Meeting Abstract
Branchiopod crustaceans have a unique phylogenetic position to make inferences about both insects and crustaceans (Pancrustacea). All branchiopods express four or more visual pigments. This brings up the question: what selective forces maintain these multiple-pigment visual systems, especially considering branchiopods have secondarily-reduced optic ganglia in comparison to “higher” order crustaceans and insects? We show that two species of branchiopods found in ephemeral pools throughout Southwestern North America use light for vertical positioning. They use multiple visual pigments for phototactic behavior. Triops longicaudatus are benthic foragers, while Streptocephalus mackini are suspension feeders that swim higher in the water column. Due to a seasonal “monsoonal” wet period in Arizona, we have described light environments of ephemeral pools over the life cycle of these branchiopods in two regions. Light within these pools attenuates rapidly with depth and is wavelength-specific to soil region. We find that regional light environments have shaped the spectral sensitivity of behavioral responses. We also find that S. mackini males may use a single photoreceptor type for positive phototaxis to maintain position in the water column above females. We compare phototactic behavior to visual system electrophysiology (extracellular ERG recordings). Using a maximum-likelihood approach, we use data from extracellular recordings to generate hypotheses about visual pigment parameters and place these results in the phylogenetic context of the ancestor of insects, which had three visual pigment classes, homologous to three of those expressed in branchiopods.
