Sponges are primitive sedentary animals that lack nervous systems but nontheless play significant roles in marine ecosystems. Sponges filter large volumes of seawater and transfer vast amounts of energy and nutrients from oceanic food sources to benthic ecosystems including coral reefs.
AIMS sponge ecologist Dr Nicole Webster has published work this year showing that sea sponges, which host a complex community of microbes in a mutually-beneficial relationship, are at risk from higher sea surface temperatures because the symbiotic relationship between the sponge and its microbes breaks down at 33 degrees Celsius.
This is identical to the temperature threshold at which corals experience bleaching, which is also a breakdown in the symbiotic relationship between the host and its microbes. Elevated sea temperatures expected as climate change progresses would threaten the survival of sponges.
In some sponges, up to 40 per cent of their body weight is made up of microscopic symbiotic bacteria, contributing a range of benefits including chemical defence systems and the processing of nutrition and waste.
The delicate equilibrium of this relationship appears to be at risk as temperatures rise. In a series of experiments in which Dr Webster and her colleagues exposed a common GBR sponge ( Rhopaloeides odorabile ) to water temperatures ranging from 27 degrees to 33 degrees, the clear line of demarcation between a continued healthy symbiotic microbial community in the sponge and a switch to harmful microbes that cause disease consistently emerged at 33 degrees.
Dr Webster has also completed the first DNA sequencing study of the microbial symbionts of sponges, funded by the International Census of Marine Microbes. The work has revealed exceptional diversity and the modes of transmission for bacterial sponge symbionts.
Another AIMS sponge ecologist, Dr Steve Whalan, with colleagues from AIMS and James Cook University via the AIMS@JCU Joint Venture, has published a paper showing that sponge youngsters hold the key to survival of the mature organism. It is only in the larval phase that sponges are mobile and they make the most of that short-lived capacity, carrying out tasks of surprising sophistication.
For a brief time also they can withstand higher temperatures than older sponges and this has both benefits and drawbacks for sponges, which only have a short time to make the transition from larvae to the next stages of growth, at which time their tolerance for high temperatures drops substantially.
Dr Whalan and his colleagues also studied R. odorabile , investigating how larvae rise through the water column in response to light, dwell at the surface of the sea briefly, then fall back down to a favoured settlement point where they start to grow.
For a creature without a nervous system, they are remarkably responsive to light and even seem to have a "memory" of light that can stay with them after the light has gone.
Larvae have a brief period after they are released from the mother sponge when they have some limited purposeful mobility and can use a rotational "corkscrew" action to propel themselves during a 24 hour period of intense activity. After propelling themselves upwards, they remain at the surface for around 18 hours before sinking to the bottom. They soon lose this swimming ability, but it is essential for getting them where they need to be near the surface of the water. Exactly why they need to go to the surface remains unclear. The scientists further found that sponges were more likely to settle on areas exposed to light. In fact, the sponges would always settle on a light-exposed surface rather than shaded crevices that appeared to be more favourable for survival.