Bivalve! At the Disco?

Imagine you are diving in a dark cave in Raja Ampat, Indonesia. Out of the corner of your eye, you spot a flashing light. By now, intrepid reader, this probably doesn’t surprise you, considering how many of the ocean’s residents create their own illumination. But you still investigate, of course, and encounter a happy little clam, it’s bright orange lips the mirrored source of that dazzling display:

This is the (aptly named) disco clam, or Ctenoides ales. Other fun names for it include electric flame scallop and electric clam, and it’s the only known bivalve with light displays. And before you think, I know why! It must bioluminesce, let me tell you: you’re wrong.

But first, really quickly, what is a clam?

Another one of Haeckel's beautiful illustrations, this time centering on Bivalvia. Source

Another one of Haeckel's beautiful illustrations, this time centering on Bivalvia. Source

Clams are a member of the class Bivalvia, which belongs to the phylum Mollusca- same as squids and octopuses! The main characteristic of this class is two hinged parts, or shells. The shells  are made of calcium carbonate, which is secreted by their body wall, known as a mantle. Other bivalves include oysters, mussels and scallops. This particular clam belongs to the family Limidae, one of 99 marine families consisting of about 9000 known species. (If you include brackish water and estuarine species this number pushes up to 106 families.)

Basic body plan of a clam: the ctenidia are the feathered structures. The gonad is the reproductive organ. The adductor muscle is very powerful, and when it contracts completely, the shell is impossible to open. When you eat scallops, you are often eating this muscle, as it is the only soft part of their body. Source

Basic body plan of a clam: the ctenidia are the feathered structures. The gonad is the reproductive organ. The adductor muscle is very powerful, and when it contracts completely, the shell is impossible to open. When you eat scallops, you are often eating this muscle, as it is the only soft part of their body. Source

Bivalvia are particulate feeders, consuming microscopic phytoplankton, zooplankton, algae and copepods. Unlike the rest of the phylum, bivalves lack teeth (or a radula); instead, they filter feed with specialized gills known as ctenidia. These feathery structures maximize surface area, allowing for a more efficient exchange of gases and water. Their filter feeding is an important facet of marine environments: bivalves can clean large amounts of water very quickly, and decreasing populations of members favored for food, such as oysters, has led to a reduction in water quality in areas such as Chesapeake Bay. We use their filter feeding ability to monitor pollutants in marine environments- as they filter polluted water, they accumulate heavy metals (such as copper and zinc) and persistent organic pollutants (such as pesticides) in their tissues, and we can examine these to figure out concentrations. We also eat them and use them for jewelry and ornaments.

Silica nanospheres, at around 300 nanometers, are an optimal particle size for scattering visible light, especially the shorter blue-green wavelengths of 400-500nm that predominate at 3-50m underwater, which is typically the clam’s habitat. Source

Silica nanospheres, at around 300 nanometers, are an optimal particle size for scattering visible light, especially the shorter blue-green wavelengths of 400-500nm that predominate at 3-50m underwater, which is typically the clam’s habitat. Source

And, like I said, only the disco clam actually puts on a flashy show. How does it do it? The reflective edge of the mantle contains nano-spheres of silica, the same stuff that quartz is made of. Silica is highly reflective on one side and absorbent on the other. The former side is so reflective, in fact, that it catches the low levels of blue light found in underwater caves- the ambient light. The clam curls and uncurls this edge, and the contrast between the white tissue and the normal red of the rest of the tissue creates the illusion of flashing!

Why does it put on this flashy show?

The initial theory was courtship, but when researchers tested this hypothesis, exactly zero of the clams forty eyes reacted. How could a male clam woo a female clam using something she couldn’t even see? So, they moved on to another, more fruitful theory: it was to attract prey. When they added phytoplankton, a tasty treat, to the disco clam’s tank, it flashed vociferously. Since phytoplankton are attracted to light, and the clams are the only source of it in dark caves, it’s a pretty effective strategy for attracting food - and they don’t even have to move!

Some of the clam's 40 eyes! Source

Some of the clam's 40 eyes! Source

Researchers also tested another theory: could the clam be warning potential predators?

A peacock mantis shrimp. They're beautiful. Source

A peacock mantis shrimp. They're beautiful. Source

They added the most terrifying creature to the tank - a styrofoam lid. Detecting the intrusion, the clam’s light began to flash at a measurably higher frequency (from 1.5 Hz to 2.5 Hz, in case you wanted specifics). However, when the styrofoam lid was replaced by the most charismatic of crustaceans, a peacock mantis shrimp, the shrimp simply ignored these warnings and went straight in for a nibble at the clam’s pretty lips. Instead of satiating itself though, the shrimp recoiled as if stung and entered a strange catatonic state. The clam’s orange color was a warning: the tentacles are soaked in sulfuric acid produced by the clam, which one can imagine gets converted into a pretty horrific tasting substance. Just imagine eating something that tastes like the smell of rotten eggs - that particular scent occurs because of hydrogen sulfide.

This deep sea disco ball deters its predators with toxic snot: I guess the ocean really does have everything!