Carbonate sediments on the inner Yucatan Shelf occur as textural belts paralleling the shoreline. Nearshore, high-energy, oolitically coated grains occur in the strait between Isla Mujeres and the mainland. Nichupte Lagoon, on the lee side of the tombolo connecting Punta Cancun and Cancun, contains fine-grained, low-energy, magnesium-calcite rich mud.

Five widely spaced quantitative (total = live + dead) benthic reconnaissance samples were collected from each of these radically different depositional settings. Ostracodes (approximately 300/sample) isolated from each sample were speciated, and ternary plots of the first 3 dominant species generated.

Plots from the low-energy Nichupte Lagoon indicate: (1) Cyprideis sp. is the first-order dominant in 80% of these samples, (2) second-order dominants are usually Xestoleberis sp., with some Paranesidea "frilled" and others, (3) other Paranesidea ("frilled" and "arched") occur as third-order dominants, and (4) the percentage spread among the 5 samples of the first-order dominants is approximately 52%, whereas it is only 24% among the second- and third-order dominants.

Ternary diagrams of the first three dominants in high-energy strait sediments reveal: (1) Paranesidea "arched" is the first-order dominant in 60% of these samples, (2) second-order dominants are almost equally divided between P. "arched" and P. "elongate," (3) third-order dominants are almost equally divided between P. "elongate" and Neonesidea longisetosa, and (4) the percentage spread among the 5 samples of the first-order dominants is approximately 32%, whereas it is only about 16% among the second- and third-order dominants.

The smaller spread (32%, 16%) of the high-energy grainstones of the carbonate strait separates these sediments from the greater spread (52%, 24%) of the low-energy lagoonal sediments of Nichupte Lagoon.

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