Readers of a certain age (or readers who have perused the comic books once belonging to readers of a certain age) will instantly recognise the image above. It appeared on almost every comic book published between 1962 and 1975, and offered a something truly mind-blowing. For a couple of bucks, you could receive a small packet in the post that, when its contents were added to water, grew into minute fish-tailed humanoids that would create their own minute society, all in one goldfish bowl sitting in your bedroom!
As Robin Ince summed up the sea monkey experience in his Bad Book Club: 'This was a lie'. You did receive a small packet in the post, the contents of the packet did hatch out in water, but you did not get the pictured anthropomorphs. What you actually got were these:
The 'sea monkeys' became labelled one of childhood's great disappointments, which I call an utter shame. Because I personally would describe them as some of the most elegant crustaceans that I've ever seen.
Brine shrimp and their relatives belong to a group called the Anostraca. The Anostraca, sometimes referred to as fairy shrimps, are a group of a little under three hundred described species. They are generally less than an inch long, though the larger species can grow to several inches. The taxon name basically means 'without a carapace', and this is one of the distinctive features of the group. The body is elongate and, behind the head, is divided into a thorax bearing feathery swimming legs and an abdomen lacking appendages except a terminal pair of uropods. Most species of Anostraca have eleven pairs of swimming legs, though the species Polyartemiella hazeni and Polyartemia forcipata have, respectively, seventeen and nineteen pairs (Weekers et al. 2002). Anostracans have a distinctive slow swimming style, lying on their back. They are found living in ephemeral or hypersaline waters where predatory fish are few or absent; in order to persist in such environments, they produce resistant eggs that are able to survive drying out, hatching when the temporary pool is refilled by the rain.
The phylogeny of Anostraca was investigated by Weekers et al. (2002), who found that they could be divided between two lineages: one including the genera Artemia and Parartemia, which are found in hypersaline waters, and the other containing the remaining freshwater genera. Most members of both lineages are filter-feeders, but some larger members of the freshwater lineage in the genus Branchinecta have become predators. The most favoured prey of these large Branchinecta? Why, smaller Branchinecta! Studied specimens of the predatory Branchinecta raptor would only deign to take other invertebrate prey if their preferred B. mackini was unavailable (Rogers et al. 2006). These predatory Branchinecta are found living in turbid, sediment-filled waters with low visibility, and mostly found their prey by coming into contact with it whilst swimming in the water column. Squeezing water out of a pipette near one would incite it to try and attack the pipette. If unable to find swimming prey, B. raptor would swim down to the sediment bed and stir it up, then attempt to find invertebrates flushed out of hiding.
The two hypersaline genera have complementary distributions: Parartemia is endemic to Australia while Artemia is found on the remaining continents (though Artemia is now present in some localities in Australia as an introduced taxon). In the past, all Artemia around the world were often treated as a single species, A. salina. However, the existence of a number of geographically distinct lineages has now been established, with these treated as separate species (A. salina proper is found in Europe). Both sexually and parthogenetically reproducing forms of Artemia exist. The parthenogenetic forms are treated as a single species, A. parthenogenetica, and derive from a single Eurasian origin, but are themselves genetically diverse, including diploid, triploid, tetraploid and pentaploid individuals (Triantaphyllidis et al. 1998). Sadly, this new-found taxonomic complexity of Artemia is in some danger of re-simplifying: the international trade in brine shrimp, used mostly as food for fish, is almost entirely based on eggs derived from the Great Salt Lake in Utah. As a result of this trade, the North American species A. franciscana has become introduced, both accidentally and deliberately, to saline waters around the world, and has been found in many localities to be replacing the native brine shrimp.
Rogers, D. C., D. L. Quinney, J. Weaver & J. Olesen. 2006. A new giant species of predatory fairy shrimp from Idaho, USA (Branchiopoda: Anostraca). Journal of Crustacean Biology 26 (1): 1-12.
Triantaphyllidis, G. V., T. J. Abatzopoulos & P. Sorgeloos. 1998. Review of the biogeography of the genus Artemia (Crustacea, Anostraca). Journal of Biogeography 25: 213-226.
Weekers, P. H. H., G. Murugan,J. R. Vanfleteren, D. Belk, & H. J. Dumont. 2002. Phylogenetic analysis of anostracans (Branchiopoda: Anostraca) inferred from nuclear 18S ribosomal DNA (18S rDNA) sequences. Molecular Phylogenetics and Evolution 25: 535-544.