Fantastic example how complicated our aquariums can be
I just found this paper while searching for something completely different but what a gem it was!
Selden et al. Waterborne cues from crabs induce thicker skeletons, smaller gonads and size-specific changes in growth rate in sea urchins. Marine biology (2009) vol. 156 (5) pp. 1057-1071
Indirect predator-induced effects on growth, morphology and reproduction have been extensively studied in marine invertebrates but usually without consideration of size-specific effects and not at all in post-metamorphic echinoids. Urchins are an unusually good system, in which, to study size effects because individuals of various ages within one species span four orders of magnitude in weight while retaining a nearly isometric morphology. We tracked growth of urchins, Strongylocentrotus droebachiensis (0.013–161.385 g), in the presence or absence of waterborne cues from predatory Jonah crabs, Cancer borealis. We ran experiments at ambient temperatures, once for 4 weeks during summer and again, with a second set of urchins, for 22 weeks over winter. We used a scaled, cube-root transformation of weight for measuring size more precisely and for equalizing variance across sizes. Growth rate of the smallest urchins (summer: <17 mm diameter; winter: <7 mm diameter) decreased by 40–42% in response to crab cues. In contrast, growth rate of larger urchins was unaffected in the summer and increased in response to crab scent by 7% in the winter. At the end of the 22-week experiment, additional gonadal and skeletal vari- ables were measured. Cue-exposed urchins developed heavier, thicker skeletons and smaller gonads, but no differences in spine length or jaw size. The differences depended on urchin size, suggesting that there are size-specific shifts in gonadal and somatic investment in urchins.
Full paper available here
What this study (or rather, the mini review in Introduction) explains is that various invertebrates react to chemical cues from their natural predators in a quite dramatic ways. Their shells can become thicker, they might alter growth rate and reproduction, their behavior might change to more cryptic etc.
If we think how small the water volume is in relation to the biomass in our aquariums it’s no wonder that we have individuals behaving differently in different aquariums with all the chemical cues flowing around. For example, I have had this long lasting “problem” with Lysmata spp. cleaning shrimps constantly hiding inside the structure while the exact same species in other tanks are very courageous. The answer to this mystery might very well be that I have some predatory crab or fish in my tank that the shrimp can sense.
This paper also highlights the point that one can not give blanket statements on invertebrate husbandry because there are just too many variables. The only way to be really successful is to study the biology and ecology of the animals you are interested in and their natural environment.