Following the light



Light-trap deployment.
Images: Kade Mills

The scientists have brought many "tools of the trade" on this field trip, with one of the most popular being the light trap.
The light trap has been used to collect specimens at night in the waters around Lizard Island.
Shawn Smith, AIMS CReefs project manager, said the lights were an effective way to collect specimens.
"It's basically a big fluorescent tube that you connect up every evening before you take it out," he said.
"It sits out in the water about two metres from the surface. It is a big perspex box with a few gaps in it so the fish can get in but it's hard for them to get out."
Shawn said that the fish were attracted to the fluorescent light, much like a mosquito to light, or indeed a moth to a flame.

Fish display "phototactic" behaviour, which is movement of an organism toward or away from a source of light.  The light entices them into the tube where, although there is room for movement, they are usually trapped.
Shawn, along with researcher Chris Glasby, has been deploying the traps at dusk and bringing them back in at dawn.
"You have to pick them up first thing in the morning, as soon as the sun's up…if they're in there too long it gets too hot for them," Shawn said.
Chris was using the device to look for polychaetes.
 

"I'm looking for the reproductive forms of a group of polychaete that normally live in the sediment. It's the most effective way to catch them as they swim into the water column," Chris said.
"We're looking at how male and female polychaetes release their eggs and sperm when they reproduce. It's interesting, as these forms that swim into the water column modify their bodies compared to the ones that live in the sediment.  They get swimming lobes and large eyes so they can detect the light – they are completely modified," he said.
Chris said the difficult part was matching the polychaetes caught in the trap to those found in the sediment. He said once you do this, you have a picture of how their body changes as they reproduce.
 


Light-trap deployment.
Images: Kade Mills

Along with the polychaete, many crustacea have been found in the traps. Normally the fish found in the device were small pelagic fish, as they swim in the water column.
Although they are effective, the traps do not always catch what the researchers need.

Jo Browne from Museum Victoria and Griffith University has used the light trap twice, to no avail.
She is trying to catch gelatinous zooplankton for her research.
"Gelatinous zooplankton often occurs in large aggregations so it can be pretty hit or miss if you find them or not. One month there might be hundreds, the next month barely anything. Often when they're really abundant in the water column you get so many at once that they fill up the light traps," Jo said.
Instead of using the light traps again, Jo has been using plankton nets - one cone shaped plankton net to do the vertical hauls and one cylinder cone net for horizontal tows. She said this is an effective method, although the specimens may be damaged during the collections.
So far, Jo has caught various creatures including marine invertebrates such as jellyfish and small transparent marine creatures called salps.
The light traps are most effective at certain times of the moon cycle when there is less ambient light, as the fish are able to focus more fully on the bright light of the trap.

"The trap is the only light in the sky so everything is attracted to it. The less light from the moon, the more attractive the light from the trap is," Shawn said.