The July 2026 Mussel of the Month is Iridea granosa
A new genus in the Family Hyriidae, but the hyriid phylogeny still has some fuzzy edges
Two open questions in freshwater mussel taxonomy are (1) the inter-generic relationships within the family Hyriidae and (2) the sister group of the Hyriidae. These are real opportunities for scientific fame and glory!
Iridea Prié & Lopes-Lima in Prié et al., 2026 is a newly described genus for one species of freshwater mussel in the family Hyriidae: Mussel of the Month Iridea granosa (Bruguière, 1792). I. granosa is one of about 65 currently recognize species of hyriids from South America, and there are another 29 species from Australia, New Guinea, and New Zealand. That biogeographical disjunction has complicated the articulation of a non-disjunct story of hyriid evolution.
The MUSSEL Project Database (MUSSELpdb) is a good place to explore the diversity of the Hyriidae generally and get data on the taxonomy, species distributions, phylogeny, and literature. Many links in this article connect to the database.
Across the board, the phylogenetic data robustly support Hyriidae monophyly. The Hyriidae is a clade — a complete branch, a thing — easily recognizable by both the genetics of its species and their anatomical characteristics (a lot more on that below). On the other hand, the prevailing classification for the included subfamilies and tribes is a house of cards: a pragmatic consensus of traditional, pre-cladistic arrangements (Parodiz & Bonetto, 1963 for the Neotropics; McMichael & Hiscock, 1958 for Australasia) and limited phylogenetic work biased toward the Australian taxa. The phylogenies summarized on the Hyriidae Cladomics page of the MUSSELpdb are wildly inconsistent. We will save that intra-hyriid jibba jabba (e.g., Alathyria) for another article.
The problem we’ll consider here — not fix but at least recognize — is the phylogenetic position of the family Hyriidae among the other five families of freshwater mussels. What taxa constitute the sister branch to the clade of hyriids? The challenge continues to be reconciling competing phylogenetic signals.
Hyriids have glochidium-type larvae like unionids.
At the dawn of the Phylogenetic Era (circa 2000), the dominant view had been that the Hyriidae should be classified with the Unionidae and Margaritiferidae because all 3 families have glochidia: minute bivalved larvae that are parasitic on fishes. The other 3 families of freshwater mussels (Iridinidae, Mycetopodidae, and Etheriidae) have lasidium-type larvae — also parasitic, but without the calcified shells of glochidia. The figure below contrasts these larval morphologies. More details are available in Graf & Cummings (2006), and additional life cycle background can be found in previous posts.
Parodiz & Bonetto (1963) grouped the glochidium-bearing families (including the Hyriidae) into the superfamily “Unionacea,” and they classified the three families with lasidia as the “Mutelacea.” Those names are modernized to the Unionoidea and the Etherioidea, respectively. Other taxonomies treated the hyriids as a mere subfamily of the Unionidae (e.g., Haas, 1969; Vaught, 1989).
Besides similar larval morphologies, hyriids have other traits in common with the Unionidae — such as sculptured shells with robust hinge teeth — that are rare among the freshwater mussels with lasidium-type larvae.
Hyriids have soft anatomy like the lasidium-bearing mussels.
The arrangement argued by Parodiz & Bonetto (1963) was a departure from the previous system of Ortmann (1921). That should be a red flag. Ortmann was the last genius to work on freshwater mussels.
Ortmann placed Mussel of the Month Iridea granosa and the other hyriids as one subfamily within the family “Mutelidae.” The other subfamily, “Mutelinae,” comprised the modern Iridinidae and Mycetopodidae. It is trivial to reconcile Ortmann’s three-family scheme with Parodiz & Bonneto’s two superfamilies, except for the affinities of the Hyriidae.
In Ortmann’s view, the widespread and diverse Unionidae could be diagnosed by at least 4 soft anatomical characters:
glochidium-type larvae brooded in the outer pair of demibranchs of their gills (or in both the outer and inner pairs),
anterior attachments of the gills to the visceral mass are distant from the labial palps,
no posterior mantle fusion between the incurrent and excurrent apertures, and
a short mantle fusion dorsal to the excurrent aperture creating a third opening to the mantle cavity: the supra-anal aperture.
Graf & Cummings (2006) provided detailed descriptions and illustrations of these traits. I have gotten jiggy with posterior mantle anatomy — “clam butts” — in previous posts.
Margaritiferids share the first 3 of those characters, and that similarity undergirds the longstanding agreement that the Margaritiferidae and Unionidae should be classified together.
However, the Hyriidae and the lasidium-bearing mussels differ on all 4 points:
larvae are brooded only in the inner demibranchs,
gills attach to the visceral mass adjacent to the labial palps,
mantle fusion between the posterior apertures, and
the mantle fusion dorsal to the excurrent aperture remains closed — i.e., it doesn’t reopen to create a supra-anal aperture.
Ortmann knew of the differences in larvae, and he was well aware that basing family-level classifications on shell similarities was asking for trouble. Shells are notorious liars.
The Hyriidae is recovered as sister to lasidium-bearing mussels in cladistic analysis of morphological characters.
As a test of these alternative arrangements, I coded morphological characters (more than just the ones we have talked here) to search for the most parsimonious tree topology and found that the Hyriidae shared a more recent common ancestor with the Iridinidae and Mycetopodidae than the Unionidae (Graf, 2000; Graf & Cummings, 2006). Cladistic analysis of the morphological data favored Ortmann’s scheme over than of Parodiz & Bonetto.
The traditional systems of Ortmann and Parodiz & Bonetto lacked a cladistic foundation. They based their implicitly evolutionary arrangements on general similarity, but they differed in which characters they favored. Phylogenetic classifications rooted in cladistic methods are explicitly evolutionary. Not only do we want our taxa to be monophyletic — clades composed of all of the descendants of a common ancestral species — but taxa should also be diagnosed by shared derived characters. Figuring out which character states are derived depends on which are ancestral. For that, we need to also consider other closely related non-unionoid bivalves — like Neotrigonia. This is best illustrated in the topology recovered by Graf & Cummings (2006).
The gist of a cladistic analysis is to discover the branching patterns that best explain the hierarchy of evolutionary transformations — that is, the trees that minimize the incongruence among the traits of the taxa. This analysis suggested that a clade of hyriids + lasidium-bearing mussels diagnosed by Ortmann’s characters [3, 4, & 5 in the tree above] is the more parsimonious explanation of freshwater mussel evolution and that lasidia [7] are derived from ancestral glochidia [1].
According to those morphological data, arranging the Hyriidae with the Unionidae rather than the lasidium-bearing mussels is analogous to grouping mice with lizards instead of bats because mice and lizards have four legs rather than wings. Legs are an ancestral character, and mice and bats share other derived traits like mammary glands and hair.
Molecular loci support different tree topologies.
That all sounds straightforward, but unfortunately, molecular phylogenetic analyses have not consistently agreed. Depending on the data (mitochondrial vs. nuclear DNA) and methods of tree optimization (parsimony, likelihood, or Bayesian), the Hyriidae is recovered as sister to the lasidium-bearing Etherioidea (aligning with Ortmann) but other times in a clade with the Unionidae and Margaritiferidae (a la Parodiz & Bonetto). The Hyriidae is also commonly placed as sister to a clade of the five other families. Instead of trying to decide if hyriids like Mussel of the Month Iridea granosa should be classified in the Unionoidea or in the Etherioidea, we have settled on giving the Hyriidae its own superfamily: Hyrioidea. The Hyrioidea cladomics page summarizes the alternative topologies to-date.
The thing is, phylogenies that don’t place the Hyriidae as sister to the lasidium-bearing Etherioidea — either basal to the other families or in a clade with the Unionidae and Margaritiferidae — up-end the interpretation of character evolution described above. For example, instead of mantle fusion between the posterior apertures being interpreted as derived within the freshwater mussels, that once-fancy trait becomes the ancestral condition for the whole order! It would follow, then, that the lack of mantle fusion dividing the in- and excurrent apertures in the Unionidae and Margaritiferidae represents a reversal — a loss of that ancestral trait.
The nature of posterior mussel mantle fusion (or lack thereof) of freshwater mussels is not the only evolutionary hypothesis left unresolved by the ambiguous position of the Hyriidae. Hopefully, as systematists continue to refine the taxonomy of Mussel of the Month Iridea granosa and other hyriids (and freshwater mussels, generally), they will adopt a cosmopolitan perspective and seek the necessary data to fill in the rest of the story. This is one of the most impactful problems facing freshwater mussel taxonomy! The MUSSEL Project is here to help.
References Cited
Graf, D.L. 2000. The Etherioidea revisited: a phylogenetic analysis of hyriid relationships (Mollusca: Bivalvia: Paleoheterodonta: Unionoida). Occasional Papers of the Museum of Zoology, University of Michigan (729): 1-21.
Graf, D.L. & K.S. Cummings. 2006. Palaeoheterodont diversity (Mollusca: Trigonioida + Unionoida): what we know and what we wish we knew about freshwater mussel evolution. Zoological Journal of the Linnean Society 148: 343-394.
Haas, F. 1969. Superfamily Unionacea. [in] R.C. Moore (ed.), Treatise on Invertebrate Paleontology Part N (1): N411-N470.
McMichael, D.F. & I.D. Hiscock. 1958. A monograph of the freshwater mussels (Mollusca: Pelecypoda) of the Australian region. Australian Journal of Marine and Freshwater Research 9(3): 372-508.
Ortmann, A.E. 1911. The anatomical structure of certain exotic naiades compared with that of the North American forms. Nautilus 24(9-11): 103-108, 114-120, 127-131.
Ortmann, A.E. 1921. South American Naiades; a contribution to the knowledge of the freshwater mussels of South America. Memoirs of the Carnegie Museum 8: 451-670.
Prié, V., S. Brosse, A. Valentini, B. Adam, M. Rhone et al. 2026. Through the prism of eDNA: exploring cryptic freshwater bivalves diversity in a mega-diverse tropical environment. Environmental DNA 8: e70319, 1-18.
Parodiz, J.J. & A.A. Bonetto. 1963. Taxonomy and zoogeographic relationships of the South American naiades (Pelecypoda: Unionacea and Mutelacea). Malacologia 1(2): 179-213.
Vaught, K.C. 1989. A Classification of the Living Mollusca. American Malacologists, Inc., Melbourne, Florida. 195 pp.