About the Cover
Cover
Marine gastropods usually develop by a process that includes an encapsulated embryonic stage followed by a feeding stage that lives independently in the plankton. Many species, however, lack the planktonic larval stage and instead develop entirely within the capsule, emerging as crawling juveniles that live in the benthos. Ancestors of our existing gastropod species are presumed to have possessed feeding larvae, meaning that the larval stage has been lost through evolution. Traditional thinking, represented by a hypothesis formulated by a French scientist in the late 19th century and known as Dollo's Law, is that evolution is not reversible. Thus a gastropod that has lost its feeding larva should not be able to reacquire it. Now, however, in work focusing on the evolutionary dynamics of the transitions between larval development and direct development in a group of gastropods, Rachel Collin and her colleagues are challenging that assumption. They base their contention that feeding larvae can be regained on the fact that the larval structures are retained in the encapsulated embryos of some species that hatch as benthic juveniles.
In pages 83 to 92 of this issue, Collin et al. present evidence that feeding larvae have been regained in a species of marine gastropod belonging to a group known as the calyptraeids, which includes slipper shells and cup-and-saucer-shells. Species in the genus Crepipatella contain some species with direct development and some with planktonic larvae.
Embryos of all species of the marine gastropod Crepipatella develop together within a transparent capsule like the one that is pictured on the cover and belongs to Crepipatella dilatata, which exhibits direct development. In this species only a few of the eggs in each capsule begin development. These embryos eventually eat the undeveloped eggs. The normal embryos seen here have eyes (visible as small black dots), tentacles, a fully formed shell, a muscular foot, and a well-developed velum-the last being a typical planktonic feature. Several of the embryos are pointing directly toward the camera. These embryos are just beginning the consumption of the undeveloped "nurse eggs," a few of which can be seen in the foreground and the bulk of which can be seen as the yellowish background. Species with direct development lose the features characteristic of planktonic larvae (such as the ciliated velum) before hatching.
Collin et al. use developmental studies and DNA sequence data to demonstrate that Crepipatella fecunda, a species with planktonic development, evolved from C. dilatata, the species shown here, which has direct development.
Credits: Photo, Oscar Chaparro (Universidad Austral de Chile); cover layout, Beth Liles (Marine Biological Laboratory).
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Copyright © 2007 by the Marine Biological Laboratory.
