Uncommon diversity in developmental mode and larval form in the genus <Emphasis Type="Italic">Macrophiothrix</Emphasis> (Echinodermata: Ophiuroidea) |
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Authors: | Jonathan D Allen Robert D Podolsky |
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Institution: | (1) Department of Biology, CB#3280, University of North Carolina, Chapel Hill, NC 27599-3280, USA;(2) College of Charleston, Grice Marine Laboratory, 205 Fort Johnson, Charleston, SC 29412, USA |
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Abstract: | Development mode in the ophiuroid genus Macrophiothrix includes an unusual diversity of planktonic larval forms and feeding types. The modes of development for seven congeners
that coexist in coral reef habitats at Lizard Island, Australia were compared using larvae generated from crosses over several
reproductive seasons from 1999 to 2003. Three species (Macrophiothrix koehleri Clark, Macrophiothrix longipeda Lamarck, Macrophiothrix lorioli Clark) develop from small eggs (<170 μm) into typical obligately feeding planktonic (planktotrophic) pluteus larvae with
four larval arm pairs. The remaining four species develop from larger eggs (≥230 μm) into either facultatively-feeding or
non-feeding (lecithotrophic) larval forms. The facultative planktotroph (Macrophiothrix rhabdota Clark) retains the ability to digest and benefit 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 simplified larval morphologies with only
a single pair of full length (Macrophiothrix nereidina Lamarck) or highly reduced (Macrophiothrix belli Doderlein) larval arms and no functional mouth or gut. This genus includes the first example of facultative planktotrophy
in ophiuroids, the first example in echinoderms of a complete pluteus morphology retained by a lecithotrophic larva, and three
degrees of morphological simplification 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. |
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