Identifying Ribbed Mussels by Marlo F. Krisberg
Posted Jan 21, 2025
Identifying the ribbed Mussels occurring in North American Western Atlantic waters
There are three genera and five species (one cryptic) of ribbed Mussels occurring in North American Western Atlantic waters. They are:
Ischadium recurvum (Rafinesque, 1820)
Brachidontes exustus (Linnaeus, 1758); a cryptic species*
Brachidontes modiolus (Linnaeus, 1767)
Geukensia demissa (Dillwyn, 1817)
Geukensia granosissima (G. B. Sowerby III, 1914)
For even experienced collectors these species can often be quite difficult to distinguish. Their morphological characters often vary significantly based upon habitat and environmental factors. This situation was exacerbated by the fact that only recently have genetic distinctions within a very common accepted species (Brachidontes exustus) been identified and it recognized that the accepted species name actually represents a cryptic species.*
Brachidontes exustus recently has been recognized to be a cryptic species encompassing five separate species.** Despite the fact that the name Brachidontes domingensis (Lamarck, 1819) has been accepted by some as a valid species, I have seen no work that takes in to consideration the findings of Bennett, Reed & Lutz** and Lee & Foighil,*** and distinguishes Brachidontes exustus, Brachidontes domingensis and the other three species in this cryptic group. Until this is done, it cannot even be accepted that the name Brachidontes domingensis has standing. In my view Brachidontes exustus must be treated as a five-component cryptic species with Brachidontes domingensis being one of the five, but unknown as to which one can be so named until all five are named and distinguished.
While it is easy to confuse the five species of ribbed Mussels at the species level, it is not at the genus level. So, the first task in identifying a ribbed Mussel is to determine which genus it is. This can be readily accomplished by examining the hinge sculpture.
If the hinge sculpture includes a smoothly curved hinge line (or no hinge line) and no teeth, you have a Geukensia. Use location data and whether ribbing is beaded (G. granosissima) or not (G. demissa) to identify as one or the other.
If the hinge sculpture includes a smoothly curved hinge line with 3-4 small, simple teeth displaced above. you have Ischadium recurvum.
If the hinge sculpture includes an irregular hinge line with 2-4 small, white simple teeth, you probably have Brachidontes modiolus. Confirm by comparing the overall shell shape to images presented on LTS and elsewhere.
If the hinge sculpture includes an irregular hinge line with 1-4 small to largish, colored simple teeth you have one of the five species in the cryptic species Brachidontes exustus.
Cryptic species - The label "cryptic species" is applied to an accepted species name (senior synonym) when it is realized (usually via DNA analysis) that the name encompasses multiple genetically separate species. This situation arises because the various species within the group (named and unnamed) were not known with sufficient certainty to be genetically separate, usually because they could not be reliably, morphologically distinguished (or morphological differences were considered to more likely the result of population variations, geographic, or other factors). The label “cryptic species” is applied to the accepted species name in those circumstances where the situation becomes recognized, but no action is taken to name and/or distinguish the various species within the group (named and unnamed). At a future time when all the species within the “cryptic group” are named and distinguished, the label “cryptic species” would no longer apply. Usually, one of the species within the group will retain the accepted species name.
Geukensia demissa and Geukensia granosissima are an example where the former was an accepted species name and the latter (named, but not accepted as a valid separate species) was not clearly recognized as being genetically different. So, when writers described Geukensia demissa they may very well been relying upon samples that may have included either or both. Geukensia demissa could have been labelled as a cryptic species that included two species if there was a strong suspicion that two species were involved. The term cryptic species came into wide use with the recent advent of DNA analysis, now accepted as a reliable method to identify genetic speciation, as opposed to the traditional methods relying on physical traits. Prior to the use of DNA analysis and the emergence of use of the cryptic species concept, essentially morphologically indistinguishable or very similar specimens suspected of differing from an accepted species named were often treated as “subspecies,” “cf. (compared to),” “aff.” or “sp. aff.” (species affinis – strikingly similar), or “forms.” DNA analysis is now enabling identification of these “similar” specimens as either genetically coincident with or separate from the accepted species name. When they are identified as genetically separate, but left undescribed and unnamed, the accepted named species is labelled a cryptic species composed of at least two separate species. In the case of Geukensia demissa, since it was uncertain as to whether Geukensia granosissima was a valid separate species, but it had been named, the two would be labelled as follows:
Geukensia demissa demissa (the nominate or nominotypical subspecies)
Geukensia demissa granosissima (autonymous subspecies)
Those who reject the subspecies approach might label the two in this way:
Geukensia demissa (the accepted species name)
Geukensia demissa form granosissima.
Once the two were confirmed to be genetically separate species and the morphological and other differences between them substantiated and their descriptions modified accordingly, both names became validated species names, and the concept of “cryptic species” no longer applied.
**Bennett, Reed, & Lutz. Jun 2011. DNA barcoding reveals Brachidontes (Bivalvia: Mytilidae) from two ecologically distinct intertidal habitats on Long Key, Florida Keys, are cryptic species, not ecotypes. The Nautilus 125(2), pg. 63.
***Lee & Foighil. 2004. Hidden Floridian biodiversity: mitochondrial and nuclear gene trees reveal four cryptic species within the scorched mussel, Brachidontes exustus, species complex. Molecular Ecology 13: 3527-3542.