance that releases the anchor cells from the substrate allowing them to either cling to the substrate or become free swimming. The body has a high surface to volume ratio and has no skeletal structure. They are triploblastic, indicative that they are composed of three fundamental cell layers – outer ectoderm, middle mesoderm, and inner ectoderm. They have no body cavities (acoelomates) other than the gut, and they lack an anus. The same pharyngeal opening takes in food and expels waste. The gut is highly branched to allow intracellular food transport to all parts of the body with no cell being too far from the outside, making a flat shape a necessity. They take in oxygen but have no formal respiration system. The nervous system is very simple, being composed of two nerve cords running down each side of the body; they have two simple brains called ganglia, that are bundles of nerves. They do not have formal eyes but have 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. This genus is distinct from other genera in the family Cryptocelidae by the presence of tentacles, and tentacular and cerebral eye clusters. California Flatworms reach a maximum of 2.4 cm (1.0 inch) x 1.4 cm (0.6 inches) in length.

Habitat and Distribution:  California Flatworms are a common but poorly studied and documented species. They are found under large rocks on gravel and sand substrates. They live from the middle intertidal zone to unknown depths. They are a subtropical, and possibly tropical, Eastern Pacific species. Some sources state that they are found in all Mexican waters of the Pacific Ocean; other sources limit this range to the west coast of the Baja Peninsula.

Diet:  California Flatworms are a poorly studied and documented species. They feed on small gastropods. They may also feed on organic matter and worms.

Predators: Predation of California Flatworms is not well-documented. Fish, such as pufferfish, and nudibranchs prey on other species of the Cryptocelidae Family.

Reproduction: California Flatworms are hermaphroditic with each individual producing both eggs and sperm. They lay eggs in cases that contain a single egg that are attached to the undersides of rocks. The eggs hatch in several weeks and quickly settle out benthically.

Ecosystem Interactions: The commensal, mutualuali and parasitic relationships of California Flatworms has not been documented.

Human Interactions: California Flatworms are occasionally used in laboratory research. Otherwise they have a very limited impact on human activities. From a conservation perspective they have not been formally evaluated however they are fairly common with a relatively wide distribution and should be consider to be of Least Concern.

Synonyms: Alloioplana californica, Planocera californica, Pseudoallioplana californica, and Stylostomum californicum.