Page last updated
5 January 2017
Mussel of the Month
The 2016 Mussels of the Month!
Lemiox rimosus (Unionidae, Nearctic)
The December 2016 Mussel of the Month is Lemiox rimosus. Lemiox is a monotypic genus endemic to the Cumberland Plateau in southeastern North America.
The thing to know about Lemiox rimosus is that it is endangered. It is one of those American mussels that is limited to the Cumberland Plateau, and even within that endemic range, extant populations are disjunct between the Duck River and the tributaries of the upper Tennessee River (IUCN Red List Account, Jones et al., 2010).
This got us thinking about the imperiled conservation status of the Nearctic freshwater mussel species in general (301 species) and the current state of the available information. Williams et al. (1993) produced an excellent assessment of North American mussel species, and that article is the source of the often-cited statistic that more than 70% of freshwater American freshwater mussels are endangered or declining. Nowadays, there are Internet initiatives to keep track of those data, such as NatureServe and the IUCN Red List, and we wondered about the progress of those efforts and any updates in conservation status that might have happened in the last 23 years. Since we have collaborated in various capacities with the IUCN (and because the NatureServe Explorer refused to load in a timely manner the day we were writing this), we decided to focus on the Red List.
This chart summarizes the species conservation assessments by Williams et al. (1993) and the IUCN Red List. The size of the circles is proportional to the number of species that fall into the ranks of each assessment, and the colors correspond to the IUCN status (and just class-up the whole thing). For all assessment ranks, the marginal totals of the number of species are provided, as are the cumulative percentages (beginning with the % extinct species and progressing through the rest of ranks). Circles above the diagonal reflect an escalation in conservation status (i.e., more threatened) from Williams et al. (1993) to the Red List, and those below indicate demotions.
Though the titles of the categories between the two assessments differ, their alignment is trivial. The Red List Threatened ranks are Vulnerable, Endangered, Critically Endangered, and Extinct, and these correspond to Threatened, Endangered, and Possibly Extinct. According to the Williams et al. (1993) assessments, 42% of the 301 freshwater mussel species in North America are in the Threatened ranks, whereas the Red List lists 36%. If the tally of species-to-be-worried-about is expanded to include Special Concern and Near Threatened, the Williams et al. and Red List percentages grow to 64% and 49%, respectively — both less than the previous summary statistic of >70%.
The Williams et al. percentage in 2016 (64%) is less than that of Williams et al. (1993) (>70%), and the Red List value is even lower (49%). This is easy to explain: 1) some species have improved in conservation status (e.g., 23 species have moved from Special Concern to Least Concern — Hooray!), and 2) the taxonomy has changed (summarized in Graf & Cummings, 2007 and updated here) so not all species have even been recently assessed. Seventy-eight species have not been evaluated for the Red List, and nine that have been assessed are labeled as Data Deficient (78 + 9 = 87; 29% of 301 species). The taxonomy has changed enough in the last two decades — even for the best-studied freshwater mussel assemblage in the world — that there are 33 currently-recognized species that were not evaluated back in 1993!
The upshot is that if we apply the current standard of dividing biodiversity into threatened, not/less threatened, and unknown species, then about 40% of the North American freshwater mussel species are threatened.
That nearly a third of the North American freshwater mussel assemblage is missing from the IUCN Red List might seem like a lot, but the dataset for the Nearctic region is the most complete in the world. Africa is the next most complete ((12 not evaluated + 16 data deficient) / 79 species = 35%). The most incomplete? Australasia ((2 + 3) / 30 = 77%) and the Neotropics ((6 + 14) / 217 = 89%).
Watch this web site for updates to conservation assessments as they become available.
Moncetia anceyi (Iridinidae?, Afrotropical)
The November 2016 Mussel of the Month is Moncetia anceyi. Moncetia is a monotypic genus endemic to Lake Tanganyika in Africa.
This month’s MotM is dedicated to the American undecided voter because Moncetia anceyi is the undecided freshwater mussel. Is it an iridinid? Is it a unionid? We can’t decide.
Traditionally, Moncetia has been classified in the family Iridinidae. But, even that arrangement was fuzzy. Haas (1969) treated M. anceyi in the genus Mutela, while Mandahl-Barth (1988) placed it under Spathopsis (= Chambardia).* The iridescent nacre is reminiscent of Mutela, but the shell outline and periostracum are more like Chambardia. We followed Daget (1998) in treating Moncetia as a monotypic genus in the Iridinidae (Graf & Cummings, 2007).
However, there is a paper by Kondo (1984) where he determined the host fish of M. anceyi in Lake Tanganyika, and he illustrated a glochidium. A what?! A glochidium. How can M. anceyi be an iridinid in the superfamily Etherioidea — a clade of mussels with lasidium-type larvae — if it has glochidia? (Graf & Cummings, 2006). At this point in the story, one can conclude that 1) someone has made a mistake or 2) we don’t have as good of a handle on unionoid evolution as we think we do (or both).
Fortunately, Takaki Kondo retained specimens and was generous enough to send us some for examination (in 2003). Before we had them in hand, we assumed the specimens would actually belong to one of the known unionids from Lake Tanganyika (e.g., Pseudospatha). Nope. The specimens we have match the types of Moncetia anceyi (and other synonyms) that we have seen in various collections.
But having the specimens has not improved our certainty. The largest female (19 mm) we have is shown in the figure to the right. In all our specimens, the posterior mantle (and adjacent adductor) was cut. It is difficult to make conclusions about whether there is mantle fusion at the diaphragm (like in iridinid) or a supra-anal aperture (like a unionid). We can see that this female is gravid, with the marsupium restricted to the inner demibranchs (iridinid), but there is a distinct gap between the anterior attachment of the demibranchs and the relatively small labial palps (unionid).
So, we are undecided about the taxonomic affinities of Moncetia anceyi because of its inconsistent anatomical positions. It’s a flip-flopper. Rather than classifying based on the lesser of two homoplasies, we are third-party punting for incertae sedis.
* If we wanted to, we could report on the opinions of Modell (1964) and Starobogatov (1970), but most discussions of freshwater mussel taxonomy are confusing enough without their jive — even when they happen to be correct.
Psorula rudis (Unionidae, Neotropical)
The October 2016 Mussel of the Month is Psorula rudis. Psorula is a genus of seven species from Central America.
There is almost nothing to say about Psorula. It has been listed as a valid genus of the family Unionidae in checklists (Frierson, 1927; Haas, 1969; Graf & Cummings, 2007) and web pages (like this one and ITIS). Internet search results tend to be a bit inflated since Psorula is also a valid genus name for a fungus.
We suspect that the richness of the genus has been inflated by splitting. The genus is distributed in southern Mexico and Guatemala, but the endemic hotspot of the currently recognized species is the Rio Usumacinta. The seven species of Psorula are represented by only 48 specimen lots in the collections we have toured, and 30 of those belong to two species. As with other Mesoamerican freshwater mussel genera (e.g., Reticulatus), we have needed a revision for almost a century.
Glebula rotundata (Unionidae, Nearctic)
The September 2016 Mussel of the Month is Glebula rotundata. Glebula is a monotypic genus from the southern United States.
Glebula belongs to a small group of monotypic genera that are largely restricted to the Gulf Coastal drainages of the United States (Graf & Cummings, 2007) — the others being Elliptoideus and Plectomerus. There are thirteen total monotypic genera in North America, 9 mostly in the in the interior basin (Dromus, Ellipsaria, Lemiox, Simpsonaias, Pegias, Obliquaria, Hemistena, Cyclonaias, Tritogonia) and Gonidea in the Pacific drainages. Cyclonaias may need to be sunk into Amphinaias, but the rest of these seem pretty taxonomically stable.
In some cases, we have been skeptical of monotypic genera. For example, the monotypic genus Arkansia was sister to the monotypic Arcidens. But why? In a case like that there is no distinction between the traits of the species and the traits of the genus. It leads to redundant taxonomy.
Glebula is recognized largely based on shell characters, though its soft anatomy is like other lampsilines (Simpson, 1914). Phylogenetic analyses have placed G. rotundata as sister to Cyrtonaias (Chapman et al., 2008; Pfeiffer et al., 2016).
Callonaia duprei (Hyriidae, Neotropical)
The August 2016 Mussel of the Month is Callonaia duprei. Callonaia is a monotypic genus endemic to the Amazon River basin of Brazil.
The Neotropical freshwater mussel fauna played an important role in the evolution of the current taxonomy of the order Unionoida, especially the family Hyriidae. That family was at the intersection of a taxonomic argument that spanned almost a century regarding the higher classification of freshwater mussels — What are the relationships among the families? Or, to put it another way, how do we arrange the families into superfamilies?
In the pre-cladistic era, Ortmann (1921; the last genius to work on mussel systematics) placed the Hyriidae in a superfamily with the Iridinidae and Mycetopodidae* based on shared characteristics of their adult anatomy. For example, all of these mussels brood their larvae in the inner demibranchs of their ctenidia (= gills), have their posterior mantle margins fused between the incurrent and excurrent apertures, and have a wide separation between the anterior attachment of the inner demibranchs and the labial palps. Species of the Unionidae and Margaritiferidae brood in the outer pair of demibranchs or all four (or rarely only the inner pair), the posterior mantle lobes are never fused, and the inner demibranchs attach adjacent to the labial palps.†
In contrast, Parodiz & Bonetto (1963) grouped the Hyriidae with the Unionidae and Margaritiferidae based on larval characteristics: All three families have glochidia while the Iridinidae, Mycetopodidae, and Etheridae all have lasidia instead. Both glochidia and lasidia are parasitic larval stages, although they have strikingly different morphologies.†
For those of us working in the era of modern systematic biology, these pre-cladistic, authoritarian classifications have value as hypotheses to be tested and as inspiration for the kinds of characteristics we should be looking at. Among the earliest relevant analyses, Graf’s (2000) phylogenetic analysis of mussel morphology (adult and larval) favored Ortmann’s system, Bogan & Hoeh’s (2000) study applying nucleotide characters supported a novel set of relationships with the Hyriidae sister to the five other families, and little support was found for Parodiz & Bonetto’s (1963) classification.‡
Skip to 2016, and there are considerably more data on the higher classification of the Unionoida and the phylogenetic position of the Hyriidae. The most comprehensive treatment to-date was published last year by Graf et al. (2015), finding strong support for the monophyly of the Hyriidae and pretty good (but not great) support for Hyriidae as sister to all the other families. Nowadays, we recognize three superfamilies of freshwater mussels:
- Hyrioidea = Hyriidae,
- Unionoidea = Unionidae + Margaritiferidae, and
- Etherioidea = Iridinidae + Etheriidae + Mycetopodidae.
This stroll down classification-history lane was prompted by the recent article by Santos-Neto et al. (2016). Whereas Graf et al. (2015) focused on the Australasian hyriid lineages, this newer paper has the broadest taxon sampling yet from South America, including Callonaia.
* Actually, Ortmann classified what we now call the Hyriidae, Iridinidae, and Mycetopodidae into a single family with two subfamilies: Mutelidae (= Etherioidea), with Hyriinae (= Hyriidae) and Mutelinae (= Iridinidae + Mycetopodidae). At the time, etheriids weren't really a part of his world-view of freshwater mussels.
† See Graf & Cummings (2006) for a detailed explanation of these technical mussel characteristics.
‡ When DLG presented his results at the 1999 meeting of the American Malacological Society in Pittsburgh, Juan Parodiz monopolized the entire question period with a vigorous argument in favor of the Bonetto & Parodiz classification.
Pressidens moellendorffi (Unionidae, Indotropical)
The July 2016 Mussel of the Monthis Pressidens moellendorffi. Pressidens is a genus of three species from the islands of Borneo and Palawan (The Philippines).
Pressidens is a genus about which we know very little. Google "Pressidens moellendorffi," and you will be asked if you mean "President Moellendorffi." Then, you will be linked to this this web site. If you try it with Google Scholar, you will find our global checklist (Graf & Cummings, 2007). All that anyone has really said about these freshwater mussels is that they exist, and there are some specimens to back that up.
There is still a lot of work left to do to fill in the details of freshwater mussel global diversity!
Pilsbryoconcha exilis (Unionidae, Indotropical)
The June 2016 Mussel of the Month is Pilsbryoconcha exilis. Pilsbryoconcha is a genus of five species from Indochina and the Sunda Islands of southeast Asia.
Pilsbryoconcha is closely related to Pseudodon (sister, fide Pfeiffer & Graf, 2015) in SE Asia. Aquarists apparently know P. exilis as the tropical swan mussel or the Thai aquaria mussel. It seems to be common enough in the business that there is a YouTube video documenting the activity of the mussel in captivity.
Pilsbryoconcha is well represented in mollusk collections, and we have accumulated more than 200 specimen records in the MUSSEL Project Database, served from this site. In fact, the link below for Pilsbryoconcha exilis will take you directly to our specimen data, including a distribution map and pictures of the individual museum lots of that species. From that page, you can browse to the other species in the genus (as well as all the species in all the freshwater mussel genera). For each species, we provide both specimen records and taxonomic information.
The other link below to Pilsbryoconcha will take you to the ITIS.gov page for the genus. That website mirrors the current version of our database. These links are provided for each Mussel of the Month.
We hope that you can make good use of the MUSSELpdb. Watch this web site for upcoming updates on Pilsbryoconcha and the other genera of SE Asian freshwater mussels.
Iheringella semisulcata (Mycetopodidae, Neotropical)
The May 2016 Mussel of the Month is Iheringella semisulcata. Iheringella is a genus of two poorly known species in South America.
Google Iheringella semisulcata and you will receive links to this web site, as well as to other online databases, that confirm that the species exists — or, more accurately, that confirm that the name exists and has been used for a species of freshwater mussel. There are also some links to some key works like Simpson (1900, 1914), Haas (1969), and Graf & Cummings (2007).
That Google search only turned up 5 pages of links, and we are responsible for almost the entirety of the first page! Conspicuously absent from that list is a link to Pereira et al. (2014). That paper is a recent comprehensive checklist of the freshwater mussels of South America. The word 'Iheringella' does not occur in their article. Did they omit it? Or has it been synonymized? The genus is so poorly known and so infrequently mentioned, it is easy to see how it could slip through the cracks. Until we know for sure, we will keep it on our list.
Reticulatus reticulatus (Unionidae, Neotropical)
The April 2016 Mussel of the Month is Reticulatus reticulatus. Reticulatus is a genus of two species from Central America.
It is the home-stretch toward the end of the semester, and we need to keep the Mussel of the Month short. Regular readers of this monthly feature will know that in situations like this we rely on Central American species — the less we know, the less there is to write! The last time we offered up a data-poor Mesoamerican species was last November with Nephritica.
Our knowledge of Reticulatus reticulatus is limited to the possible fact of its existence. Simpson (1900) described it as a species of Nephronaias based on a single specimen from Honduras (the one in the picture above). Frierson (1927) sequestered the species into its own subgenus, Nephronaias (Reticulatus), and then other authors occasionally mentioned the species (Thiele, 1934; Haas, 1969). We (Graf & Cummings, 2007) elevated Reticulatus to a full genus. R. reticulatus is classified in the Tribe Pleurobemini based on tradition since that is where Nephronaias still resides, be it would not surprise us in the least to see it move following more/some/any study.
In addition to the type, our extensive tour of collections has only turned up 4 more lots in three North American collections.
Hyridella glenelgensis (Hyriidae, Australasia)
The March 2016 Mussel of the Month is Hyridella glenelgensis. Hyridella is a genus of 7 species found in Australia, New Guinea and the Solomon Islands.
Back in 2009, we first shared Hyridella through this venue. We are bringing that genus back with H. glenelgensis to honor our colleague, the late Dr. Keith Walker of the University of Adelaide, South Australia. The following was offered by our friend and collaborator, Dr. Hugh Jones of Sydney, Australia
The Glenelg mussel is unusual among the Australian Hyriidae in that most of the surface of the adult shell is strongly sculptured with v-shaped ridges extending outwards from the umbos. It is restricted to a few, small streams in the lower Glenelg River catchment in the southern edge of Australia and is federally listed as critically endangered. The decline in H. glenelgensis is thought to be linked to increasing salinization of rivers resulting from land use change, indirect effects of forestry and the direct impacts of domestic livestock access to rivers. Genetic studies are currently underway to resolve the taxonomic status of H. glenelgensis with respect to its congener, H. narracanensis.
Dr. Keith Walker was a university professor at the University of Adelaide since 1975, focussed on improving understanding the ecology of the River Murray, especially the impact of river regulation. Keith had a major interest in freshwater molluscs and made a valuable contribution to knowledge of the two dominant freshwater mussels in the Murray River, Alathyria jacksoni and Velesunio ambiguus. He also observed the decline in gastropods in the river, notably the viviparid Notopala hanleyi. I recall Keith pointing out the many shells of this “extinct” mollusc littering the banks and floodplain of the lower Murray. Then one day, an irrigator arrived in his office with a box full of shells of this species, claiming that they were clogging his irrigation pipes and wanting to know how he could get rid of them! This sparked several years of research on this species resulting in its listing on federal and state threatened species legislation. Dr. Walker was also instrumental in the rediscovery and federal listing of Hyridella glenelgensis as critically endangered and undertook basic research into the biology of this species. He was currently contributing to research into its species status.
Etheria elliptica (Etheriidae, Afrotropical)
The February 2016 Mussel of the Month is Etheria elliptica. Etheria is regarded as a monotypic genus from Africa and Madagascar, but recent work is changing that view.
Etheria elliptica was first given the spotlight as Mussel of the Month way back in September 2003 — it was the 2nd freshwater mussel so honored by us. But, 2003 was forever ago and pre-dates YouTube.com, Facebook.com, and Graf & Cummings (2006, 2007), for crying out loud! Moreover, the early online edition of a forthcoming paper by Elderkin et al. (2016) highlights the need to re-examine this species. E. elliptica also exhibits such highly divergent morphologies that it certainly merits two pictures on this list.
Etheria elliptica is the sole Afrotropical species of the Etheriidae, the cementing freshwater oysters. It has been hypothesized that it is the only species of the Etheriidae and that the other oyster-types are the result of convergent evolution, but we have beat that topic to death under previous monthly mussel entries (Acostaea rivolii, Pseudomulleria dalyi, Bartlettia stefanensis), as well as in print (Graf & Cummings, 2009, 2010). Classification aside, E. elliptica is a very oyster-like freshwater mussel. Though dimyarian,* the foot is highly reduced, reflecting the shift from the mostly sedentary life-style of most freshwater mussels to the completely sessile habit of oysters cemented to their substrate.
Etheria elliptica (as currently classified) has the broadest geographical range of any Afrotropical freshwater mussel species, reported from 38 freshwater ecoregions in both Africa and Madagascar (Graf & Cummings, 2011).** Given such a wide distribution and its high degree of shell polymorphism (i.e, compare the specimen figured here with the one from our previous E. elliptica entry), it would be fair to assume that the current taxonomy of E. elliptica is “lumped” — that is, the taxonomy recognizes fewer species than actually exist in nature. Recently there are more data that suggest we might need to do some “splitting.”
Bauer (2013) hypothesized two distinct but sympatric populations of E. elliptica in the upper Blue Nile based on differences in brooding pattern, and now Elderkin et al. (2013) have found at least three cryptic species in the Congo Basin using phylogenetic methods. What about West Africa? If the Etheria in the Niger, Volta or other rivers also harbors reproductively and/or genetically distinct lineages, there may be multiple species carving up the broad distribution of the genus! This could have implications for the prioritization of conservation efforts since species with smaller ranges face higher risks of extinction.
We were there with Dr. Elderkin in Zambia in 2008 when he sampled some of the specimens that he and his colleagues used for this study, and we are glad that this work is now available. Congratulations to Curt et al.!
* Marine oysters are monomyarian — they have only a single adductor muscle rather than two like most bivalves. The other freshwater oysters, Acostaea and Pseudomulleria, are monomyarian.
** The next-most widespread mussel is Chambardia wahlbergi from 25 ecoregions, though absent from Madagascar.***
*** The IUCN Red List reported that E. elliptica may have been extirpated from Madagascar, but see Graf & Cummings (2009) or click here for a review of the evidence that it ever actually inhabited that island.
Cyclonaias tuberculata (Unionidae, Nearctic)
The January 2016 Mussel of the Month is Cyclonaias tuberculata. Cyclonaias is a monotypic genus, widespread in the Interior and Great Lakes basins of eastern North America.
Cyclonaias tuberculata is a rather distinctive mussel species for its combination of warty sculpture and colored nacre. It’s a good-looking shell. However, classification of this species has been trouble from the get-go, and it is likely that during the next round of generic revisions, Cyclonaias will be replaced with Amphinaias or maybe Rotundaria. It is kind of a long story, so get a beverage and make yourself comfortable.
Going into the 20th century, this freshwater mussel went by the name Rotundaria tuberculata. Rafinesque (1820)* described Obliquaria (Rotundaria) with three included species: O. tuberculata, O. subrotunda (= Obovaria subrotunda), and O. pusilla (= Fusconaia ebena?). O. tuberculata was listed first, and, by the nomenclatural standard applied by Simpson (1900) and others, that was good enough to regard it as the type species — though it was never explicitly designated by Rafinesque.
However, Ortmann & Walker (1922) discovered that Herrmannsen (1848) had been explicit in listing O. subrotunda as the type of Rotundaria, and they accepted that as a subsequent designation of the type species. Thus, Rotundaria became a junior synonym of Obovaria (also named by Rafinesque), and Ortmann & Walker (1922) did their best to resolve the problem by taking the name Cyclonaias from their friend Henry Pilsbry. Since 1922, the name has been Cyclonaias tuberculata.
A new wrinkle with the nomenclature of C. tuberculata came from Campbell & Lydeard (2012) when they reported that Valenciennes (1827) had selected O. tuberculata as the type of Rotundaria. However, that is not exactly what Valenciennes said. Here is the full quote referred to by Campbell & Lydeard (they only quoted the bold-faced part):
“Nous avons reçu au cabinet du Roi une autre Mulette remarquable par sa forme arrondie, et par les nombreux tubercules dont sa surface est couverte. M. Rafinesque l'a envoyée comme le type d'un nouveau genre qu'il nomme Rotundaria. Les dents de la charnière sont disposées exactement comme celles des Mulettes. Ainsi nous croyons devoir laisser cette coquille dans le genre Unio; et, en donnant la description et la figure de cette espèce remarquable, nous aurons encore occasion d'en séparer deux qui ont été confondues en une seule: elles viennent de l'Ohio. Je laisserai à l'une d'elles le nom sous lequel M. Rafinesque nous a envoyé les deux espèces, et je la caractériserai par la diagnose suivante.”
Valenciennes received more than one shell from Rafinesque belonging to two different species. One he did refer to as Unio tuberculata (not subscribing to Rafinesque’s genus), but the other he named U. verrucosa non Barnes (= Amphinaias pustulosa). Notice that Valenciennes did NOT explicitly say “tuberculata is the type of Rotundaria.” The whole sentence in the bold-faced part of the quote basically reported that Rafinesque sent him shells that Rafinesque called the type of Rotundaria. But, Valenciennes referred to that “type” as two different species. Moreover, Valenciennes listed his new species first, and that was the one that he figured. This sort of fuzziness is not the kind of data on which to base nomenclature, but we concede that while we haven’t seen the fire yet, there is smoke.
After this long tale, you may be thinking, “Who cares?” (You may have screamed it out loud before clicking back to a web page in which you are interested.) Why would anyone get so cranked up about the unconfusing nomenclature of one species? The vexing issue is that phylogenetic studies have nested C. tuberculata in a clade with species of Amphinaias Crosse & Fischer, 1894 (e.g., A. pustulosa) (Boyer et al., 2011; Campbell & Lydeard, 2012). In this taxonomic twilight zone, Cyclonaias tuberculata should be Amphinaias tuberculata, but if it is Rotundaria tuberculata then all species currently classified as Amphinaias should also be transferred to Rotundaria! Or they are all just Quadrula (also named by Rafinesque)... **
It is a crazy world. Someone should sell tickets.
* From the mention of that name, you know this story is going to be more complicated than it is worth.
** We have discussed the lumping/splitting of Quadrula with previous Mussels of the Month Quadrula quadrula and Theliderma cylindrica.