Rostanga pulchra: One Species or Three, Differentiating Morphology Within a Cryptic Species Complex

Abstract

63 pages.The nudibranch Rostanga pulchra, first described by F.M. MacFarland in 1905, has long been regarded as a single, morphologically variable species distributed along the Pacific coast of North America. Recent DNA barcoding, however, reveals that R. pulchra represents a cryptic species complex comprising at least three genetically distinct operational taxonomic units (OTUs, a term which is commonly used when species are being identified by DNA barcoding alone) occurring sympatrically in Oregon. Delineating species, whether cryptic or not, is fundamental to biology as it underpins accurate biodiversity estimation, ecological interpretation, and evolutionary inference. This thesis integrates DNA barcoding with comprehensive morphological analyses to identify diagnostic characteristics that distinguish the three genetically distinct groups, in a revival of the golden age of biological observation. While molecular tools are powerful for detecting hidden diversity, the ability to differentiate species morphologically is equally essential for practical identification, ecological study, and formal taxonomic description. Live external morphological surveys revealed differences in the structure of dorsal caryophyllidia —specifically relative spicule height and spicule position—as well as in minute spot aggregation and density. Radular morphology, examined using scanning electron microscopy (SEM), revealed two useful diagnostic features: the angle of the inner pleural tooth base and length of the inner pleural tooth basal marginal facing extension. Egg diameter was also measured and evaluated as a potential species-level character. Together, dorsal caryophyllidium microstructure, minute spot aggregation patterns, radular metrics, and egg diameter provide a cohesive morphological framework for distinguishing members of the Rostanga pulchra species complex and form the foundation for future species descriptions. Moreover, with newly identified characteristics of each OTU, a hypothesis of the true identity of Rostanga pulchra is made. More broadly, the framework presented here offers a methodological template for resolving additional cryptic species complexes within Discodorididae and other nudibranchs