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Accumulating data on the increasing degradation of coral reefs worldwide has led to a focus on the unique mesophotic coral ecosystem (MCE) as a potential refuge for threatened shallow-water species (i.e., the deep reef refugia hypothesis: DRRH). Sponges play a crucial role in coral-reef functioning but are often overlooked in benthic surveys. This knowledge gap is especially true for the Red Sea, where data on sponge abundance, coverage and species composition are scarce. Furthermore, to date, no study has examined mesophotic sponges in the Red Sea. Here we compared sponge abundance and diversity metrics between shallow and mesophotic reefs at two sites in the Gulf of Aqaba, northern Red Sea. We also examined the role of biotic and abiotic parameters in determining sponge variability between sites and depths. Sponge diversity metrics significantly varied with increasing depth and between sites. Sponge species composition also differed significantly between depth and sites. However, parameters measured in the water column, that is, Total Organic Carbon (TOC), Particulate Organic Carbon (POC), NOx, etc., did not differ between depths and sites, except for Synechococcus and Prochlorococcus concentration, which did. The findings indicated that site characteristics and biotic interactions may play an essential role in determining sponge diversity and community composition. Between-site differences in sponge fauna revealed in the study highlight the importance of considering the variability in the benthic communities and the factors controlling it when designing management tools for coral reefs, particularly for the northern Red Sea.

Climate change is predicted to have detrimental impacts on sessile invertebrates, including sponges. Mesophotic ecosystems have been suggested to play a major role as refugia for coral reef sponge species, however knowledge regarding the ability of mesophotic sponges to cope with thermal stress is scarce. In this study we compared the response of the sponge Diacarnus erythraeanus, a widespread Red Sea sponge, from the shallow and mesophotic reef, to moderate and acute temperature elevation (2°C and 6°C, respectively) for short and long term periods (two and 35 days, respectively) by measuring physiological parameters (respiration, oxygen removal, pumping rates, and photosynthetic efficiency), and the microbiome composition change. The results indicated that mesophotic and shallow populations of D. erythraeanus are highly tolerant to both moderate and acute heat stress, demonstrating a high survival rate (100%) across the experimental treatments, with no visible signs of bleaching or necrosis. Exposure to heat stress resulted in significant alterations in the physiological parameters of sponges, including higher respiration rate and lower photosynthetic efficiency. These alterations were accompanied by correspondingly significant microbial adjustments, thus emphasizing the essential role of the microbiome in the host’s ability to persist when facing essential environmental stress. Moreover, while shallow and mesophotic sponges showed similar physiological tolerance to heat stress, their microbial response differed: while the microbiome diversity of the mesophotic sponges remained stable throughout the experiment, the shallow one significantly changed. This result suggests that their underlying coping mechanisms might differ between mesophotic and shallow populations. Since the associated-microbiome is largely regulated by the sponge-host genetics, difference in microbial adjustments to stress between populations, could indicate genetic variability between hosts. Therefore, while the results of this study support the hypothesis that mesophotic coral reefs could serve as thermal refugia for some sponge species, it raises the question regarding the validity of MCEs as a refuge for shallow populations. Finally, it emphasizes the crucial need to elucidate the underlying mechanisms governing the sponge-microbiome interactions, specifically in the context of the anticipated climate change scenarios.

Polychaetes are among the most common marine organisms, and in many habitats they dominate both in species richness and abundance. They are often found in association with other organisms. Specifically, sponges with their complex three-dimensional internal architecture, are known to host a great diversity of polychaetes. Due to the fact that a large number of sponge-associated polychaete species are known to be common on other substrates, most studies agree that they represent an opportunistic part of the sponge-inhabiting assemblage, without any selection of species tightly associated with sponges. The current study aimed to shed light on polychaetes affinity to sponges. We applied Clarke and Warwick’s taxonomic distinctness indices and test for random assembly, to a dataset compiled of 13 publications that provided polychaete species lists from massive-sponges across the Mediterranean Sea, and compared them with benthic-polychaete species lists from all of the Mediterranean and its zoogeographical regions. These indices are considered to be independent of sampling efforts and setting, and can be applied on presence/absence data, making it possible to compare data from multiple studies. We further compared the trophic structure of sponge-associated polychaetes between the Mediterranean’s zoogeographical regions. Our results show that the trophic structure of the sponge-associated polychaete community, was found to be stable across the entire Mediterranean Sea. Moreover, randomization tests showed that at almost all observational scales (e.g., study, region, Mediterranean) the phylogenetic diversity is not assembled at random, and that sponge-associated polychaete communities are composed of taxonomically related species. These results support the statement that polychaete assemblages inhabiting sponges are not a transient facultative assembly of species, but rather stable, diverse and specialized communities which are well adapted for life in this habitat.

Sponges have long been considered as “living hotels” due to the great diversity and abundance of other taxonomic groups often found in association with them. Sponges are the dominant components of benthic communities in the Levant Sea; and especially in the recently discovered mesophotic sponge grounds off the coast of Israel. However, almost no data exist regarding their associated macrofauna. The current study sought to identify the macrofauna associated with massive sponges along the Israeli Mediterranean coast; and to compare the role of sponges, as ecosystem engineers, or “living hotels,” in both the shallow-water and mesophotic habitats. Sixty-four massive sponge specimens, from 10 different species, were collected from shallow and mesophotic habitats by SCUBA diving and Remotely Operated Vehicle, respectively. Sponge volume was estimated, specimens were dissected, and the associated macrofauna were identified. Our results reveal that the sponges supported a diverse assemblage of associated macrofauna. A total of 61 associated taxa were found, including species reported for the first time in Israel. A clear, differentiation existed in the structure of the associated assemblage between the two habitats, which is mainly attributed to four species (two polychaetes, a crustacean, and a brittle star). The trophic composition remained stable across the two habitats. No correlation was found between sponge volume and the associated fauna community parameters. The highest richness of associated fauna was found in the mesophotic habitat, where sponge diversity is also higher. In contrast, a greater endobiont abundance and density were recorded in the shallow habitat, where massive sponges may be a limiting factor due to their lower richness and abundance. Our findings emphasize the importance of sponges as ecosystem engineers, and suggest that sponge diversity may be an important factor that contribute to benthic biodiversity in these regions.

Determining demosponge reproductive strategies is essential to understanding their ecology and life history, as well as for the management of benthic marine environments. This is especially important in mesophotic ecosystems, which have been suggested to serve as a refuge for shallow-water populations and for which knowledge is lacking. Here we compared the reproductive strategies of two common Mediterranean demosponges species: Chondrosia reniformis and Axinella polypoides, which can be found in both shallow and mesophotic habitats along the Israeli coast. Samples were collected over 2 years, via SCUBA diving from the shallow coast (2–32 m) and Remotely Operated Vehicle from the mesophotic sponge grounds (95–120 m). A. polypoides and C. renifornis differed in oocyte morphology and development, but both demonstrated reproductive plasticity with regard to temperature and depth: temperature appears to regulate their reproduction in the shallow water, as both species exhibited seasonality; whereas, in the mesophotic zone, where the temperature amplitude is much smaller, seasonality was not observed for either species. Furthermore, in the mesophotic zone, C. reniformis exhibited low fecundity and probably invests more in asexual reproduction by budding. Mesophotic A. polypoides, in contrast, exhibited enhanced and continuous asynchronous sexual reproduction year-round. Our findings suggest that reproduction plasticity may be a general rule for sponge species that inhabit different habitats; such plasticity could allow sponges to thrive in different environmental conditions. These findings further emphasize the importance of protecting the mesophotic sponge grounds, which can serve as a refuge in the face of the adverse impact of anthropogenic disturbances and rising seawater temperatures.

Sponges hold a key role in benthic environments, and specifically in the Mediterranean Sea. Past events of mass mortality in sponge communities have been linked to extended periods of high-temperature anomalies, yet it is unknown how a gradual change, such as the constant rise in global seawater temperatures, will affect biodiversity. Here we present a case study of Agelas oroides, a common massive sponge in the Mediterranean Sea, found at a wide depth range of 1–150 m. Last documented in the 1970s, A. oroides was considered lost from the Israeli coastal fauna. However, its recent rediscovery in mesophotic depths, where environmental conditions are stable, provided an opportunity to examine whether it can survive the present conditions in the shallow Israeli coast – where temperatures increased by 3°C during the past 60 years, while the nutrients concentration decreased following the damming of the Nile River. To test this hypothesis, A. oroides individuals were collected during winter from mesophotic sponge grounds (100–120 m) and transplanted to a shallow rocky habitat (10 m). Control individuals were transplanted back to the mesophotic habitat. Sponge survival, temperature, and nutrient concentrations were measured in both habitats. The shallow-transplanted sponges’ survival decreased only when the ambient temperature exceeded 28°C. In contrast, the control group at the mesophotic depth, where the temperature never rose above 20°C, survived the duration of the experiment. Our findings suggest that a prolonged period of high temperatures may constitute a major factor in A. oroides survival and disappearance from the Israeli shallow habitats.