Thallus ontogeny, morphology and molecular phylogeny of Madagascaria atlantica sp. nov. (Compsopogonophyceae, Rhodophyta), a diminutive crustose species uncovered in the Atlantic Ocean

Abstract

Madagascaria is a crustose monotypic genus belonging to the class Compsopogonophyceae, so far known only from the Indo-Pacific region. The taxonomy within this group is difficult since it includes genera with small dimensions (<5 mm) and few diagnostic characters. In this study, we combined thallus development in culture with morphological and molecular data to uncover a new species of Madagascaria and the first record in the Atlantic Ocean (Piauí coast, northeastern Brazil). The new species was isolated from rhodoliths cultured under laboratory conditions, and its thallus ontogeny was analyzed. Morphological observations, along with molecular data based on rbcL sequences, allowed us to identify the small crustose red algae as closely related to M. erythrocladioides from the type locality (Madagascar). Madagascaria atlantica sp. nov. from Brazil is characterized by crusts with initially filamentous growth, free marginal cells and asexual reproduction by monospores. Our analyses support three evolutionary lineages in Madagascaria, corresponding to isolates from Brazil (M. atlantica), Japan and Madagascar. The rbcL sequence divergence between M. atlantica and samples from Madagascar and Japan was 2.4% and 6.9%, respectively. Madagascaria atlantica and M. erythrocladioides are indistinguishable with overlapping features. Our findings evidence the existence of cryptic species diversity and suggest that Madagascaria is probably a widely distributed genus. Molecular evidence based on rbcL gene supports the recognition of M. atlantica as a new cryptic species. Since the small size and resemblance to other crustose rhodophycean, data from other regions are needed to evaluate the distributional range of Madagascaria.