Stephen Moore, Landcare Research, Auckland
Many terrestrial and seawater invertebrates around the world have the ability to glow in the dark, but there is only one fresh water invertebrate with this ability, and it is only found in the North Island of New Zealand. This mysterious creature is the freshwater limpet Latia neritoides.
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Millions
of years ago a group of land snails decided to return to the water (where
all snails came from originally) and one New Zealand descendant of that group
slowly developed a limpet-like shell. This limpet can be found in many stony
streams, and if you find their jelly-like egg capsules you can still see the
land snail shape of the embryos (see Top photo).
The
limpet-shaped shell of up to 12mm in length, and the powerfully adhesive foot
allow Latia to cope with fast-flowing waters, as long as they have something
firm to attach to.
Being
limpet shaped is unusual enough for a freshwater snail, but the most amazing
thing about Latia of course is the ability to produce a glow-in-the-dark slime.
When disturbed Latia releases a slime containing the substrate and enzyme
chemicals luciferin and luciferase, which react together to produce a bright
green light. If you find a stream with a dense population of Latia, and if
for some strange reason you happen to stand in the stream at night, you'll
notice that your footprints are glowing bright green (the light is much brighter
than that of glow worms).
Because this luminescence is only released when the limpet is threatened,
it must be a defence mechanism. One theory is that a night time predator (presumably
a native fish), having attempted to dislodge the limpet will see the glowing
slime being carried away by the stream current and will chase the moving light
rather than the limpet.
It's also possible that some of the glowing slime may stick to the predator - probably somewhat confusing or alarming. Native fish feed mostly at night for one reason; they're less likely to be seen by even bigger predators - but how are you supposed to hide in the dark from bigger predators when you have a brightly glowing bogey clinging to your snout?
There's also a potential use of this luminescence for cunning Pollution Detectives. Anything that upsets Latia can trigger the release of the luminescent slime. That includes any nasty chemicals being illegally tipped into streams at night. Who knows, some young Pollution Detectives on night shift could find themselves following a trail of luminescence towards some sinister, previously undetected pollution source. Bright eh?
