11 May 2015 – Things that go snap in the night

It’s 8 AM and the ground is still swaying under my feet.  Just over twelve hours ago, three people in my research team – Laís, Sophie and Brendan – and I loaded a small skiff with recording equipment, empty bottles, anchors, a dozen life jackets, all manner of ropes and a hatful of sweets, and we left the dock of the Cape Eleuthera Institute.  We headed east into the vast, shallow bay of Rock Sound with one goal in mind: to record the sounds of coral reefs at night.

We anchored near a reef patch, next to which we had rigged up a ‘tripod’ the previous day.  This contraption, made up of an upturned empty bleach bottle held midwater by ropes tied to cement blocks, was meant to make it easy to hook up our hydrophone and keep it off the bottom.  At least, that was the theory.  So we waited, and waited, and tried to doze a little in nests of life jackets, despite the wind and waves, till the sky turned pitch black.  I stared into the darkness and saw a shooting star.

Finally, around 11 PM, it was time to spring into action. BrendanBren Lais small and Laís slipped into the water, swam the hydrophone, at the end of its 20 m cable, to the bottle and hooked it on with an elastic band… which promptly broke. Dang! But they had a spare and the second one held. Phew! Meanwhile, Sophie hooked up her fancy DJ-like switchboard and started listening.  Crinkling of the nose, squinting of the eyes, shaking of the head.  No, something’s not rightSophie small, something’s rubbing.  With my flashlight, I illuminated a thumb down to the snorkelers.  Laís, the queen of the rubbing cables, worked her magic with another rubber band.  Crinkling of the nose, squinting of the eyes, smile.  It’s a thumb up this time.  The swimmers got back in, and recording began.  While we were quiet on the boat, down below, there was a cacophony of noise.  What does a coral reef sound at night?  Think frying bacon, or milk poured on Rice Krispies, with the odd low grunt added by fish for good measure.  Better still: click here to hear it for yourself. The first violins in this nocturnal orchestra are snapping shrimp, which can make sounds as loud as a jet engine at take-off!  Luckily, they’re never THAT close to the hydrophone.

When the time was up, Brendan and Laís retrieved the hydrophone and returned to the boat.   We lifted anchor, slowly made our way to the next reef (hats off to Polynesians for crossing large expanses of ocean by starlight!), and did it all over again, five more times.  Why?  To see if reefs with lots of invasive lionfish sound different than reefs with few of these invaders.  Reef noise reveals a lot, like how much coral there is, and fish and invertebrate larvae, at the end of their journey in the plankton, use it as a cue to find a good habitat to settle in and turn into their adult form.   If lionfish change reef noise, they could have a major impact on how attractive reefs sound to potential newcomers.

By 4 AM, we had finished our last recording of the night. Finally a Parrot smallchance for me, the boat captain until then, to see for myself what these reef patches look like at night.   What a sight!  Small grunts were hanging out like a halo around the reef; big white urchins, some crowned with bits of coral or algae, sat in plain view, as did all manner of enormous crabs; sleepy parrotfish were ensconced in their night dens and large ocean triggerfishes peeked at me from below cover.  And of course, there were lionfish, hunting on the reef and over the adjacent seagrass meadow, perhaps making the reef a much quieter place.   We’ll soon find out.