Today, I've noticed a few that are different. These photos don't show everything because they're top-down views. (I haven't worked out how to take profile shots with a microscope without the aid of zero-gravity).
I guess these guys are post-larvae. They're benthic, and occasionally zoom around (forwards) when startled. I'm surprised, because they are so small. Mike Noren's article (currently off-line as best I can tell) described the postlarvae as being 8mm long -- this article http://caridina.japonica.online.fr/English/index.html describes late-stage larvae as 'less than 1cm long'.
Mine are much smaller than this, as you can see. I'm not sure what to make of that -- perhaps the postlarvae continue to grow in seawater?
Thanks for the pictures Many of us have not seen these post-larvae before. They are so small at about 3mm, but you can see the shrimp body shape clearly.
They can live for weeks in salt water, so don't get stressed. If they are benthic they are post larval. When you are ready scoop them out and start the transition back to fresh water.
Since I was expecting them to be more than twice as big, I'm not positive. But the behavioral changes (not evident in the photos, obviously) were pretty dramatic.
Because the larvae are in a huge tank, I check on them by switching off all the lights and luring them into a spot close to the front glass with a flashlight. The larvae drift gradually towards the light, hanging head down, and then start to bounce around nervously when they bump into the glass or each other.
This happened yesterday, as usual, but then a few other somethings zoomed up to the light, swimming 10x faster than the others and zipped around crazily, sending all the other larvae into a scattering panic. The active swimmers were lighter in color and had slightly different posture and swimming style. When I scooped out a few to look at more closely, the clear ones sat on the bottom of the cup while the orange onces floated in the water column.
And, I suspect that in profile they'd look pretty different in a photo too. I'm not sure how to get profile photos, though, without squashing the little ones under a coverslip.
Anne..."post-larva" (this is the singular of "larvae", which is plural) just means that the shrimp have completed all their larval stages and are miniature versions of their parents. They will have legs and craw around instead of floating, i.e. they are what's called "benthic."
What eraserbones describes above sounds like post-larvae to me. Post-larvae are usually very obviously different from the larvae, so there should be no confusion. Maybe eraserbones just didn't have a chance to take a good look at their shapes.
However, there is chance that they might just be the last or next to the last larval stage. At that stage the larvae don't float around anymore but usually sit on the ground, the tank sides or anything else that happens to be in the aquarium. Either way...they still have the larval shape, so they are still easily distinguishable from post-larvae.
Good idea, Anne -- I put the larvae in a specimen box and balanced everything on its side. I have no idea how to embed a video in this post, but here are a couple of clips.
Many of us have not seen these post-larvae before. They are so small at about 3mm, but you can see the shrimp body shape clearly.
