The closure of the late Neogene interoceanic seaways between the Western Atlantic (WA) and Tropical Eastern Pacific (TEP)—commonly referred to as the Central American Seaway—significantly decreased nutrient supply in the WA compared to the TEP. In marine invertebrates, an increase in parental investment is expected to be selectively favored in nutrient‐poor marine environments as prolonged feeding in the plankton becomes less reliable. Here, we examine turritelline gastropods, which were abundant and diverse across this region during the Neogene and serve as important paleoenvironmental proxies, and test whether species exhibit decreased planktotrophy in the WA postclosure as compared to preclosure fossils and extant TEP species. We also test for differences in degree of planktotrophy in extant sister species pairs. Degree of planktotrophy was inferred by measuring the size of protoconchs, the species' larval shell that represents egg size. Protoconch size was compared between extant postclosure WA and TEP species and preclosure fossil species. To compare extant sister species, we reconstructed the phylogeny of available WA and TEP species using one nuclear (H3) and three mitochondrial markers (12S, 16S, and COI). Compared to the preclosure fossils, protoconch size increased in WA species but remained the same in the TEP species. In the two extant sister species pairs recovered in the phylogenetic analysis, the WA species are inferred to be nonplanktotrophic while the TEP species are planktotrophic. This suggests that decreased nutrient availability and primary productivity in the WA may have driven this change in developmental mode, and was the primary selective force resulting in postclosure turritelline extinctions.

M illepore morphology is h ighly variable and shows signs of phenotypic plasticity. Two species of Millepora are present around the islands of the Bahamas: one exhibit ing a strong, blade-like structure, Millepora complanata, and the other having a delicate branch-like structure, Millepora alcicornis. The phylogenetic relationship of these corals has been under considerable debate for many years. The existence of a range of intermediate growth forms exh ibit ing characteristics of both recognized species has led to the re-examination of this species complex. Several methods were emp loyed to examine the taxono mic relationship including ecological abundance surveys, morphological thin-section analysis, and sequencing of rDNA internal transcribed spacer (ITS) reg ions. Abundance surveys showed a demarcat ion of growth forms by depth at two sites but an intermingling of growth forms at a third site. Morphometric analysis resulted in discrimination between M. alcicornis, M. complanata and the intermediate g rowth forms. However, rDNA sequence differences revealed the presence of two distinct clades, each containing members of the two currently recognized species as well as intermediate growth forms. The sequence analysis suggests the presence of two, phenotypically plastic cryptic species. Although limited in scope, our results indicate that caution should be exercised when describing species based on morphology alone and that multip le characters, including genetic information, should be used when describing species relationships.

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