Reproductive Patterns in Six Species of Lepidochitona (Mollusca: Polyplacophora) from the Pacific Coast of North America

¹ Museum of Zoology and Department of Biology, University of Michigan, Ann Arbor, Michigan 48109

Reproductive patterns are documented and compared in six morphologically similar members of the chiton genus Lepidochitona from the west coast of North America (Oregonian and Californian Provinces). Three of the six species studied brood embryos: L. thomasi (Pilsbry, 1898), L. caverna Eernisse, 1986, and L. fernaldi Eernisse, 1986. The offspring of brooders are able to crawl away. In contrast, L. dentiens (Gould, 1846), L. hartwegii (Carpenter, 1855), and the less common L. berryana Eernisse, 1986 are free spawners whose offspring are obligate dispersers. The dispersal consequences of brooding or not brooding are exemplified by Lepidochitona, without major complications due to differences in larval size or larval feeding ability. Developmentally, brooders and free spawners in Lepidochitona differ primarily in stage (i.e., age) at which larvae hatch from their egg capsules. Larval size and morphology differences are present but not as extreme as in other taxa.

As in many other taxa there is a link between brooding and particular life history traits, especially small adult size and self-fertilization. Size comparisons match the expectation that, as adults, brooders are generally as small or smaller than free spawners. The two smallest of the three brooders, L. caverna and L. fernaldi, are also simultaneous hermaphrodites, based on examination of gonads. These are the only known hermaphroditic chiton species, and are apparently fully capable of self-fertilizing multiple broods based on evidence from animals isolated for up to nine months in the laboratory. The third brooder, L. thomasi, is more typical of chiton species including those that brood; it has separate sexes and does not produce viable broods in isolation. Based on comparisons among chitons and among other groups that normally have separate sexes, hermaphroditism is argued to be a consequence of brooding, rather than the reverse. A mechanism is suggested that would link hermaphroditism, but not small adult size, to the consequences of crawl-away offspring. Small adult size could alternatively be attributed to the morphological constraints imposed by brooding.

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