Poecilogony and population genetic structure in Elysia pusilla (Heterobranchia: Sacoglossa), and reproductive data for five sacoglossans that express dimorphisms in larval development.
ABSTRACT Credible cases of poecilogony, the production of two distinct larval morphs within a species, are extremely rare in marine invertebrates, yet peculiarly common in a clade of herbivorous sea slugs, the Sacoglossa. Only five animal species have been reported to express dimorphic egg sizes that result in planktotrophic and lecithotrophic larvae: the spionid polychaete Streblospio benedicti and four sacoglossans distributed in temperate estuaries or the Caribbean. Here, we present developmental and genetic evidence for a fifth case of poecilogony via egg-size dimorphism in the Sacoglossa and the first example from the tropical Indo-Pacific. The sea slug Elysia pusilla produced both planktotrophic and lecithotrophic larvae in Guam and Japan. Levels of genetic divergence within populations were markedly low and rule out cryptic species. However, divergence among populations was exceptionally high (10-12% at the mitochondrial cytochrome c oxidase I locus), illustrating that extensive phylogeographic structure can persist in spite of the dispersal potential of planktotrophic larvae. We review reproductive, developmental, and ecological data for the five known cases of poecilogony in the Sacoglossa, including new data for Costasiella ocellifera from the Caribbean. We hypothesize that sacoglossans achieve lecithotrophy at smaller egg sizes than do related clades of marine heterobranchs, which may facilitate developmental plasticity that is otherwise vanishingly rare among animals. Insight into the environmental drivers and evolutionary results of shifts in larval type will continue to be gleaned from population-level studies of poecilogonous taxa like E. pusilla, and should inform life-history theory about the causes and consequences of alternative development modes in marine animals.
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ABSTRACT: Species of the genus Elysia Risso, 1818 have relatively few external characters useful for species identification, and features such as papillae and/or color markings on the body surface are often used to classify and identify species. Elysia atroviridis Baba, 1955 and E. setoensis Hamatani, 1968, for example, have been described as distinct based mainly on such characters. Close examinations of 46 specimens identifiable as either species collected from nine localities in Japan revealed that many specimens exhibited mixed features of the two species. Detailed comparisons of radular morphology could also not differentiate the two species. With phylogenetic analyses based on COI and 16S rRNA (1072 bp in total), all of the examined specimens formed a single clade with very little variation among specimens. The AMOVA of the specimen haplotypes showed there was no significant genetic differentiation in relation to differences in external morphology, geographic region, or algal host. Our results indicate that E. atroviridis and E. setoensis are conspecific. In contrast, cryptic species were found in two other clades of congeners, indicating that further taxonomic scrutiny is needed within the genus.American Malacological Bulletin 02/2013; 31(1):25-37. · 1.00 Impact Factor
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ABSTRACT: Research on sea slugs production has steadily increased in the last decades as a result of their use as model organisms for biomedical studies, bioprospecting for new marine drugs and their growing demand for academic research and the marine aquarium trade. However, standardized methods for culturing sea slugs are still limited to a reduced number of species. The main bottlenecks impairing sea slugs aquaculture are the lack of knowledge on suitable larval diets and settlement cues that can induce metamorphosis in competent larvae. Additionally, the stenophagous feeding regime displayed by several species requires the collection and/or culture of their prey, which commonly impairs large-scale production. Nevertheless, significant breakthroughs have been achieved in recent years through the development of innovative culture techniques. The present review summarizes the major issues impairing the culture of sea slugs and presents relevant biological and ecological data that can assist on the development of suitable culture protocols. Information on the most suitable husbandry, larviculture and grow-out techniques are critically discussed, with emphasis to their application on some of the most relevant groups of sea slugs from an academic and commercial point of view: sea hares (Aplysia spp.), nudibranchs (e.g., the marine ornamental species Aeolidiella stephanieae) and the "solar powered" sacoglossan (e.g., Elysia spp.).Aquaculture 05/2013; 408-409:1-14. · 2.01 Impact Factor
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ABSTRACT: DNA barcoding can highlight taxa in which conventional taxonomy underestimates species richness, identifying mitochondrial lineages that may correspond to unrecognized species. However, key assumptions of barcoding remain untested for many groups of soft-bodied marine invertebrates with poorly resolved taxonomy. Here, we applied an integrative approach for species delimitation to herbivorous sea slugs in clade Sacoglossa, in which unrecognized diversity may complicate studies of drug discovery, plastid endosymbiosis, and biological control. Using the mitochondrial barcoding COI gene and the nuclear histone 3 gene, we tested the hypothesis that three widely distributed "species" each comprised a complex of independently evolving lineages. Morphological and reproductive characters were then used to evaluate whether each lineage was distinguishable as a candidate species. The "circumtropical" Elysia ornata comprised a Caribbean species and four Indo-Pacific candidate species that are potential sources of kahalalides, anti-cancer compounds. The "monotypic" and highly photosynthetic Plakobranchus ocellatus, used for over 60 years to study chloroplast symbiosis, comprised 10 candidate species. Finally, six candidate species were distinguished in the Elysia tomentosa complex, including potential biological control agents for invasive green algae (Caulerpa spp.). We show that a candidate species approach developed for vertebrates effectively categorizes cryptic diversity in marine invertebrates, and that integrating threshold COI distances with non-molecular character data can delimit species even when common assumptions of DNA barcoding are violated.Molecular Phylogenetics and Evolution 07/2013; · 4.07 Impact Factor