Heterostroma nereidiis gen. et sp. nov. (Rhodophyta), a dorsiventral rhodomelaceous marine alga from Western Australia

Abstract

Heterostroma nereidiis gen. et sp. nov. (Rhodomelaceae, Ceramiales) is described from Yanchep (type locality) and the Abrolhos Islands in Western Australia. The bases of the plants are prostrate, laminar, and anchored by cables of consolidated rhizoids. Decumbent to erect, linear, deeply serrate blades arise from marginal initals along the bases. The vegetative fronds are 1–4 times pinnate, with a lateral branch primordium borne on every axial tier in a ¼ spiral arrangement. Dorsal and ventral branch primordia are generally dormant, only the distichous lateral primordia growing out into determinate or indeterminate axes. Four uncorticated pericentral cells are formed, two lying dorsally and two ventrally. Ventral pericentral cells divide longitudinally, and the derivatives along adjacent axes form secondary pit connections, giving rise to membranous thalli. Dorsal pericentral cells divide transversely, obliquely and longitudinally, and cover the ventral thallus layer, three or four derivatives corresponding roughly to each subtending ventral pericentral cell or pericentral-cell derivative. Trich-oblasts not associated with reproductive structures or axes are apparently absent. Fertile axes are short, terete, bear a single trichoblast or trichoblast scar-cell on every tier in a spiral series, and extend from the blade margins by renewed and transformed growth of generally second-or third-order lateral-branch apices. The tetrasporangial laterals are stichidia-like and bear lengthy, non-pigmented, deciduous trichoblasts. The tetrasporangia are spirally arranged, one per fertile tier on a pericentral cell adjacent to the trichoblast basal cell, and are associated with three cover cells. Spermatangial heads occupy one branch of the basal dichotomy of lengthy fertile trichoblasts. Ostiolate cystocarps develop from procarps on the epibasal cells of fertile trichoblasts. A comparison is made between Heterostroma and other dorsiventral members of the Rhodomelaceae. It is concluded that Heterostroma is probably not closely related to superficially similar genera such as Periphykon, Symphyocladia and Placophora, but may show stronger links with genera that do not have congenital branch fusions, such as Aphanocladia. Heterostroma appears to represent independently evolved dorsiventrality from precursors somewhere between the Pterosiphonia and Polysiphonia levels of organization.