Where Do The Shells Go? Seaside Recycling

Wednesday, September 2, 2009

Nature Notes

by Bob Thomas

Have you ever wondered if and how seashells are broken down in nature? The answer is more interesting than you may think.

Other than simply wearing down from being jostled about in a gritty environment by wave energy, seashells are broken down by a long list of organisms whose niche in nature includes the work of biodegrading empty shells into the tiny pieces we see on the beach.

None of the critters whose activities result in turning seashells into calcareous sand, or simply putting the calcium carbonate back into the ecosystem, are directly nourished by the seashells. Instead, they either break the shells up to get to the living animal inside or they use them for their homes.

Some larger animals, such as black drum and stone crabs, crush mollusks during their routine feeding activity.

Even more intriguing and much less known are a myriad of small creatures that bore into the shells, thus weakening the walls and often providing habitat for a new wave of invaders. The blister worm (Polydora), a polychaete, uses a rasping technique coupled with self-produced acids to etch a groove in the shell that eventually becomes a deep pit where the worm lives. In oysters, they wage an epic battle that includes the polychaete creating a U-shaped tunnel, followed by the oyster producing more shell in an attempt to cover the entrance (a thin bump that resembles a blister), with the polychaete then reopening. Over time, their grooving and tunneling may result in shell destruction.

The bright yellow boring sponge (Cliona) may attack living shells, but is most often found along our coast in non-living shells. Boring sponges secrete chemicals and release a marauding army of amoebocytes (cells that move like an amoeba) that physically lift chips of shell to construct surface holes that open into a labyrinth of tunnels in the host's shell. This network of tunnels is produced during the early infestation period. If infestation is extensive enough, the boring sponges may cause the shell's walls to collapse entirely, especially as the sponges mature.

A species that specializes in oyster shells is the oyster piddock (Diplothyra smythii). This critter is a boring clam that might bore completely through the oyster! Typically it settles inside the oyster shell and lives as a commensal (gaining from its association with the oyster while the oyster is unaffected). Another bivalve (Gastrochaena) makes an oval-shaped penetration of another bivalve's shell and then spends its life embedded in its host's shell.

There are a number of predatory marine snails that begin the degradation of seashells by first drilling (actually, rasping with its thread-like radula) a hole in the prey's shell and then consuming the prey, leaving the shell(s) we find washed up on the beach. One example is the shark eye (Neverita duplicata), which makes a nicely beveled hole, usually near the prey's hinge (umbo). Thick-lipped drills (Eupleura caudata), common offshore around peninsula Florida, make a slightly beveled hole, the inside diameter more than half that of the outer diameter. Straight-sided holes are diagnostic of oyster drills (Stramonita haemastoma). Look closely at a bunch of oyster shells and you will likely see an oyster drill hole.

In living mollusks, any and all of these infestations may gradually weaken the living host (which has to continually replace shell) to the point where it dies.

The next time you're at the beach, examine shells closely and you will certainly find evidence of shell recycling. In its absence, we would be up to our ears in seashells.

Also pubilshed in Nature Profile, The Times Picayune, February 29, 1984.

                                  

The elongate groove made by a blister                                            A shell bearing evidence of a predatory 
worm (Polydora), created by a combination                                     snail, such as the beveled hole made by
of scraping with its rough body anatomy                                         a shark eye (Neverita, formerly known
and secreted chemicals.                                                                as Polynices). Also, note the oblong hole
Photo by Bob Thomas.                                                               probably made by the bivalve Gastrochaena.
                                                                                                     Photo by Bob Thomas.

                                  

This shell has the calcareous tube of a serpulid                               A shell fragment riddled with boring sponge
worm, as well as evidence of other destructive                               (Cliona) holes. Much of the original shell has
penetrations of its surface.                                                            crumbled away.
Photo by Bob Thomas.                                                                Photo by Bob Thomas.

An age-old tradition of young beach lovers
is to tie a string through the hole made by a
carnivorous snail and enjoy a shell necklace.
Photo by Bob Thomas.