Uncommon diversity in developmental mode and larval form in the genus Macrophiothrix (Echinodermata: Ophiuroidea)

Abstract

Development mode in the ophiuroid genus Macrophiothrix includes an unusual diversity of plank- tonic larval forms and feeding types. The modes of development for seven congeners that coexist in coral reef habitats at Lizard Island, Australia were com- pared using larvae generated from crosses over several reproductive seasons from 1999 to 2003. Three species (Macrophiothrix koehleri Clark, Macrophiothrix longi- peda Lamarck, Macrophiothrix lorioli Clark) develop from small eggs (<170 m) into typical obligately feed- ing planktonic (planktotrophic) pluteus larvae with four larval arm pairs. The remaining four species develop from larger eggs (¸230 m) into either faculta- tively-feeding or non-feeding (lecithotrophic) larval forms. The facultative planktotroph (Macrophiothrix rhabdota Clark) retains the ability to digest and beneWt from food but does not require particulate food to complete metamorphosis. Among the lecithotrophic species, Macrophiothrix caenosa Hoggett retains the pluteus morphology with four pairs of larval arms, but is incapable of feeding, depending instead on maternal provisions for larval development. The remaining two lecithotrophs have simpliWed larval morphologies with only a single pair of full length (Macrophiothrix nerei- dina Lamarck) or highly reduced (Macrophiothrix belli Doderlein) larval arms and no functional mouth or gut. This genus includes the Wrst example of facultative planktotrophy in ophiuroids, the Wrst example in echi- noderms of a complete pluteus morphology retained by a lecithotrophic larva, and three degrees of morpho- logical simpliWcation among lecithotrophic larval forms. Egg volume varies 20-fold among species and is related to variation in feeding mode, larval form, and development time, as predicted for the transition from planktotrophic to lecithotrophic development.