Noetia ponderosa is one of the most common shells in Southwest Florida. Its shell is not very attractive at first glance (at one time I called it the “Ugly Duckling Clam”), but it is still one of my local favorites. Keep reading to find out why!

Noetia ponderosa, Sanibel, Florida. Illustration: José H. Leal

A striking feature of this species is its dark-brown or black, velvet-like periostracum (above), the organic shell layer that covers a part of or the entire shell. Visitors to Southwest Florida beaches often assume that the periostracum of the Ponderous Ark is oil, tar, or another strange substance accidentally deposited onto the otherwise white shell. This false impression was frequent after the 2010 BP Oil Spill in the Gulf of Mexico. The image above also explores a different angle and lighting to show cool features inside the Ponderous Ark shell. Observe the hinge, with many small “teeth,” and the intricate pattern of different scars indicating muscle attachment sites in the living clam.

Noetia ponderosa associated with a colonuy of Leptogorgia hebes, Sanibel, Florida

Another feature that sets the Ponderous Ark apart from other local species of bivalves is its occasional association with the Regal Sea Fan (Leptogorgia hebes), a marine colonial organism related to sea whips, sea anemones, and corals (above). Regal Sea Fans attach themselves to the posterior region of Ponderous Arks and may profit from an enhanced food supply (zooplankton) facilitated by the water currents created by their filter-feeding host bivalves.

Noetia ponderosa, empty valves hosting boring Stimpson’s Chimney Clams, Sanibel, Florida. See text below for explanation

The thick Ponderous Ark shells (ponderous = heavy), when empty, serve to host boring organisms, including other bivalve mollusks such as the Stimpson’s Chimney Clam (Rocellaria stimpsoni). The image on above left shows a Stimpson’s Chimney Clam nested within its dwelling within a left valve, while the two views of the right valve (on center and right) show a large number of boreholes and related dwellings made across the valve by several Stimpson’s Chimney Clams. Red lines connect external (center) and internal (right) views of each borehole/dwelling on opposite sides of that same valve.

Noetia ponderosa with blister pearl in two views (arrows), Sanibel, Florida

Last but not least, the illustration above shows two views of a blister pearl (arrows) in a Ponderous Ark shell. Blister pearls occur when a particle enters the space between the mollusk’s shell and the soft mantle, remaining very close to the shell. The mollusk’s mantle will eventually coat that particle with shell material, forming a pearl. Blister pearls grow attached to the inner surface of the shell, rather than “floating freely” within its soft tissue. If the internal shell layer is nacreous (“mother-of-pearl”), the pearl will be lustrous, “pearl-like”. If the internal layer of that species is not nacreous, the shell will be dull, opaque, as is the case with the Ponderous Ark. I hope now you can join me as a member of the Ponderous Ark Fan Club!

All illustrations by José H. Leal

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