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 croceus, 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.