Floating around in their ocean home, Australian sea urchin(Holopneustes purpurascens) larvae are in a race against the clock to settle on sub-tidal algae and metamorphose into their adult form before they use up their precious internal food supplies or are eaten themselves. About 5-7 days after fertilisation, the larvae become `competent': they develop tiny tube feet for attachment and they are able to detect histamine. This chemical is a settlement cue, much like a `homing' signal, released by the preferred host alga that they will dine and hide on. A previous study by Jane Williamson and Peter Steinberg found that younger larvae metamorphosed in response to red algae only, while older larvae metamorphosed not only in response to red, but also to brown algae, which contains lower levels of histamine than reds. Because the older larvae may be responding to lower histamine levels, or another compound altogether, Rebecca Swanson from the University of New South Wales, Australia, and her colleagues Dustin Marshall and Steinberg, tested if older Australian sea urchin larvae would metamorphose in response to lower concentrations of histamine than younger larvae().
The team fertilised sea urchin eggs and allowed them to develop in large beakers in the lab. Once they were competent, after 5-7 days, the team let groups of larvae develop for different lengths of time – 7, 14, 21 or 28 days – before testing their responses to histamine.
To find out how larvae of different ages responded to histamine, the team added histamine of different concentrations to small test dishes containing seawater before adding larvae of different ages and measuring if they settled at the bottom of the dish and metamorphosed or not. They found that the percentage of larvae that responded to lower concentrations of histamine increased as the larvae got older. For example, at 10 nmol l–1 histamine after 72 h exposure, ∼45% of 28 day-old larvae metamorphosed compared to only 5% of 7 day-old larvae. `The increased sensitivity of older larvae to histamine occurred gradually with age,' says Swanson.
The team also found that the amount of time that larvae of different ages were exposed to histamine affected their transition to metamorphosis. When exposed to 10 μmol l–1 histamine, 60% of newly competent(7 day-old) larvae settled after 20 min exposure. However most of these larvae started swimming again when placed in histamine-free seawater. The young larvae needed at least 3 hours of continuous histamine exposure to induce metamorphosis in all of them. But, older larvae only needed 30 min histamine exposure for all of the settled larvae to metamorphose. This suggests that younger larvae are more flexible when they settle, and can resume swimming if conditions change. Older larvae, however, are committed to metamorphose, even if they no longer detect histamine.
By becoming more sensitive to ever decreasing concentrations of histamine as they develop, the sea urchin larvae are potentially able to detect a wider range of host algal species, over greater distances, meaning that they are more likely to find a suitable place to settle, increasing their chances of survival. `What we really need to do next is work out the mechanism', Swanson says.
