Raising C. Multidentata: The French Guy Method
First of all, credit where credit is due: After watching a few hundred tiny zoeas perish in a jar of fresh water (back in 2005 or so), I googled a bit and immediately found Mike Noren’s fine article. I didn’t have much luck with his technique because I’m not as well-equipped or compulsive as he was, but his write-up is well researched and well-thought-out and clears up a bunch of misconceptions that seem to live on among hobbyists. Mike includes a link to a site in France (now translated into English) which outlines a much simpler rearing technique — a process which, due to the lack of attribution on the site, is inevitably referred to as “The French Guy Method”: shrimp larvae and microfood are cultured together in the same container without any supplemental feedings.
A Yamato shrimp’s life is similar to that of a salmon but whereas a salmon swims upriver only in the last days of life, Yamato shrimp spend their entire lives in a steady, uphill climb. This tendency is obvious in an aquarium — any time I add fresh water they swim about frantically, and if a current is present they will swim against it (even, at times, trying to force their way up a filter outlet.)
Yamato shrimp are amphidromous. The adults live their entire lives in freshwater, and spawn frequently during their multi-year lives. Females carry eggs on their swimmerettes for several weeks. Unlike their landlocked relatives (e.g. cherry or bee shrimp), the eggs don’t develop into fully-formed shrimplets, but instead hatch into late-stage plankton called ‘zoeas’. The zoeas can propel themselves slowly, but for the most part they’re carried with the current. They’re drawn to light which, presumably, keeps them in the river’s current and increases their chances of being swept out to sea.
After a month or so in the ocean (where in the ocean, I can only guess — mangroves, perhaps. They don’t live in intertidal areas, as brackish water seems to kill them, but it seems unlikely that they venture far into the open water) the zoeas develop into miniature adults with powerful swimming skills, at which point they begin their long lifetime of swimming against the current.
Reproducing this experience on a micro-scale is not especially complicated. You will need a tiny river (which, if you keep freshwater aquaria, you have already), and a tiny ocean. You’ll also need to simulate the first bits of the long swim from ocean to mountaintop, but that doesn’t require much planning. If your shrimp are carrying eggs but they’re only a week or two old, then it’s not to late to prepare a little patch of ocean for their offspring.
I use 2.5-gallon tanks, although lately I’ve been suspecting that jars (or even two-liter bottles) would work better, since changes in salinity due to evaporation would be minimized. Everything I’ve read, and everything that I’ve learned from experimentation, suggests that the key element to keeping the zoeas happy is complete consistency of conditions. Specifically:
– Constant water motion. Easy to accomplish with aeration. I use a bare airline tube rather than an airstone because on occasion I’ve found zoeas trapped on the surface by tiny airbubbles. (It’s possible that I’m confusing cause with effect, though, and that they were floating because they were already dead or impaired.)
– Constant lighting. I have a bright flourescent bulb over the tanks which is left on 24-hours a day. If the lights turn off, larvae start dying. My suspicion is that without light to keep them swimming they gather in the corners and suffocate.
– Clean water. This is the part that makes actually feeding the zoeas so tricky — add a bit of food, and the water quality swings all over the place leading to rapid mortality. Fortunately, the French Guy Method provides a perfect solution for this.
– Food. Mike Noren fed his zoeas lots of fancy high-protein plankton food. I, again in deference to The French Guy Method, feed mine one thing only: Tetraselmis.
I’m not very rigorous about maintaining my algae cultures, so before raising a batch of shrimp I generally buy a new algae starter from an aquaculture supplier. Tetraselmis is my default choice because the cultures seem to hold out the longest, but I’ve raised other plankton species with Nanochloropsus as well. If you have access to a source of phytoplankton or are one of the lucky folks whose tanks produce it by accident, you’re probably good to go regardless of variety.
The bright light that the zoeas like will please the algae as well. With a few drops of fertilizer and a bit of a plankton starter-culture, you should have an ideal shrimp-larvae habitat in a week or so.
I don’t exactly feed the Tetraselmis to the zoeas. Instead, I drop the zoeas directly into the algal culture. It works like magic: the shrimp larvae get to live in a never-ending algae buffet, and their wastes are immediately reprocessed by the remaining algae into more food. Nitrate and ammonia never have a chance to accumulate and, as long as you keep the lights and air turned on, oxygen is abundant.
Meanwhile, back in the river, you’ll need to collect the zoeas. I usually move the expectant mothers into a jar or bowl so that the newly-hatched plankton don’t get sucked into a filter or eaten by tank-mates. Unfortunately, hungry shrimp aren’t entirely above collecting and eating their own larvae, so it helps to keep an eye out and remove the adults right after hatching.
Some reports suggest that the zoeas are are sturdier if they’re left in freshwater for a few days. I don’t have any first-hand evidence either way — it’s a topic for future experiments. If the zoeas hatch in a bowl, gathering them is easy: I just pour the entire contents of the bowl through a brine-shrimp net. If yours hatch in a larger tank, you can wait until nighttime and lure the zoeas into a corner with a flashlight and then siphon them into a net.
Now it’s time for the zoeas to drift downstream into the ocean. In my experience, you don’t need to ease them into it — they can transition from fresh to salt-water abruptly, and will immediately begin swimming, eating, and growing. Just rinse out your zoea net in the saltwater tank, and you’re done.
At this point, restrain yourself from messing with anything — the larvae have plenty of food, and feeding them anything else will just mess with the water chemistry. If possible, leave the country for a month or so. Just don’t forget to pay the electric bill.
Sometimes, everything dies. Much of the time, a dozen or so of the original thousand zoeas will grow and develop. Occasionally hundreds will. If luck is with you, then after three or four weeks you will start to notice that the zoes are big, fat, and orange.
Towards the end of their childhood, the zoeas start to resemble their parents. They don’t swim like their parents, though — they still have the tentative, hovering, backwards-swimming posture that they were hatched with. As long as they’re acting like that, they need to stay in the ocean — reductions in salinity will kill them. After a few days, though, they’ll start to settle on the bottom, and act like shrimp — resting, and then zooming forward, and then resting.
A day or so after that, they’ll lose their orange shade, and turn completely transparent.
Now they look and act like miniature duplicates of their parents. They may be as long as 1cm, or as little as half that — size doesn’t seem to correspond very closely with development. In any case, now they’re ready to head upriver.
Separating the postlarvae (who want to move into fresh water) from almost-postlarvae (who will die in freshwater) requires patience. I find that the postlarvae are almost impossible to catch, so typically I scoop out as many hovering zoeas as I can, then pour the whole tank through a net, and then return the zoeas to the now empty tank. If you are sharp of eye and steady of hand, sucking up the postlarvae one by one with a syringe can work as well.
The postlarvae now need to transition gradually into freshwater. This is hard, and I always lose quite a few at this stage. Move them too quickly and they’ll die of shock — move them too slowly, and they’ll foul their brackish water and die from that. The best possible solution is to transfer the postlarvae into new, clean seawater (without the algae, which will die during the transition) and then set up a slow drip of freshwater which will triple the water volume over a couple of days. A simpler method that works reasonable well is to scoop your postlarvae into a cup or quart jar and perform partial, daily water changes for three or four days, then dump the jar into fresh water.
Unless your freshwater tank is very sparsely decorated, your postlarvae will now disappear from view. They’re tiny, transparent, and shy. I always spend a few weeks fretting before the tiny shrimp are big enough and brave enough to eat in the open. They’re durable, though — once they’ve survived the shift to freshwater they’re as sturdy and hungry as their parents, and in six months or so will be producing plankton of their own.