Morphoclines of living operculinid foraminifera based on quantitative characters

Abstract

Growth-independent test characters of the genera Operculina, Planoperculina, and Planostegina were investigated to determine relations to water depth in form of morphoclines. All characters describing test form – thickness, initial vector of the test spiral that is correlated to proloculus size, and radius expansion rate describing test contour – are correlated. The investigated Operculina and Planoperculina species (O. ammonoides, O. elegans, O. complanata, P. heterosteginoides) demonstrate significant morphoclines in these characters, which can be used for gradient estimation using regression analyses. Thick forms of Operculina with intensively coiled spirals predominate in shallow water (−20 to −40m); in the deeper parts of the euphotic zone (−120m) they transform into thin forms possessing a weakly coiled spiral. Planoperculina heterosteginoides, with thin tests and therefore restricted to the deeper euphotic zone (starting at −80m), is the only symbiont-bearing larger foraminifer that can extend its distribution to just below the euphotic zone, where it develops extremely thin tests. Both Planostegina species, similar to shallow specimens of Planoperculina heterosteginoides in test form and thickness, do not show morphoclines and are thus restricted to depths between −30 and −90m (P. operculinoides) and −70 and −90m (P. longisepta). Chamber numbers, represented by the growth-independent parameters ‘chamber number of the first whorl’ and the ‘chamber number increase rate’ differ between Operculina species on the one hand and the Planostegina Planoperculina group on the other. Although both parameters are weakly correlated with characters describing test form, they do not show significant morphoclines either in the Operculina species or in the Planostegina Planoperculina group.