Taxonomy and distribution of Didiscus and Myrmekioderma (Demospongiae: Axinellida) off the mouths of the two largest rivers in Brazil, with description of four new species

Abstract

Didiscus Dendy, 1922 and Myrmekioderma Ehlers, 1870 are closely related genera of axinellid sponges, distinguished mainly by the presence of didiscorhabd microscleres in Didiscus and of trichodragmata in Myrmekioderma. Their family-level classification, however, is highly controversial, in part because their specific and morphological diversities are still poorly known. Here, we redescribed Didiscus oxeatus Hechtel, 1983 based on new specimens and revision of the holotype, described two new species of Didiscus and two new species of Myrmekioderma from off the Amazon and Sao Francisco river mouths in Northern and Northeastern Brazil. These two areas in the Tropical Atlantic (TA) are still understudied, especially when considering their high environmental and economic importance. We observed for the first time the presence of tylostyles in the holotype of D. oxeatus, as well as in the new specimens from Sergipe State. Didiscus pseudoverdensis sp. nov. has thick, strongly spined didiscorhabds with sharp ends; Didiscus raraediscus sp. nov. has small microspined oxeas; Myrmekioderma guajajara sp. nov. has microxeas and verrucose small oxeas; and Myrmekioderma tenax sp. nov. has both raphidiform and sinuous oxeas. The morphological diversities of both genera are expanded by the addition of new spicule types and ornamentations. Morphological data indicated that both Didiscus and Myrmekioderma should be classified in the family Heteroxyidae, in contrast to previous molecular studies, which suggest that they could belong at least in part to Raspailiidae. These two families and the genus Myrmekioderma are probably polyphyletic and need revision. The number of species of Didiscus and Myrmekioderma worldwide is raised to 11 and 13, respectively, of which five species of Didiscus and six of Myrmekioderma occur in the TA. Most species have narrow distributions, probably reflecting the low sampling effort. This study provides a more accurate picture of the biodiversity and distribution of Didiscus and Myrmekioderma in the TA and reinforces the importance of scientific explorations in understudied areas such as off the mouths of the Amazon and Sao Francisco rivers.