Hatchery in Progress
As described a while back, all steps in the culturing process seems to be going pretty well, except for one bottleneck. The adult broodstock is (are?) happy to lay eggs, the eggs hatch consistently, and the veligers settle. However, they will not develop much farther after settling in a controlled environment. Oddly, the settled veligers will develop if left on their own in a large tank full of algae. Although I have now reared E. clarki from egg to adult, it is not really possible to plan experiments based on when slugs may or may not decide to develop in a display aquarium. A more systematic approach was needed.
The hatchery is an attempt at making the juveniles happier during and after settling. Egg masses will still develop in glass crystallization dishes, but they will be placed in the new setup just before hatching. There were a few issues that may have impeded development, and they should be addressed by the new setup.
The tank is an acrylic “half-ten” from Glasscages.com. It is essentially a half-height 10-gallon tank (10″ W X 20″ L X 6″H). I had originally planned on using a standard 10 gallon, but it became clear that it would be clumsy and result in a lot of wasted space.
One problem that arises with free-swimming veliger larvae is that they are positively phototactic (attracted to light). This may not be a problem in the open sea or a large aquarium, but in the small dishes I was using for hatching, it meant that they would swim to the surface and promptly get stuck in the air-water interface. This would leave little rafts of veligers on the surface. These floaty veligers were capable of settling, so it was not a complete disaster, but it could not be good for them. Some authors (e.g., Dionisio et al, 2013) go so far as suggest rearing them in the dark. My solution is to illuminate from below. Marineland makes a nice little submersible LED light that can be used to keep the veligers swimming downward. Below are views of a prototype hatching chamber (2″ PVC pipe, with 50 micron nylon mesh to retain the veligers and larvae), showing the light coming from underneath.
Secondly, the presence of a relatively large food plant (large enough to be certain all of the little slugs can climb on) might have altered water chemistry, either through the process of photosynthesis (raising pH, e.g.) or by releasing chemicals that inhibit the slugs’ feeding or development. Continuously recirculating ASW (artificial seawater) from the larger system through their hatching chambers should reduce or eliminate this problem. In order to keep voracious invertebrates from entering the chambers, ASW will pass through a UV sterilizer before being distributed in the hatchery.
The manifold for distributing the ASW to the chambers is made from a few PVC pipe fittings. Once the cement has cured, it will be drilled to accommodate valves to control the flow to each chamber.
Naturally, water arriving in the tank needs to leave, so I drilled it and added a bulkhead to drain to the sump.
Once the manifold is finished, and the UV unit arrives, it will be ready to hook up and accommodate the next available brood. Stay tuned.