About broken shells

The lighting around my aquarium always varies, from morning sunshine to rainy-day blues, to snowy whites, to indoor yellows. I take a few photos at random, just to check the lighting of the moment. This was one of them.

Hermits (3; can you find the 3rd?), holdfast, red algae, broken shells, marine "snow".

The littlest hermit, in the centre, has a couple of big holes in his shell; from one angle, I can see through and out the other side. He seems happy enough in it.

That orange mass is the remains of an old kelp holdfast. I added it to the tank months ago, still wearing its outer skin, and populated with diatoms and assorted bits. The hermits cleaned it completely in a day or two, as usual. Now it looks bare; just the clean, orange twigs, but every time I pull it out, thinking to discard it, there are several hermits and a clan of amphipods on it, so they must be finding something to eat there. And I put it back.

The big shell in the back is a moon snail shell I added two summers ago. It was intact then, but even without wave motion and rolling rocks to break them, shells gradually disintegrate in salt water, so that picking up the shell now, even gently, causes edges to break off. About half the shell is gone now. This deterioration of a shell provides calcium for the living snails' shells.

To build shells, organisms extract calcium ions (Ca2+) and carbonate ions (CO32-) from seawater, which combine into the solid crystals of calcium carbonate (CaCO3) that shells are made of.  (Woods Hole Oceanographic Institution)

Acidity in the water (such as the water I'd brought home from the "improved" boat launch) causes shells to dissolve more rapidly, both on leftover shells and the growing shells of live snails. A new study by Woods Hole OI examined the effects of rising CO2 on marine invertebrates. Some tolerate it, others don't. Some don't survive, as their shells provide less protection from predators and other stresses.

Ries and colleagues found that species with more protective coverings on their shells and skeletons—crustaceans, the temperate urchins, mussels, and coralline red algae—are less vulnerable to the acidified seawater than those with less protective shells, such as conchs, hard clams, and tropical urchins.
All of the test organisms continued to create new shell throughout the experiment, Ries said, but some suffered a net loss of shell because older, more massive portions of their shells dissolved under the highest CO2 conditions. (Woods Hole)

Just another reason to monitor the acidity of my aquarium.

(Also see: As The Oceans Become More Acidic, Sea Snail Shells Could Dissolve.)