Functional Morphology of Melonis Barleeanum and Hoeglundina elegans: a Proxy for Water-Mass Characteristics

Morphometric study of Melonis barleeanum and Hoeglundina elegans was carried out on 15 core top samples from the Indian Ocean. Length to breadth ratios and wall and septal thicknesses of the largest tests of both the species from each sample, along with δ¹³C and δ¹⁸O values of Cibicides wuellerstorfi were measured. Both the species show equal growth rates of the test in their normal habitat. However, the high organic carbon preference species M. barleeanum shows more elongation of the test during food scarcity. This effect is not evident in H. elegans, which varies in its wall and septal thicknesses with bottom-water oxygen levels of the deep water mass up to 2000 m, probably to maintain the required rate of osmosis for the intake of dissolved O₂. Below this depth both parameters show parallel relationship with deviation indicating that oxygenation may play some role in the variation of wall and septal thicknesses. Thinning or thickening of the wall and septa in M. barleeanum and H. elegans has no relation with the water depth, indicating no relation with either the overlying pressure effect or nutrients as each deep water mass has a different nutrient budget. Depletion in δ¹³C and enrichment in δ¹⁸O below 2000 m water depth suggests that up to 2000 m depth, the Indian Ocean is bathed by the welloxygenated, low-nutrient North Atlantic Deep Water (NADW), whereas below 3000 m cold, nutrient-rich Antarctic Bottom Water (AABW) is dominant. Between 2000 and 3000 m water depths, the water mass in the Indian Ocean is a mixture of NADW and AABW.