Is no one interested in breeding nutibranchs? Well this one is easy. I've observed 3rd generation Elysia Crispata obtained with very little effort in the right system. Unfourtunatly this was over 5 years ago and I dont have pictures so I can't make a proper journal. Here is the thread on the sea slug forum where I explained some of my results.
http://www.seaslugforum.net/find/17626 The seaslug experts questioned my identification (because of lack of a picture) but I am certain my slugs were Elysia crispata not clarki because their white spots (no chloroplast areas) remained as large distict spots as characteristic of crispata. On clarki the spots are small and much more numerous. The experts said "Unfortunately the food source for juveniles is unknown." but I know it!
These sea slug experts did not get a second generation of crispata, only the WC nutibranchs laid eggs. Perhaps this was because their water quality was not as good as mine and perhaps some chemical or lighting que was missing. They kept the nutibranchs in small bare tanks with lots of water changes and adding algae to
feed. Mine were in well established reef tanks with lots of coral, fish, and macro-algea. My tanks were controlled with custom designed (by me) ATS filters or macro-algea and required no water changes ever (for decades) so water quality was always perfect. (still have to add KI trace or frequent molting things like shrimps tend to die mid-molt and pulseing Xenia stop pulsing)
The tank size was not critical. Elysia crispata reproduced in 90 gal, 55 gal, and 24 gal (nanocube) systems. But having a gentle path to the sump for the larvae is critical. They went over an overflow to a sump where they settled in all 3 tanks. Larvae did not survive going back up through the pump and were eaten by the many filter feeders and fishes in the tank if they remained. They are lecithotrophic (non-feeding larvae) and pelagic for 5 days according to the experts. I did not specifically identify or seperate larvae, but I saw them hatch and swim out into the tank. Since they dont need to eat until they settle its easy you just have to provide the right algae and some of them will land on it.
Bryopsis is the favorite food of adults and juveniles did grow out eating it. However I got he best results raising juvenile crispata on cheatamorpha exclusively. The adults will eat pretty much any caleurpas also, but they dont get fat and lay eggs without bryopsis or cheatomorpha. They only choose to eay caleurpas after they are getting thin if they run out of tastier food. It is hard to tell how many larvae made it to settlement because they are about 1 mm long when they settle. But it looked like pretty much any that settled on the right aglae in the saftey of the sump grew out. Growth rates varied widely depending on if they got a good spot (close to the light) or not. They seem get most of their energy for growing from the chloroplasts they already stole rather than from sucking down more algea juice. One sump area had a CFL fixture stuck right to the side of the glass (velcro glued on) So the nutibrachs were about 1-2 cm from the bulb. This was a 28 W bulb with a split daylight/actinic spectrum. It was on 24/7. These guys grew the fastest, were adult size by 2 months and laid eggs a few weeks later. Others randomly living in the sump here or there or even a couple in the 55 gal tank also grew to adult size much slower (up to 6 months depending on conditions).
This was almost entirely a "hands-off" breeding success. The only exception being that eggs were moved from the 90 to the 55 and from the 55 to the 24 gal tanks. Adults didn't seem to live very long. Its hard to be certain about generations or individual lifespans because once you have hundreds of them they all look the same, but I bet adults did not live for more than 2 years. So like seahorses you really need to breed them to keep any long term. I know there were at least 3 generations because of the tank seperation but it could have easily been more. The population survived for at least 4 years until a couple house moves, power outages, and the back actually coming off the big 90 gal acrylic killed them off. (28 year old custom acrylic tank! beware weak looking weld seams on old systems!) Again I did not ever make a scientific count, but I would estimate that from a given hatch anywhere from 2 to 25 nutibranchs became adults. My Dad was able to move eggs successfully by razor blading them off the side of the tank into a specimen container. Egg location was usually high up on the glass in an area with strong but indirect flow. I dont recall ever counting eggs. I would guess something like 100-200 are layed.
Other details...all systems were full salt (1.026 salinity) Lighting for the adults was on an 8 hour day (full metal halide daytime was about 500 W/square meter) and 30 min (evening morning actincs only). Fish were all non-threatening to adults (tangs and angels). Though there were small mantid shrimps that concievably could have eaten adults and a big carpet anenome. But it didnt react if a nutibranch brushed up against it; maybe if one fell in it would have eaten it. They dont swim very well; they pretty much just float and wiggle.
I think thats all that is relevant. If you have any other questions I'll be happy to help. Good luck with your nutibranchs!