Abstract
Holothuroids (sea cucumbers) are one of the most ubiquitous groups of benthic animals found across diverse marine ecosystems. As echinoderms, they also occupy an important place in the evolutionary hierarchy, sitting close to vertebrates in the deuterostome clade, making them valuable multidisciplinary model organisms. Apart from being ecologically and phylogenetically important, many species are commercially exploited for luxury seafood markets. With the global rise of aquaculture and fisheries, management and protection of these valuable species relies on a better understanding of how their immune systems respond to environmental and anthropogenic stressors. Here, the cellular, hormonal and behavioral indicators of stress in the North Atlantic sea cucumberCucumaria frondosawere examined. The immediate and carry-over (post recovery) effects of a 1-hour exposure to low salinities or to emersion (at two temperatures) highlighted that morphoplasticity inC. frondosawas accompanied by shifts in all monitored indicators. From baseline levels measured in controls, densities of free coelomocytes increased, showing successions of specific cell types and subsequent coelomocyte aggregations, combined with a rise in cortisol levels. These responses mirrored increased fluctuations in cloacal opening rates, decreased force of attachment to the substrate, and enhanced movements and active buoyancy adjustment with increasingly severe stressors. The findings suggest that many systems of sea cucumbers are impacted by stresses that can be associated with harvesting and handling methods, with likely implications for the quality of the processed products. Gaining a deeper understanding of immune and hormonal responses of sea cucumbers is not only of broad ecological and evolutionary value, but also helpful for the development of sustainable fisheries and aquaculture practices, and conservation programs.
Highlights
Sea cucumbers (Echinodermata: Holothuroidea) are globally fished as a luxury seafood and many populations worldwide are fully or overfished (Purcell et al, 2013)
Free Coelomocytes Individuals exposed to stressors, globally displayed higher densities of free coelomocytes in the Polian vesicle (PV) fluid than their respective controls (Figure 1)
An increase in coelomocyte density occurred after 1 h in three of the four treatments (17◦C air, 15 and 22 psu salinities, Figures 1A,C,D) while it occurred only after 24 h under the 5◦C air treatment (Figure 1B; for statistics see Supplementary Table 2)
Summary
Sea cucumbers (Echinodermata: Holothuroidea) are globally fished as a luxury seafood and many populations worldwide are fully or overfished (Purcell et al, 2013). While the anatomy of sea cucumbers appears quite simple, they have developed many unique adaptations that allow them to thrive in different marine environments across the globe. They can be suspension feeders, using branching oral tentacles to capture particulate matter from the water, or deposit feeders ingesting sedimented organic matter. The internal anatomy of sea cucumbers includes two main coeloms, the perivisceral cavity and the hydrovascular system The former is the central body cavity holding most organs. The latter is comprised of the ampullae of the tube feet, the vesicle of the tentacles and the Polian vesicle, as well as numerous canals and the madreporite, which aid in a variety of processes including locomotion, feeding, and immunity (Li et al, 2013)
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