Hydroclathrus (Scytosiphonaceae, Phaeophyceae): Conspectus of the genus and proposal of new species from Australia and Hawaii

Abstract

SUMMARY Representatives of the two current species of Hydroclathrus, Hydroclathrus clathratus (C. Agardh) Howe (the generitype) and Hydroclathrus tenuis Tseng et Lu, are compared to recent collections of the genus from isolated localities in the central and south Pacific: Necker Island and Lord Howe Island, respectively. Although published descriptions of the virtually pan-tropical/warm-temperate H. clathratus portray a species highly variable in the habits and soral distribution patterns of the macro-phases of its life history, our observations support the hypothesis that the newly discovered Pacific island populations represent new species. Hydroclathrus steph-anosorus Kraft, sp. nov., from Lord Howe Island, differs from seemingly typical H. clathratus by the low-domed profiles of its surface cortical cells, aggregates of moniliform hair primordia that are almost always associated with plurangial sori, and particularly by the configurations of the sori themselves, which form discrete, nearly circular rings around the central hair tufts. Hydroclathrus tumulis Kraft et Abbott, sp. nov., from two deep-water localities in the northwestern Hawaiian Islands, has subacutely papillate cortical cells, scattered single, paired or laxly aggregated hair primordia of distinctive obcuneate morphology, and discrete, angular plurangial sori with no predetermined relationship to hairs, the plurangia being relatively laxly aggregated by virtue of their often arising on pedicels formed by the peaked crests of the cortical bearing cells. H. tenuis, although making the most striking visual impression of any of the species because of its exceedingly narrow, fibrous membranes, seems otherwise closest to H. clathratus in cortical cell, hair and soral features, absolute morphological boundaries between the two species being perhaps difficult to draw at times.