Time to metamorphosis of blue crab Callinectes sapidus megalopae: Effects of benthic macroalgae

There is growing evidence that postlarvae (megalopae) of the blue crab Callinectes sapidus can slow the progression through the molt cycle while in offshore nursery grounds, and delay metamorphosis until reinvasion of coastal habitat occurs. However, the cues that trigger metamorphosis of megalopae are not well known. This study tested the hypothesis that the time to metamorphosis (TTM) from the postlarval megalops stage to the first crab stage is shortened in the presence of 2 potential macroalgal settlement substrates, Ulva lactuca (Chlorophyta), and Gracilaria spp. (Rhodophyta). Megalopae and test water were collected from 3 locations (offshore, at a coastal inlet, and inside a coastal lagoon) and tested in a completely crossed factorial experiment with algal type and location as main effects. TTM was longest in offshore treatments (mean TTM 4.72 d) and similar in the inshore treatments (mean inlet TTM 2.73 d, lagoon TTM 2.51 d). TTM of offshore megalopae was reduced in the presence of Ulva lactuca, but macroalgae had little effect on the inshore treatment groups. The effect of algal cues may be masked once megalopae have initiated premolt prior to invading coastal lagoons.