Rhodoliths or maërl are calcareous nodules of coralline red algae growing unattached to the substrata. They sustain a high biodiversity, being one of the most important marine benthic environments dominated by macrophytes. Sabellariidae are tube-dwelling filter feeding marine polychaetes that build their tubes by secreting cement from their thoracic glands. Some species are solitary, while others build tubes together, forming large aggregates. This study analyses Sabellariidae collected in rhodolith beds along the subtidal zone of the coast of João Pessoa and Cabedelo, Paraíba, north-eastern Brazil. A total of 100 individuals of six species were identified. Four are reported for the first time for the north-east coast of Brazil: Phragmatopoma caudata Mörch (1863), Sabellaria bella Grube (1870), Sabellaria nanella Chamberlin (1919), and Sabellaria wilsoni Lana & Gruet (1989). The first occurrence of Sabellaria pectinata Fauvel (1923) along the western Atlantic is provided as well as the description of a new species of the genus Sabellaria.

Coralline algae are important reef-builders which can form nodules, known as rhodoliths, occurring worldwide in beds sustaining a high biodiversity. Although considered a non-renewable resource, they are exploited as a source of calcium carbonate used mainly for agricultural purposes. In Brazil between 96,000 and 120,000 metric tonnes of rhodoliths are extracted per year. Besides the direct impact caused by removal on the coralline bed, the dredge process may also produce a plume of fine sediment, which can change the primary production of the remaining organisms. In this study, four treatments, with three replicates, were used to acquire Lithothamnion sp. net photosynthetic rates with and without a sediment layer using a Clark-type oxygen microelectrode and micromanipulator. The results demonstrated that, under controlled conditions, the addition of a thin sediment layer resulted in a 30% reduction of the irradiance, decreasing the Lithothamnion sp. net production in 70%. For this reason direct and indirect effects of mechanical exploitation of the rhodolith beds should be included in future studies that focus on environmental impacts of dredging activity, whether it is linked to the extraction of these algae.

Gelidium maggsiae sp. nov. is described from plants found in shallow subtidal maërl beds off the west coast of Ireland, where it attaches to detached coralline algae by peg-like rhizoids. Rhizines are more abundant in the medullary region than in the cortex and erect axes display a prominent central row formed by the axial and periaxial filaments. Gametophytes are bisexual and protandrous and were found only in culture. Post-fertilization and cystocarp development are similar to that of G. pteridifolium and cystocarps are biloculate. Tetrasporophytes are morphologically similar to gametophytes; tetrasporangia are irregularly arranged in pod-shaped terminally inserted stichidia, borne on erect axes and arranged unilaterally. Gametangial plants reproduced under short-day (8[ratio ]16 h) conditions at 20–25°C in enriched media, but gametangial formation was independent of photoperiod when plants were kept in unenriched medium. No tetrasporophytic plants were produced from germinated carpospores, but plants originally collected in a vegetative state formed tetraspongia at 20°C, 16[ratio ]8 h. The validity of several characters previously used in species delimitation within the Gelidiales is discussed, and only cystocarp structure and the bisexual nature of the gametophyte are considered to be reliable characters.