By Donna M. Recktenwalt
Variously known as green marl or Jersey Green Sand, greensand is a fine, altered dark-green sedimentary material (probably of oceanic origin, since shark’s teeth are sometimes found in it) that is mined and sold as a holistic fertilizer and soil conditioner for plants. It differs from silica sand by being extremely fine grained and extremely dense; its high iron content gives it a blue-green/black color.
Eggs are collected from greensand either by stirring the spawning material and then netting out the floating eggs, or by slowly pouring the sand through a fine sieve into a second container of water.
A number of breeders have used greensand for spawning their annual killifish. The benefits of the material are that you know exactly how many eggs are contained; and you know when the eggs should be ready to hatch.
But greensand has its downside, as well. It is a fine, powdery sand, difficult to clean and messy to work with. Even after numerous vigorous washings, it tends to cloud the water, and it continually leaches salts and minerals. Its use can also result in egg loss or damage from abrasion, and in lowered viability, possibly due to interference with fertilization or to leachates in the water.
For some time, Wright Huntley used greensand extensively for spawning Fundulopanchax occidentalis, Nothobranchius guentheri and N. korthausae. He had what heconsidered mixed success with the spawning medium, and encountered apparent species specific infertility. With N. guentheri the fertility rate was about 95%; with F. occidentalis 80%; and for N. korthausae it dropped to 5%. Over peat, spawning fertility for the same pair of KOR was about 80%.
Barry Cooper has also used greensand extensively to spawn Nothos, and has also observed that fertility varies. He puts eggs collected from greensand on peat for a period of observation before storage, and has seen fertility rates as high as 95% and as low as 20% when using the material. “What is difficult to compare is what the fertility is in peat,” he says. “When eggs are collected from greensand you see every egg. That’s not the case when [eggs are] spawned in peat, although you can get an indication by looking for abundance at collection time, and looking again some weeks later.”
There apparently is a cost in terms of fertility to spawning over greensand, but the causative agent is as yet unclear: it might be something in the sand itself; it might be abrasion, either at spawning time when the egg is soft and particularly vulnerable, or later during harvest. It also may be a problem of fertilization in the closely packed sand.
“My advice,” says Barry Cooper, “is not to depend exclusively on greensand for [spawning] critical species.”
— G.C.K.A. Newsletter, January 1998