- MIKE: Here in SoFla we call them "thorny oysters" and they are abundant, especially on wrecks. I'm told they are all the same species (S. americanus) but the variations between specimens is huge, mostly depending on substrate and exposure to current. Deep inside the wrecks, the spines are widely separated, long, and flat. The shell has pronounced curvature and is generally white. On exposed areas of the reef, the spines are close together, short, and rounded. The shell is flatter and dark red to light orange.

- PEGGY: Correction, Mike: the deeper water shells are Spondylus americanus. They can be white, pink, orange, red or yellow. The ones from about 50 feet that are brick red with white speckles on the umbo are Spondylus ictericus. They have fewer spines, are smaller, and the spines may be spatulate or "fingered".

- MIKE: I suspected the brick red ones were a different species. They are common on the reef but not on the wrecks. But I have what I am sure are S. americanus, some with short spikes, some with long spatulate spikes. The difference seems to be exposure to current.

- DAVID: I'm just wondering how frequently Spondylus americanus has been identified when S. ictericus is actually present. Judging by my attempts to ID Spondylus from Thailand, distinguishing variability from species-differences in this genus has got to be among the most difficult in conchology. Even after I got some assistance, I still couldn't be confident in diagnosing them. Who are the Spondylus experts and what do they rely on?

- PEGGY: Spondylus americanus is common in Florida on both coasts, mostly at about 100 feet. Farther south in the Caribbean it is shallower, and I've found it intertidally even in the Abacos (Bahamas) and in Honduras. S. ictericus in Florida is usually at about 50 feet or a little less or more. I've found it intertidally in Bermuda. It is ALWAYS at least partly brick red and usually has those white spots on the umbones. I do have one with yellow spines but the shell itself is brick red. Any other color combination is probably S. americanus. S. americanus can be very spiny (inside wrecks) with a huge "skirt" along the base of the lower valve or have fewer, longer spines (outside wrecks and on reefs). The best ones are covered with sponge when found - the spines don't break and grow long. The intertidal ones are ugly, with very short and sometimes no spines.

- DAVE: I'm not sure how variable or consistent the muscle scar patterns within Spondylus shells are, but such patterns might aid in distinguishing individual species.

[click on pics to load larger views]

These three S. americanus all came from the same wreck, but different exposures to current.
The center one came from a chain in the steering locker which was very difficult to access and completely shielded from currents. The other two were both in an open hold, exposed to currents.

Two more, same shells, showing the variation in attachment of one to a narrow chain, the other to a flat plate. It appears to me that the spines on the attached side morph into an attachment best suited to the environment: a narrow, high pedestal on a precarious perch in no current, versus a low, broadly crenelated skirt on plate in high current.

...That it seems that S. americanus and S. ictericus had different depth ranges:
S. americanus lives in deeper waters, from 50-100 feet deep in the north to shallower waters in the south, while S. ictericus lives from intertidal down to something around 50 feet deep.
The colouration of the two species is variable, but ictericus is said to be always at least partly brick red.
S. americanus varies from white to a dark and bright red, and it has been supposed that the colouration depended of the depth and/or the strength of the streams, directly or not (Mike said that they're white in sheltered places inside wrecks and reddish on the exposed reefs). But the problem is that the shallower the place, the uglier the shell will be.
And a red and ugly shell with poor spines could be confused with an ugly ictericus...
Hopefully, ictericus is supposed to often have a white spotting in the umbonal areas... Often !

Click on pics to load larger views.
At left, an americanus. The right one has been identified by Peggy William as an ictericus.
According to Mike's observations, the americanus may have lived in the upper part of its depth range, while the ictericus reached its lower limit.
It seems that americanus does not occur lower than 9-10m deep in Martinique, when ictericus can be fished here at low tide and, of course, is usually ugly, lacks spines etc.
The one pictured here may represent an extreme variation.