Tropical sponges

They are sea creatures that have no neural system, just sit still in one place, and are far less studied and understood than more charismatic species such as corals. But a new scientific study has revealed just how remarkable tropical marine sponges really are – and how their young perform amazing feats to ensure the survival of new generations.

AIMS sponge ecologist Dr Steve Whalan, with colleagues from AIMS and James Cook University via the AIMS@JCU Joint Venture, has published a paper inMarine Ecology Progress Series1documenting the first ever study of sponge larvae from release to settlement and so sheds new light on what happens in the early stages of the lifecycle.



The research shows 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, an unexpected finding published in another paper2by Dr Whalan and colleagues. 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 studied the common Great Barrier Reef sponge species Rhopaloeides 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.

This is a mysterious process and the project turned up some surprising sponge activities that confirmed that there is a lot more going on with sponges than meets the eye.

"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," Dr Whalan said.

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, using tiny hairs called cilia to beat through the water. Work by other scientists has shown that these cilia also contain light sensitive cells that enable the larvae to respond to light.

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.

Even at its most vigorous, sponge swimming ability is feeble and no match for water currents that can easily catch hold of them, so currents do play a role in dispersal of sponge larvae.

"Without the stimulus of light, the larvae behave differently, dispersing throughout the water in a haphazard way," Dr Whalan said However, the scientists found something amazing – that an initial exposure to light provided sufficient "imprint" to direct upward movement even when the light is quickly removed. The larvae "remembered" the light and travelled towards where it had been, even when it was gone.

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.

This work points to light being essential to sponges for their survival, although more work needs to be done to understand this better.

The scientists concluded that the long-term fate of a sponge is determined by its young self, based on its capacity to carry through an initial period of travel through the water to the sea surface, followed by a controlled drop back down to settle on the sea floor.

1TheMarine Ecology Progress Seriespaper, written by Steve Whalan, Piers Ettinger-Epstein, Chris Battershill and Rocky de Nys is titled "Larval vertical migration and hierarchical selectivity of settlement in a brooding marine sponge". Go tohttp://www.int-res.com/articles/meps2008/368/m368p145.pdf2TheCoral Reefspaper written by Steve Whalan, Piers Ettinger-Epstein and Rocky de Nys is titled "The effect of temperature on larval pre-settlement duration and metamorphosis for the sponge,Rhopaloeides odorabile". Go to:http://www.springerlink.com/content/9890736214173185/fulltext.html

For further information, please contact:

Dr Steve Whalan, AIMS

Phone: 07 4753 4174; 0419 770 999

E-mail:s.whalan@aims.gov.au

Wendy Ellery , AIMS Media Liaison

Phone: 4753 4409; 0418 729 265

E-mail:w.ellery@aims.gov.au

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