Rare algae hides in plain sight

Two marine biologists have found a red algae species that resembles a branching coral.

Western Australian Museum coral biologist Zoe Richards says "there are lots of animals that mimic plants, and plants that mimic other plants, it's much less common in the oceans to have a plant that models an animal."

She found one of the latter on a recent survey of Ashmore Reef, a biodiversity hotspot off the Kimberley.

Reaching for what she thought was a staghorn coral, she was surprised to find it soft and yielding to her touch.

Dr Richards says she initially doubted her own perceptions.

"Finding this 'hard coral' that was soft, I started to wonder if I was nuts, succumbing to nitrogen narcosis," she says.

Western Australian Herbarium algae biologist John Huisman, who was also on the trip, identified it as a red algal species (Eucheuma arnoldii) that was first described four decades ago in the Philippines.

"I would have just swam over the top of it thinking it was a coral," he says.

Dr Huisman says it had been collected only once before in WA waters, at One Arm Point in the West Kimberley, by amateur naturalist Brian Carter.

Mr Carter sends specimens of algae from the tidal reef to Dr Huisman.

"When I get them they're dried specimens pressed down on sheets: I don't get them fresh…I identified it at that point but it doesn't look as distinctive as it does when you see it in the field," Dr Huisman says.

He says it was known to be a coral look alike because of the description in the original paper.

Dr Richards says this is an example of Batesian mimicry, whereby a species resembles something else to deter predators.

In this case the algae avoids predation by herbivorous gastropods and fish because it resembles an unpalatable hard coral Acropora vaughani, which she found growing just a few metres away.

"A lot of the herbivorous fishes avoid the coral and they are just looking for the soft algae to feed on, so it's avoiding predation by doing this," she says.

She says its exact range is unknown, probably because researchers may have mistaken it for a coral.

Dr Richards is using the discovery to illustrate a need for further marine research in Western Australia.

"Ashmore Reef has been researched for a number of years but there's still interesting finds like this out there," she says.

"This is just one little example."

Z. T. Richards, J. M. Huisman "Coral-mimicking alga Eucheuma arnoldii found at Ashmore Reef, north-western Australia." Coral Reefs, January 2014 DOI: 10.1007/s00338-013-1121-2