Coral-eating Sea Slug, Phestilla lugubris, Bane of a Coral Keeper's Tank

View of the Porites compressa on Monday morning. Photo - M. Heckman 

Time for an update on one of our less desirable community tank inhabitants!

Trying to recreate nature in an aquarium is always a tough thing to do. We strive to keep a relatively harmonious mix of creatures in our touch/observation table. But with raw seawater coming in and clumps of new sponge continuously being added for the tiger cowries to eat, it is inevitable that some less desirable characters will appear.

Such was the case again recently - we have been having an infestation, which seems to happen to us at least once a year. When I leave on Friday for the weekend, I always check the tank and typically the corals look good. But about once or twice a year when I come in Monday morning, pieces of the finger coral (Porites compressa) have gone from healthy to compromised. They have big white blanched areas down near their base. 

Phystilla sea slug, C. Pittman photo

The culprit is easy to guess, the white sickly looking area is living tissue devoured by the coral eating sea slug Phestilla lugubris. This is an animal feared by those that try and raise Porites (finger and lobe) corals in captivity. Outbreaks can be devastating if not checked. A quick examination of any coral in question typically reveals a large (well fed) specimen of Phestilla settled between two of the coral's branches.

Note sea slug at top, areas that the sea slug has eaten and the 
various egg masses that it has laid - all in one weekend.

Closer view - M. Heckman Photo

Phestilla tend to hide under corals or rocks during the day and come out at night to feed on living corals.  In the wild, they are rarely a major issue for coral health since many predators will feed on them. A study by Dr. Aeby (from HIMB) and colleagues found that the threadfin butterflyfish (Chaetodon auriga)  would eat small specimens and the old woman wrasse (Thalassoma ballieui)  would eat any large Phestilla slugs that made the mistake of wandering up in the open during the day. 

Old woman wrasse

Threadfin butterflyfish

As to those hidden during the day, they would be sharing their habitat with various crabs and mantis shrimp that Dr. Aeby and group has found would also feed on the slugs. 

Gonodactylaceus mantis shrimp from
Introduced Marine Species of Hawaii

Areolated xanthid crab

The slugs are not without their own defenses. Although they are not able to transfer the coral's stinging cells to their skin (like some of their relatives) they do exude a slightly noxious substance when disturbed.

These are very interesting animals. They are one of the few sea slugs that scientist have figured out their entire life history - think egg to adult in less than 40 days, adults live only a few months, laying thousands of egg a day, then die a couple of weeks later. Contrast this to an adult sea cucumber that might live decades. Some invertebrates move slow and live slow, some move slow and live fast. Phestilla  may take down 10 square inches of coral a day, lay thousands of eggs and pass away, leaving their progeny to continue on.  

Crawl slow, eat large, live fast. 

For some great sites on these animals (and some of my references for this article), see:

Bill Rudman photo

The Phestilla lugubris page at the Sea Slug Forum - a great site, bits of info, questions with pictures from readers and answers to those questions. See: http://www.seaslugforum.net/showall/pheslugu

Pauline Fiene photo

Sea Slugs of Hawaii by Cory Pittman and Pauline Fiene - excellent site - simple, straightforward information with life history and more. A good place to go to learn about our local sea slugs (also, Cory was one of the first marine biologists I encountered as an undergraduate - ask me about the meeting sometime). See: http://seaslugsofhawaii.com/species/Phestilla-lugubris-a.html

Dana Riddle Photo

A recent article (this month) in Advanced Aquarist by Dana Riddle on coral eating sea slugs. Broad coverage, information for those who keep corals in captivity and the most up to date information. See: //www.advancedaquarist.com/2012/6/inverts

Other references
Control of populations of the coral-feeding nudibranch Phestilla sibogae by fish and crustacean predators. D.J. Gochfeld and G.S. Aeby. MARINE BIOLOGY, Vol. 130, Number 1 (1997)