Divergent responses to warming of two common co-occurring Mediterranean bryozoans

Abstract

Climate change threatens the structure and function of marine ecosystems, highlighting the importance of understanding the response of species to changing environmental conditions. However, thermal tolerance determining the vulnerability to warming of many abundant marine species is still poorly understood. In this study, we quantified in the field the effects of a temperature anomaly recorded in the Mediterranean Sea during the summer of 2015 on populations of two common sympatric bryozoans, Myriapora truncata and Pentapora fascialis. Then, we experimentally assessed their thermal tolerances in aquaria as well as different sublethal responses to warming. Differences between species were found in survival patterns in natural populations, P. fascialis showing significantly lower survival rates than M. truncata. The thermotolerance experiments supported field observations: P. fascialis started to show signs of necrosis when the temperature was raised to 25–26 °C and completely died between 28–29 °C, coinciding with the temperature when we observed first signs of necrosis in M. truncata. The results from this study reflect different responses to warming between these two co-occurring species, highlighting the importance of combining multiple approaches to assess the vulnerability of benthic species in a changing climate world.

Response diversity in Mediterranean coralligenous assemblages facing climate change: Insights from a multispecific thermotolerance experiment

“…Regardless of the possible mechanisms behind these differences, which in the case of Parazoanthus axinellae could include the presence of highly bioactive secondary metabolites only in the “slender” morphotype as chemical defences induced for coping with environmental changes (Cachet et al, ; Reverter et al, ), our results represent a good example of the diversity of responses to warming found among structurally, functionally and taxonomically related organisms dwelling in coralligenous outcrops. Previous studies dealing with habitat‐forming emblematic species from these assemblages, such as the red gorgonian Paramuricea clavata (Risso, 1826), the white gorgonian Eunicella singularis (Esper, 1791), the red coral Corallium rubrum (Linnaeus, 1758) or the bryozoans Myriapora truncata (Pallas, 1766) and Pentapora fascialis (Pallas, 1766), have already pointed to such diversity (Crisci et al, ; Linares et al, ; Pagès‐Escolà et al, ; Torrents et al, ). However, the low number of studied species impeded the assessment of whether the response diversity was limited or widespread at the community level.…”

“…As the response variable, we measured the percentage of necrotic tissue in each specimen, which was visually monitored on a daily basis. This variable has been widely used for coralligenous assemblages both in field mortality assessments and laboratory experiments as a proxy of partial and/or total mortality following disturbance (Cebrian, Uriz, Garrabou, & Ballesteros, ; Cerrano et al, ; Crisci et al, ; Garrabou et al, ; Kersting, Bensoussan, & Linares, ; Pagès‐Escolà et al, ). Following these similar previous studies, we considered necrotic tissue to be all areas with evident signs of irreversible damage, which may include the following: the loss of tissue, denudation/exposure of the skeleton, the loss of coloration derived from the total or partial loss of living tissue and/or the colonization by saprophytic microorganisms (see Supporting information Figure ).…”

“…Regardless of the possible mechanisms behind these differences, which in the case of Parazoanthus axinellae could include the presence of highly bioactive secondary metabolites only in the "slender" morphotype as chemical defences induced for coping with environmental changes (Cachet et al, 2015;Reverter et al, 2016) (Crisci et al, 2017;Linares et al, 2013;Pagès-Escolà et al, 2018;Torrents et al, 2008). However, the low number of studied species impeded the assessment of whether the response diversity was limited or widespread at the community level.…”

“…Garrabou, & Ballesteros, 2011;Cerrano et al, 2000;Crisci et al, 2017;Garrabou et al, 2009;Kersting, Bensoussan, & Linares, 2013;Pagès- Escolà et al, 2018). Following these similar previous studies, we considered necrotic tissue to be all areas with evident signs of irreversible damage, which may include the following: the loss of tissue, denudation/ exposure of the skeleton, the loss of coloration derived from the total or partial loss of living tissue and/or the colonization by saprophytic microorganisms (see Supporting informationFigure S2).…”

The constraint of ignoring the subtidal water climatology in evaluating the changes of coralligenous reefs due to heating events

“…Overall, thermal tolerance has been highlighted mainly for gorgonians, bryozoans and sponges species, and lethal and sublethal responses (e.g. necrosis or decreased polyps activity) to the exceeding of heating thresholds or to manipulated duration of heating events have been identified [43][44][45] . Overall, the survival horizon of specific taxa as well as of the whole coralligenous reef is considered strongly dependent on heat waves occurrence 46 .…”