3rd September 2012
Over the last few days, starting Thursday 30th August, we’ve been experiencing what appears to be a run of daily ostracod spawning events. I say ‘appears to be’, as I’ve been unable to find much in the way of details about marine Ostracod reproduction but have found reference to eggs hatching into nauplius larvae which already have a hard shell (Barnes). Otherwise descriptions of ostracod reproduction seems to be vague, describing eggs as being laid in the water as plankton or being attached to vegetation or to the substrate, with some species brooding the eggs inside the parent’s shell. With thousands of species of ostracod it makes identification difficult for anyone other than an ostracod specialist.
Each morning we’ve encountered large numbers of ostracods floating on the surface of a single tank, part of a 1000+ litre, 12-tank critter system. Roughly one millimetre in length and half a millimetre in depth they are forming what I’d describe as an egg raft, with up to a couple of hundred individuals clumping together.
To be more precise, I should say that they are floating on the surface of the water. This is odd as most small organisms in this size range often have difficulty at the water surface owing to surface tension, a boundary that they are unable to cross, either water to air or air to water. Without intervention small organisms remained trapped by surface tension; localised stirring can breakdown this boundary enabling small organisms to penetrate it and enter the water column.
What is difficult to understand is why these ostracods are on the airside of the water surface as the feed to the tank is below the surface and I can see nothing about the layout of the system that would allow them to be just ‘poured’ onto the surface.
In the reef aquarium ostracods are most commonly seen close to the viewing pane moving around on (and in) the top layer of the substrate. Once harvested from the surface of the water of this single productive tank on the critter system, and mixed into the water column, they quickly settle to the substrate exhibiting the commonly observed behaviour as above.
If we can find out how to intentionally harness this form of reproduction it may prove to be useful in mariculture as another natural food for larval fishes or early juveniles.
I’ve been feeding a proportion of each day’s harvest to our resident population of young clownfishes who are eagerly consuming them.
If anyone out there has an explanation for what is occurring or has had any experience of this phenomenon, please get in touch.
To learn more about ostracods, see Reef Ramblings ‘Zooplankton in the Reef Aquarium – Ostracods’