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The hexactinellid sponge Anoxycalyx (Scolymastra) joubini is the largest and most conspicuous of Antarctic sponges, with some individuals growing over two meters tall. It is found throughout Antarctica and the South Shetland Islands at depths from about 15 m to more than 400 m. Individuals can be up to 2 m high and 1 m in diameter, although they are usually smaller. In the Southern Ocean, hexactinellids are the dominant sponges in terms of biomass even though they are represented by just a few species, with demosponges accounting for by far the majority of Antarctic sponge species (Cerrano et al. 2000 and references therein). Anoxycalyx joubini is among the dominant, structure-forming species in the McMurdo Sound region of Antarctica. Antarctic hexactinellid sponges are generally found at depths greater than 30 m, making them difficult to study. Life history patterns of growth and reproduction are poorly known for most hexactinellid sponges, although growth and reproduction are generally believed to be slow relative to demosponges. (McClintock et al. 2005; Dayton et al. 2013)
Given the large size of A. joubini individuals, the lack of observations of settlement or growth have led to estimates of extreme longevity. Dayton et al. (2013), however, reported on observations of remarkably rapid episodic recruitment and growth and apparently high mortality of A. joubini in McMurdo Sound. They note that these observations call into question the validity of the widely held view of this species as slow-growing and extremely long-lived and they suggest that accelerated growth may be associated with shifts in plankton composition. Dayton et al. suggest that A. joubini may respond quickly to certain environmental shifts, but that resulting population increases may be relatively brief (on the scale of decades rather than centuries). Given the observations of Dayton et al., it seems likely that the more extreme longevity estimates for A. joubini (lifespans of as long as 15,000 years have been suggested) are unrealistically high, although these sponges may indeed have very long life spans.
Sponges often host large numbers of symbionts, mainly cyanobacteria and dinoflagellates, with the sponges using the photosynthetic symbionts as a food source to complement their filter feeding. Studies of Antarctic sponges have documented the presence of symbiotic diatoms as well. Cerrano et al. (2000) studied the invasion by large populations of the diatom Melosira sp. into A. joubini individuals. Although the nature of this relationship is not entirely clear, some evidence suggests that it is an example of a mutualism shifting to parasitism, whereby the diatoms obtain their nourishment not only by photosynthesis, but also from organic carbon derived from the sponge host.