climate change, dead zones, eutrophication, evolution, hypoxic events, marine benthic
hypoxia, oxygen depletion, phenotypic plasticity, Porifera, sessile organism
Ocean deoxygenation is one of the major consequences of climate change. In coastal
waters, this process can be exacerbated by eutrophication, which is contributing to
an alarming increase in the so-called ‘dead zones’ globally. Despite its severity, the
effect of reduced dissolved oxygen has only been studied for a very limited number
of organisms, compared to other climate change impacts such as ocean acidification
and warming. Here, we experimentally assessed the response of sponges to moderate and severe simulated hypoxic events. We ran three laboratory experiments
on four species from two different temperate oceans (NE Atlantic and SW Pacific).
Sponges were exposed to a total of five hypoxic treatments, with increasing severity (3.3, 1.6, 0.5, 0.4 and 0.13 mg O2 L
−1, over 7–12-days). We found that sponges
are generally very tolerant of hypoxia. All the sponges survived in the experimental
conditions, except Polymastia crocea, which showed significant mortality at the lowest
oxygen concentration (0.13 mg O2 L
−1, lethal median time: 286 h). In all species except
Suberites carnosus, hypoxic conditions do not significantly affect respiration rate down
to 0.4 mg O2 L
−1, showing that sponges can uptake oxygen at very low concentrations
in the surrounding environment. Importantly, sponges displayed species-specific phenotypic modifications in response to the hypoxic treatments, including physiological,
morphological and behavioural changes. This phenotypic plasticity likely represents
an adaptive strategy to live in reduced or low oxygen water. Our results also show that
a single sponge species (i.e., Suberites australiensis) can display different strategies at
different oxygen concentrations. Compared to other sessile organisms, sponges generally showed higher tolerance to hypoxia, suggesting that sponges could be favoured
and survive in future deoxygenated oceans.