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.
Acrylic “Half-Ten” tank from Glass Cages. 5/19/15
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.
Light from below, with prototype hatching chamber. Front view. 5/19/15
Light from below, with prototype hatching chamber. 5/19/15
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.
Components of the manifold. 5/19/15
Manifold, assembled but not drilled. 5/19/15
Naturally, water arriving in the tank needs to leave, so I drilled it and added a bulkhead to drain to the sump.
Tank drilled and bulkhead installed for drain. 5/19/15
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.
Good luck! I don’t understand why it’s a problem to have the veligers swim up. Do they get stuck at the air-water interface and can’t free themselves because of surface tension? Why doesn’t this happen in the ocean? Can they grow while at the surface to get large enough to overcome surface tension? Would putting a mesh on the water help?
It’s interesting that they will swim down if light is down, but seems like an odd situation for them. Are they able to detect gravity? Do they know it’s “down”?
Are you planning to try to figure out what is inhibiting development?
Good luck raising the young’uns.
I don’t understand why it’s a problem to have the veligers swim up. Do they get stuck at the air-water interface and can’t free themselves because of surface tension?
Yep they get stuck in the surface tension. Or maybe get air stuck in their little shells.
Why doesn’t this happen in the ocean?
I think it’s because the ocean is more than an inch deep. The veligers can buzz around in the algae beds all day without hitting the surface. Probably the same reason juveniles get past the bottleneck in the 20″ deep slug tank. I just needed them to be able to develop in a small enough container to keep track of them.
Can they grow while at the surface to get large enough to overcome surface tension?
They don’t grow as veligers, but eventually they crawl out of their shells, which seem to be the actual problem. I am not sure whether the shells are more apt to stick to the surface layer, or whether they trap air, but the little slugs can crawl away and get onto the plants once they shed their shells. I am just not sure how much it stresses them to bob around on the surface for several days.
Would putting a mesh on the water help?
It might, but it seems easier technically to put mesh on the bottom and attract them down there. Doesn’t mean it will work.
It’s interesting that they will swim down if light is down, but seems like an odd situation for them. Are they able to detect gravity? Do they know it’s “down”?
They probably use both light and gravity to figure out which way is up. I don’t think they are particularly concerned about moving upward, but my impression, based on what is essentially anecdotes from the literature, is that light is very attractive. If light comes from above, they move toward it and get stuck at the surface of the little dish. If it comes from below, they will move down and bounce harmlessly around on the mesh on the bottom of the chamber.
Are you planning to try to figure out what is inhibiting development?
That’s the idea behind the new recirculating system. If it’s some chemical building up in the little dish, then changing to a recirculating system, with a much larger effective volume and activated carbon filtration, should get us past the hump.
Good luck raising the young’uns.
Thanks!