My 100-gallon reef tank was recently overtaken by hair algae, an unfortunate plight that befalls many older reef tanks. I set up this tank in 1972 and cleaned it thoroughly only once, about 25 years ago. Putting off a long overdo cleaning, I decided to add an animal that was advertised as something that would eat this menacing algae. To this end I acquired what was sold as a "lettuce nudibranch" and placed it in the tank.
To my surprise, it was not the least bit interested in eating the hair algae--or as far as I could tell, anything else. Still, the newcomer was interesting to me in other ways and moved around quite a lot. Since this "nudibranch", which I later found out is actually a slug, did not help with my algae problem, I knew it was inevitably time to clean the tank.
I use mostly artificial sea water with a proportion of natural sea water collected locally here in New York. I have to admit that I was kind of curious to see what I would find under my reverse undergravel filter plates, which were state of the art in technology in 1972. I often add amphipods and anything else I find under rocks from muddy Long Island beaches.
I placed the live rock in the dark in plastic garbage pails containing the existing tank water. While cleaning the tank, I temporally relocated the live coral and the lettuce sea slug to a few small, unheated tanks. The tank was emptied except for a few inches of water and an extra large diatom filter was run while I stirred the vintage dolomite gravel. The filter required twelve cleanings. The small tank with the slug cooled about ten degrees this may or may not be a factor in this story. I returned the coral and the slug to the tank. The rock remained in the dark for a week to kill the algae; it was then also returned to the tank.
Now my tank looked pristine and the animals stood up with renewed vigor, all except the slug, which I perceived to have shrunken drastically. Every time I passed the tank, the formerly two-inch long slug seemed to be getting significantly smaller. It was now only just over 1/4"-long. I just figured that like sea anemones, it was starving and shrinking, but it amazed me that, in a six-foot long tank, I always spotted my ever shrinking "lettuce nudibranch" immediately.
I decided to watch it for a while to see if it was eating. I did not have to watch it long before I found, to my surprise, another one, then another, and so on! I counted 50 of them, ranging from 1/8 inch to about 1/2 inch. I have also found egg cases, but they were already hatched. They look like 3/8" long spirals similar in appearance to the piece of metal waste you get when you drill into metal with a sharp bit.
It has been two months since I acquired the original "mother" slug, which I have not been able to find recently and there are much too many of "her" progeny to count. The largest ones are almost one inch long and I know they are still multiplying because of the numerous 1/8 inch babies. The critters are multiplying and growing so fast that I have started looking for recipes for slug soufflé!
But how do they eat? According to Bill Rudman on the online Sea Slug Forum, in a process known as kleptoplasty, they suck the chloroplasts out of certain Caulerpa and incorporate these chloroplasts into their own tissue. (With strong lighting, the chloroplasts continue to generate food for the animal through photosynthesis).
As for these slugs eating hair algae, forget it. I performed a test where I placed six of them of various sizes in a two-pint enclosure in my tank with two different kinds of Caulerpa, hair algae, lettuce and parsley (just because you never know). They have been there a week and nothing, including the algae, was eaten. They do seem to rest on the hair algae, but if they are eating it, it is growing faster that they can consume it, which I doubt. Keep in mind, there are other types of sea slugs which definitely will consume hair algae.
The biggest problem with these animals is that they really do not "stick" to rocks like land slugs do, and if the current is too strong for them, they get swept up into filter intakes rendering them into slug jello. Of course, current aside, they will also crawl into the intakes. There are three ways that I can see to solve this problem. The easiest is to screen the openings. This will do little to keep the babies, which have a very short planktonic swimming stage I have yet to witness, out. You can also place the power heads in the tank in such a way that they do not make contact with the sides of the tank underwater, thus barring the slugs' access since they swim about as well as they eat hair algae.
The last way to deter accidental slug suicide is to place the pump intakes in the dark. These slugs rarely venture into the dark, and looking at them at night with a flashlight, (talk about my having no life) you can see that they do not move in the dark. Of course, the best way would be to incorporate all three of these methods.
I really cannot report on the longevity of the species, but, at the rate they grow, I would imagine it should not be more than a few months. Time will tell.
Rudman, W.B., 1998 (October 11) Solar-powered Sea Slugs Sea Slug Forum. http://www.seaslugforum.net/factsheet.cfm?base=solarpow