Fig. 1.  Poorly fed (left) and well-fed (right) sibling echinopluteus larvae of the sea urchin Lytechinus variegatus on day 4 of development.  Note greater investment in ciliated band and internal skeleton under low food conditions.  Photo by JS McAlister.

Environmental heterogeneity creates a special challenge for character expression, because a given phenotype may not be optimal across all conditions.  Phenotypic plasticity allows many organisms to adaptively match trait expression to particular environments.  We have been taking advantage of a classic example of adaptive plasticity to examine tradeoffs that govern the expression of plasticity.  Echinoid larvae allocate more to growth of feeding structures (ciliated larval arms, and the internal skeleton that supports them) when food is scarce, but shift allocation to developing post-larval structures when food is plentiful.  Justin McAlister, then a grad student and now a faculty member at the Holy Cross, tested for possible constraints on the evolution of this form of plasticity.  In previous experiments with the echinoid Lytechinus variegatus, he demonstrated significant genetic variation for larval plasticity (see Fig. 1).  He then used a quantitative genetic breeding design and measures of growth rate to test an important theoretical prediction of a cost to plasticity: that more "plastic" genotypes, which have a fitness advantage across environments, will experience lower fitness relative to more "fixed" genotypes within environments.

Our work with Macrophiothrix brittlestars also provided the first test of whether larval plasticity is expressed in ophiuroids.  This question is of special interest because the pluteus larvae of echinoids (Fig. 1) and

Fig. 2.  Ophiopluteus larva of Macrophiothrix longipeda.  Armspan approx 1.7 mm.  Photo by JD Allen.

ophiuroids (Fig. 2), although morphologically and functionally similar, appear from phylogenetic and developmental evidence to be independently derived.  We have found that the expression of developmental plasticity in these ophiuroids is similar in timing, persistence, and magnitude to results previously published for echinoids.  Our demonstration of plasticity in larval skeletal growth is one of first examples where plasticity for a convergently-evolved trait appears itself to be convergent.