Ontogeny of phototaxis and geotaxis during larval development of the sabellariid polychaete Phragmatopoma lapidosa
Thorson generalized that the larvae of intertidal animals should remain photopositive throughout larval life to facilitate encounters with shallow surfaces at settlement. We tested this idea for the sabellariid polychaete Phragmatopoma lapidosa by investigating ontogenetic changes in phototaxis and geotaxis of well-fed, laboratory-reared larvae. Larvae at each of 5 age groups were exposed to (1) 8 different light intensities from a directional light source in a horizontal trough and (2) complete darkness in a vertical chamber. Phototaxis changed with age. Half-day old larvae were not responsive to light, 1 d old larvae were positively phototactic, and all older larvae to 28 d age were negatively phototactic. Larvae from 0.5 to 28 d age were positively geotactic with the exception of 5 d old larvae, which lacked a defined response. Consequently, larvae of the predominantly intertidal P. lapidosa did not conform to Thorson's suggestion of continuous positive phototaxis in intertidal species.
McCarthy, DA; Forward, RB; Young, CM
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