Nudibranches and acoel flatworms

September 30, 2008

The above photograph of acoel flatworms on porites coral was shot in January 2008. Acoel flatworms reproduce primarily asexually, by fission, and can quickly attain high densities with sufficient light (they have symbiotic algae) and abundant copepod prey. They are not parasitic, but can reportedly damage corals by shading. Though no easily available predator of these worms is known, aquarists have found that certain nudibranchs of the genus Chelidonura feed exclusively on acoel flatworms. These attractive nudibranchs measure 5 cm in length and are interestingly shaped similar to their flatworm prey, and possess two streamer-like appendages and a hammer-shaped head. Aquarists have described their feeding actions as best compared to a vacuum cleaner. As they come upon a flatworm, they fold part of their body and form a straw-like proboscis that they use to “suck” up the worms.

These flatworms were first noticed on Hantu Islands corals in August 2007. They were repeatedly observed on corals that shows signs of bleaching in Nov 2007, then later in April and May this year. It is not apparent what caused them to multiply this visibly in Hantu waters and since there have been no previous observations, it is difficult to pin point when exactly they began their significant presence and what caused it.

Sources: Sea Slug Forum, Advanced Aquarists Online

Posted in Biology