Rhodolith seabeds function as ‘ecosystems engineers’, which globally provide a range of ‘ecosystem services’. However, knowledge on the structure, composition and distribution of rhodolith seabeds is still lacking. This Special Issue comprises six articles, addressing specific questions of rhodolith seabeds, and covering a wide range of topics. Two papers provide new large-scale information on the presence, structure and distribution of rhodolith beds at two southern hemisphere areas, in particular continental shelfs off South Africa and Brazil. Another two studies contributed to the discovery on new algal species from rhodolith beds, including Sporolithon franciscanum, a new rhodolith-forming species from Brazil, and the small benthic alga Schizocladia ischiensis. In terms of associated fauna, the taxonomic composition and patterns of abundance of decapod crustaceans are described in another article, including the description of a depth-partitioning in the abundance of juveniles and adults of the crab Nanocassiope melanodactylus. Rhodoliths are often present in fossilized deposits, so we can track changes in their presence with climate fluctuations. High temperatures during the Eocene and widespread oligotrophic conditions are finally connected with low abundances of rhodolith beds at mid and high latitudes, despite a larger presence at equatorial regions. Full article

Emerging evidence of rhodolith bed complexity and heterogeneity poses a challenge to monitoring strategies and questions about the role of abiotic factors in controlling the observed morphostructural diversity. Mollusk thanatocoenoses quickly respond to environmental conditions, expressing fidelity to biocoenosis and representing, thus, a useful ecological/paleoecological tool to unravel this heterogeneity. In this research, we studied three distinct rhodolith beds from the Tyrrhenian Sea (Italy), in a range between 40 and 100 m of water depth, together with their mollusk thanatocoenoses, sediment size, and oceanographic conditions. The beds are all heterogeneous and rarely correspond to a specific rhodolith morphotype and shape. On the contrary, the study of the associated thanatocoenoses led to distinguish at least five different assemblages within the known variability of the Coastal detritic (DC) association. DC and deep mud (VP) mollusk species dominate hanatocoenosis A, which is associated with mixed sediment and a low hydrodynamic regime. Thanatocoenosis B corresponds to an assemblage in which DC species mixed with species of adjacent vegetated bottoms in sandy sediment with a medium hydrodynamic. Thanatocoenosis C includes species related to muddy coastal detritic (DE) and offshore detritic (DL) associated with sand and a variable proportion of praline and branch morphotypes. Thanatocoenoses D and E include a wide range of detritic species (DE DC, DL) together with VP and coralligenous (C), associated with exclusive praline and gravelly sand (Thanat. D), or a mixed proportion of branch and praline and mixed sediment (Thanat. E). Our results demonstrate that the study of mollusk thanatocoenoses provides insights into the diversity within and among heterogeneous rhodolith beds. Moreover, rhodoliths, as part of the sediment, create microhabitats suitable for a wide range of mollusk species that contribute to the formation of complex thanatocoenoses. Full article

Rhodolith beds are biogenic benthic habitats mainly formed by unattached, non-geniculate coralline algae, which can be inhabited by many associated species. The Brazilian continental shelf encompasses the largest continuous rhodolith bed in the world. This study was based on samples obtained from seven sites and videos taken by a Remotely Operated Vehicle (ROV) at four transects off the Sergipe-Alagoas Coast on the northeast Brazilian shelf. ROV operations and bottom trawl sampling revealed the occurrence of rhodolith beds between 25 and 54 m depths. At the shallower depths, fruticose (branching) rhodoliths (maërl) appear in troughs of ripples, and other non-branching rhodoliths occur associated with corals and sponge patches surrounded by bioclastic sand. Rhodoliths also occur in patches from 30 to 39 m depth; some are fused, forming larger, complex tridimensional structures. At deeper depths, from 40 to 54 m, the abundance of rhodoliths increases and occur associated with fleshy macroalgae on a smooth seafloor; some rhodoliths are fused into complex structures, locally some are fruticose (maërl), and others are partially buried by fine-grained sediment. The collected rhodoliths vary from fruticose in two sites to encrusting to lumpy, concentric and boxwork nodules in the rest; their size ranges from small (<1.5 cm) to large (~6 cm) and are mostly sub-spheroidal to spheroidal. A total of 16 red algal morpho-taxa were identified in the study sites. Two phases of growth can be distinguished in some rhodoliths by changes in color. The brownish inner cores yielded ages of 1600–1850 cal years before the present, whereas outer layers were much younger (180–50 years BP old). Growth layers appeared to have been separated by a long period of burial in the seafloor sediment. Other rhodoliths have ages of hundreds of years. Full article

Mediterranean rhodolith beds are priority marine benthic habitats for the European Community, because of their relevance as biodiversity hotspots and their role in the carbonate budget. Presently, Mediterranean rhodolith beds typically occur within the range of 30–75 m of water depth, generally located around islands and capes, on flat or gently sloping areas. In the framework of a collaboration between the University of Milano-Bicocca and the Marine Protected Area “Capo Carbonara” (Sardinia, Italy), video explorations and sampling collections in three selected sites revealed the occurrence of a well developed and heterogeneous rhodolith bed. This bed covers an area >41 km² around the cape, with live coverage ranging between 6.50 and 55.25%. Rhodoliths showed interesting morphostructural differences. They are small compact pralines at the Serpentara Island, associated with gravelly sand, or bigger boxwork at the Santa Caterina shoal associated with sand, whereas branches are reported mostly in the Is Piscadeddus shoal, associated with muddy sand. Both in the Santa Caterina shoal and the Serpentara Island, rhodoliths generally show a spheroidal shape, associated with a mean value of currents of 4.3 and 7.3 cm/s, respectively, up to a maximum of 17.7 cm/s at Serpentara, whereas in the Is Piscadeddus shoal rhodolith shape is variable and current velocity is significantly lower. The different hydrodynamic regime, with a constant current directed SW, which deviates around the cape towards E, is responsible for such morphostructural heterogeneity, with the site of the Serpentara Island being the most exposed to a constant unidirectional and strong current. We can associate current velocity with specific rhodolith morphotypes. The morphostructural definition of the heterogeneity of rhodoliths across large beds must be considered for appropriate management policies. Full article

Crustaceans are a key component of the fauna living in rhodoliths, but patterns in their distribution and abundance remain largely unknown. This paper assessed spatio-temporal variability of Brachyura associated with rhodoliths. A seasonal study was conducted at three depth layers (18, 25, and 40 m), throughout two years (December 2015 to October 2017) at Gran Canaria Island (eastern Atlantic Ocean). A total of 765 crabs belonging to 10 species were collected. A larger abundance and richness of crabs at 25 m correlated with a larger biomass of epiphytic algae attached to rhodoliths. A seasonal pattern was also observed, where a higher richness of crabs occurred in the summer. The Xanthid crab, Nanocassiope melanodactylus, dominated the assemblage (83%); juveniles of this species were more abundant in deeper waters (40 m), while adults were more abundant on the shallower depth layers (18 m and 25 m). The species Pilmunus hirtellus was restricted to 25 m. Nevertheless, Pisa carinimana and Achaeus cranchii did not show any spatio-temporal pattern. In summary, this study demonstrated that two conspicuous crabs, N. melanodactylus and P. hirtellus, associated with rhodolith beds are bathymetrically segregated. Full article

This paper describes Sporolithon franciscanum, a new rhodolith-forming species of non-geniculate coralline algae found at depths between 47–52 m near the São Francisco river mouth, the second largest and the most extensive drainage basin in Brazil, and also at the Abrolhos Bank, in the world´s largest rhodolith beds. DNA sequences from plastidial psbA and rbcL markers indicate that the species is unique compared to all other Sporolithon species that have thus far been sequenced. Since morpho-anatomical features of the new species are shared with some other Sporolithon species, its identification was only confirmed by DNA sequences. Full article

Rhodolith beds have not previously been recorded in South Africa. A multidisciplinary research effort used remote sampling tools to survey the historically unexplored continental shelf off the Eastern Cape coast of South Africa. A rhodolith bed, bearing both living and dead non-geniculate coralline red algae, was discovered in the 30–65 m depth range off the Kei River mouth in the newly proclaimed Amathole Offshore Marine Protected Area. Some of the rhodolith forming coralline algal specimens were identified as belonging to at least three genera based on their morphology and anatomy, namely, Lithophyllum, Lithothamnion and a non-descript genus. Rhodolith mean mass and diameter were 44.85 g ± 34.22 g and 41.28 mm ± 10.67 mm (N = 13), respectively. Remotely operated vehicle (ROV) imagery revealed a suite of epibenthic red macroalgae associated with the rhodolith bed. Taxonomy, vertical structure and distribution of rhodoliths in South Africa require further investigation. Full article

During the greenhouse conditions prevailing in the early–middle Eocene, larger benthic foraminifers (LBF) spread out on carbonate platforms worldwide while rhodolith beds were scarcely represented. This reduction in rhodolith beds coincided with a relative decrease in coralline algal diversity and with a drastic decline of coral reef abundance. Middle Eocene rhodoliths from two tropical (San Jacinto Fold Belt in northern Colombia and Bahoruco Peninsula in the Dominican Republic) and two mid-latitude (Salinas Menores Ravine and Sierra del Zacatín in Southern Spain) localities were studied. Rhodolith rudstones in the tropical areas accumulated on relatively deep (several tens of meters) platform environments and were also redeposited in deeper settings downslope. In Salinas Menores, rhodoliths are dispersed in planktic foraminifer-rich marls. Miliolids are common in the infilling of constructional voids in these rhodoliths, indicating that they originally grew in shallow-water inner-shelf settings and afterwards they were transported to deeper environments. In Sierra del Zacatín, rhodoliths are scarce and coralline algae mainly occur as crusts attached to and intergrowing with corals. Here, LBF dominated shallow-water carbonate platforms. In terms of taxonomic composition, coralline algae of the order Hapalidiales are the most abundant in the study areas, followed by Sporolithales. The order Corallinales is poorly represented except in Salinas Menores, where it is relatively abundant and diverse. The impact of high temperatures due to high levels of atmospheric CO₂ during the Eocene and widespread oligotrophic conditions, which favored formation of LBF-rich lithofacies, might account for the low abundance of rhodolith beds at mid and high latitudes. In contrast, the more productive equatorial regions would have favored the formation of rhodolith beds. Full article

Substratum collected during diving surveys of sublittoral communities off the Greek island of Rhodes (Dodecanese, South-East Aegean) in late 2015 was incubated in the laboratory. Among the emerging macroalgal germlings, there was the second-ever record and isolate of the small benthic multicellular alga Schizocladia ischiensis of the poorly known monotypic Schizocladiophyceae, the sister group of the brown algae (Phaeophyceae). Its nuclear ribosomal small subunit, Rubisco spacer (rbcL, psaA, and psbC sequences (in total 5237 bp)) were similar to those of the only previous isolate of the species from Ischia, western Mediterranean. Our new strain formed branched upright thalli attached to the substratum by an amorphous substance secreted at the bottom of the basal cell. It is possible that S. ischiensis is a common member of the infralittoral and circalittoral communities in the Mediterranean and generally overlooked because of its minute size. Germling emergence appears to represent the method of choice to reveal benthic algae of this small size. Full article

The world’s largest rhodolith beds have been reported from the Brazilian continental shelf. Highly biodiverse beds are located in Southeast Brazil, but ecological aspects of these beds remain unknown. Despite their ecological importance, rhodolith beds (RBs) have recently been subjected to a severe threat, when more than 35 million cubic meters of mining residues slid down a mountainside on 5 November 2015, after a collapse of a gigantic dam upstream (the Mariana disaster), causing a huge impact on the Doce River. Our aim is to assess rhodolith beds and adjacent coralline formations on the Doce River Shelf (DRS) after the dam collapse. This paper describes the distribution, abundance, vitality, size and shape, as well as unmapped bryozoan rich sediment formations in this area, serving as baseline knowledge for environmental monitoring. Four distinct biogenic sea bottom habitats (bryozoan bottoms, rhodolith beds, carbonate concretions, and reefs) were recognized at different depth ranges with distribution indicated to be mostly related to the local sedimentary regime. Mud sediments dominated the seafloor up to 35 m depth. On the mid shelf, bryozoan bottoms were recorded from 35 to 45 m depth. Crustose coralline algae (CCA) occurring as rhodoliths and carbonate concretions extend over 1953 km² in the mid and outer shelf. Rhodolith beds predominate in these areas, totaling 1521 km² of sea bottom and were more abundant at depths between 45 and 65 m, occupying an extensive area south of the Doce River mouth. Northward, rhodolith beds are less abundant or absent likely due to the long-term deposition of fine sediments in this region. Carbonate concretions and reefs covered by CCA occupy sparse areas on the outer shelf (65–105 m depth). Differences in rhodolith features recorded, including coverage, density and size, may be related to the Doce River sedimentation and related factors (e.g., hydrodynamics, depth, and light). However, since there are no previous detailed studies on RBs along the DRS, we could not assess the impact of sedimentation of dam wastes on RBs’ abundance and density. In any case, these are valuable results for the further monitoring of long-term effects. Considering that the growth of these rhodoliths is relatively slow, and that they are affected by the sedimentation from the Doce River, the implementation of a management and conservation plan for this area is necessary in order to preserve this ecosystem. Full article