Life history dynamics of nudibranch sibling species in the genus Dendronotus in the Gulf of Maine
Two different larval types have been described for the nudibranch Dendronotus frondosus (Ascanius, 1774) in the Gulf of Maine. A taxonomic review of adults with planktrophic larvae and those with lecithotrophic larvae revealed regular differences in radula morphology, and no predictable differences in the reproductive system. These data and previous data on a behavioral reproductive isolation mechanism suggest a new species designation for populations of Dendronotus with lecithotrophic larvae. Laboratory studies were used to compare the larval dispersal potential of the two types of Dendronotus. D. frondosus planktotrophic larvae had a potential planktonic period of approximately 11 weeks at 10&deg;C and underwent significant larval shell growth prior to metamorphosis. This is the first example of a nudibranch larva with a Type 2 protoconch (Thompson) to have an extended planktonic period and require shell growth. The lecithotrophic larvae of the Dendronotus sibling species had a potential for delayed metamorphosis of only up to approximately one week. Finally, field observations indicated a southwestern coastal distribution limit of Dendronotus with lecithotrophic larvae in the Gulf of Maine in the Penobscot Bay region. This distribution potentially correlates with high seasonal stability of hydroid prey resources in the northeast Gulf of Maine and seasonally ephemeral hydroid populations in the southwest region. Higher temperature ranges in the southwest Gulf of Maine correspond with this trend of trophic stability. The significantly different dispersal potentials of the two larval types may correlate with these differences in seasonal resource availability, suggesting that trophic stability is a selective agent for their distribution and divergence of life history traits. Although fitness peaks for these two habitat regimes still must be quantified, this is a potential example of natural selection through ecological opportunity as part of an adaptive radiation. Understanding the life history dynamics of Dendronotus species in the Gulf of Maine will provide insights into the evolution of ecological characters of invertebrate larval types and life history strategies.
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