Distinguishing Morphotypes of Foraminifera Orbulina Universa Using Shell Morphometrics

Abstract:

Several recent genetic studies have revealed that many morphologically defined planktonic foraminiferal species contain multiple distinct cryptic or pseudocryptic genetic variants. In some cases, such as with the genetic variants of Globigerinoides ruber, past taxonomic “lumping” may be easily revised within the framework of the morphospecies concept. However, some planktonic foraminiferal cryptic species are not easily differentiated from one another based on identifiable morphological differences, as is the case with Orbulina universa cryptic species (Morard et al., 2009). Based on recent findings, it is believed that several of these O. universa cryptic species differ in their habitat preferences (Darling et al., 1999,2000; de Vargas et al., 1999, 2001), and the lumping of these cryptic species could possibly account for a significant amount of the noise observed in various paleoclimate records derived from this species (Kucera and Darling, 2002). The current study uses foraminiferal morphometrics (area density, Marshall et al., 2013, thickness and weight-diameter realtionships) to identify distinct groupings of individual Orbulina universa specimens collected from sediment trap samples in the Cariaco Basin, Venezuela. The study reveals three distinct groups of individuals, varying in their test thicknesses and pore space distribution, which likely correspond to the previously identified cryptic species of O. universa. We find significant differences in the isotopic compositions of these groupings – suggesting that the identified morphotypes differ in their habitat preferences. These morphotypes also exhibit different relationships with the measured hydrographic parameters and their relative distribution can be linked to changes in upwelling and non-upwelling hydrographic regimes. We agree with the results of previous studies that suggest different cryptic species should not be treated as ecophenotypic variants and need to be distinguished from one another during proxy development and application.