LITERATURE SUGGESTS CRYPTIC SPECIES ARE RARE IN EXTANT SHELLED MARINE GASTROPODS: IMPLICATIONS FOR INTERPRETING SPECIES IN THE FOSSIL RECORD

Cryptic species (CS) – traditionally defined as species that differ genetically but not morphologically - are routinely ignored by paleontology. Yet to the degree that paleobiological theories about macroevolution are about species, some understanding or assumption about the relative frequency of CS is required. A standard paleontological approach to this would be assessment of CS frequency in modern taxa, and application of the result to fossil relatives. Although a few species have been tested (and much cited by paleontologists), there are few broad surveys.

From Web of Science articles mentioning CS between 1995 and 2021 (n = 79), we reviewed how CS were studied and how they were taxonomically treated in extant shelled marine gastropods. Most reviewed publications were not focused on identifying or delimiting CS. The species discussed had been selected for a variety of reasons, including potential cryptic status and problematic taxonomic status. Approximately 460 putative species from 110 genera in 35 families were represented. Caenogastropods were the most studied subclass (~75%), followed by heterobranchs (20%).

One of the most conspicuous patterns in this literature was that most studies did not explicitly define their uses of the term ‘cryptic species’, which made their meaning ambiguous and application unclear. This difficulty notwithstanding, ~70% of species discussed were confirmed not to be cryptic (meaning they were separable both morphologically and genetically). The remaining ~30% of species were judged potentially cryptic based on molecular differences with little or no diagnostic morphology, or their species status was unclear. On further analysis, 33 of these were named as valid species based on both molecular and morphological differences. Only 3 species were described based solely on molecular differences (<<1%).

Our review thus indicates that for living shelled marine gastropods, CS are present but rare. Their proportion differs among families and habitats, although this is difficult to disentangle from biases in study effort. To the degree that this sample is representative of extinct taxa, this suggests that a high proportion of shelled marine gastropod species are identifiable for study in the fossil record.