Lethal thermal limits in embryos of two intertidally brooding nassariid species, and evolution of a protective developmental mechanism conditioned by maternal experience.

Tritia obsoleta and Phrontis vibex are distant relatives in the neogastropod clade Nassariidae (1). Both species are common along coastline from Maine to the Caribbean; females deposit clutches of zygotes in capsules cemented to solid surfaces in intertidal flats (2), with swimming veliger larvae emerging after about a week of development. In this study, embryos of T. obsoleta (from Massachusetts and Florida) and P. vibex (from Florida) were exposed to temperatures ranging from 33{degrees} to 37{degrees}C. In both species, very young embryos are especially sensitive to thermal stress. Brief early heat shock did not disturb spiral cleavage geometry but led to variable, typically severe defects in larval morphogenesis and tissue differentiation. In T. obsoleta but not P. vibex, early heat shock resulted in immediate slowing or arrest of interphase progression. This reversible arrest is correlated with improved prognosis for larval development, and (in Massachusetts snails) depends on parental exposure to warm temperature. Embryos from Massachusetts snails housed at lower temperature exhibited cytokinesis defects in response to even mild warming. Thus, parents informed by experience may or may not endow their young with a developmental braking mechanism that lets them withstand occasional perilous conditions during the most vulnerable stage of life.