Ontogenetic shift of spectral sensitivity in the larval phototaxis of two sympatric caridean shrimp, Lysmata wurdemanni and L. boggessi (Decapoda: Lysmatidae)

Abstract

Phototactic behaviors have not been previously examined in the caridean shrimp family Lysmatidae. Informal observations suggest that their larvae exhibit a pronounced and unusual phototaxis. We subjected larvae of two sympatric species (Lysmata wurdemanni and L. boggessi) to a range of light spectra and intensities to examine their phototactic responses. Early- (zoea 1), middle- (zoea 6), and late-stage (zoea 12) larvae were examined under conditions of either light adaptation or dark adaptation. Each unique experiment (e.g., dark-adapted zoea 1 larvae of L. wurdemanni) had five replicate runs per treatment (e.g., n = 5 for each spectrum or intensity), with ten larvae per run. Larvae were less sensitive to light than some other decapod crustaceans, typically requiring a light intensity of 10¹⁴ photons m⁻² s⁻¹ to elicit a response. Both species displayed similar peak spectral preference shifts from 480 to 520 nm as they progressed from early to late zoeal stages. Spectral preference shifts during ontogeny are previously unknown in crustaceans and, to our knowledge, other marine taxa. The phototaxis of these larvae is also interesting in light of the reevaluation of the taxonomic status of western Atlantic Lysmata. These newly recognized separate species have distinct but overlapping ranges and habitats, and an adaptive interpretation of their respective phototactic behaviors may provide insight into their unique habitats and life histories. The apparent spectral shift observed in both species is likely a reflection of a migration from bluer (offshore or deeper) water early in larval development to shallower coastal waters as metamorphosis approaches.