Habitat types and megabenthos composition from three sponge-dominated high-Arctic seamounts.

Seamounts are isolated underwater mountains stretching > 1000 m above the seafloor. They are identified as biodiversity hotspots of marine life, and host benthic assemblages that may vary on regional (among seamounts) and local (within seamounts) scales. Here, we collected seafloor imagery of three seamounts at the Langseth Ridge in the central Arctic Ocean to assess habitats and megabenthos community composition at the Central Mount (CM), the Karasik Seamount (KS), and the Northern Mount (NM). The majority of seafloor across these seamounts comprised bare rock, covered with a mixed layer of sponge spicule mats intermixed with detrital debris composed of polychaete tubes, and sand, gravel, and/or rocks. The megabenthos assemblages consisted of in total 15 invertebrate epibenthos taxa and 4 fish taxa, contributing to mean megabenthos densities of 55,745 ind. ha-1 at CM, 110,442 ind. ha-1 at KS, and 65,849 ind. ha-1 at NM. The faunal assemblages at all three seamounts were dominated by habitat-forming Tetractinellida sponges that contributed between 66% (KS) and 85% (CM) to all megabenthos. Interestingly, taxa richness did not differ at regional and local scale, whereas the megabenthos community composition did. Abiotic and biogenic factors shaping distinct habitat types played a major role in structuring of benthic communities in high-Arctic seamounts.

A community perspective on the concept of marine holobionts: current status, challenges, and future directions.

Host-microbe interactions play crucial roles in marine ecosystems. However, we still have very little understanding of the mechanisms that govern these relationships, the evolutionary processes that shape them, and their ecological consequences. The holobiont concept is a renewed paradigm in biology that can help to describe and understand these complex systems. It posits that a host and its associated microbiota with which it interacts, form a holobiont, and have to be studied together as a coherent biological and functional unit to understand its biology, ecology, and evolution. Here we discuss critical concepts and opportunities in marine holobiont research and identify key challenges in the field. We highlight the potential economic, sociological, and environmental impacts of the holobiont concept in marine biological, evolutionary, and environmental sciences. Given the connectivity and the unexplored biodiversity specific to marine ecosystems, a deeper understanding of such complex systems requires further technological and conceptual advances, e.g., the development of controlled experimental model systems for holobionts from all major lineages and the modeling of (info)chemical-mediated interactions between organisms. Here we propose that one significant challenge is to bridge cross-disciplinary research on tractable model systems in order to address key ecological and evolutionary questions. This first step is crucial to decipher the main drivers of the dynamics and evolution of holobionts and to account for the holobiont concept in applied areas, such as the conservation, management, and exploitation of marine ecosystems and resources, where practical solutions to predict and mitigate the impact of human activities are more important than ever.

Size Is the Major Determinant of Pumping Rates in Marine Sponges.

Sponges play an important ecological function in many benthic habitats. They filter large volumes of water, retain suspended particles with high efficiency, and process dissolved compounds. Nevertheless, the factors that regulate sponge pumping rate and its relation to environmental factors have been rarely studied. We examined, in situ, the variation of pumping rates for five Mediterranean sponge species and its relationship to temperature, particulate food abundance and sponge size over two annual cycles. Surprisingly, temperature and food concentration had only a small effect on pumping rates, and the seasonal variation of pumping rates was small (1.9-2.5 folds). Sponge size was the main determinant of the specific pumping rate (pumping normalized to sponge volume or mass). Within the natural size distribution of each species, the volume-specific pumping rate [PR V , ml min-1 (cm sponge)-3] decreased (up to 33 folds) with the increase in sponge volume (V, cm3), conforming to an allometric power function (PR V = aVb ) with negative exponents. The strong dependence of the size-specific pumping rate on the sponge size suggests that the simplistic use of this value to categorize sponge species and predict their activity may be misleading. For example, for small specimens, size-specific pumping rates of the two low-microbial-abundance (LMA) species (allometric exponent b of -0.2 and -0.3) were similar to those of two of the high-microbial-abundance (HMA) species (b of -0.5 and -0.7). However, for larger specimens, size-specific pumping rates were markedly different. Our results suggest that the pumping rate of the sponges we studied can be approximated using the measured allometric constants alone in conjunction with surveys of sponge abundance and size distribution. This information is essential for the quantification of in situ feeding and respiration rates and for estimates of the magnitude of sponge-mediated energy and nutrient fluxes at the community level. Further work is required to establish if and to what extent the low seasonal effect and the strong size dependency of pumping rate can be generalized to other sponges and habitats.