Larval phototaxis in barnacles and snails associated with bathyal sea urchins

Larvae of many shallow-water marine invertebrates show phototactic behaviors which may allow them to exercise some control over their positions in the water column. Such behavior should be useful only in regions where light provides a consistent directional cue and might not, therefore, be expected in species found exclusively in deep water where light levels are extremely low. In this paper, we investigated larval phototactic responses in two barnacle and one prosobranch gastropod species that occur epifaunally on tropical, bathyal sea urchins. Newly released larvae of the gastropod Pelseneeria sp. did not respond to light but Stage I nauplii of both barnacle species (Verruca floridana and Paralepas pedunculata) showed strong positive phototaxis at light intensities greater than 2.66 μmol m−2s−1. Larvae are expected to encounter the threshold intensities at depths no greater than 90 m in Caribbean waters. As both barnacle species were collected at much greater depths, phototaxis (at least in the early larvae) should have little effect. The ability to detect and respond to light may foreshadow functional photoresponses in later larvae that move to shallower depths. Alternatively, the presence of photoreceptors and light responses may be interpreted as evidence that these barnacles, which now reside in the deep sea, may have originated in shallow water.