Competition to attract fish boosts species diversity among freshwater mussels – ScienceDaily

The freshwater mussels of North America are both impressively diverse and highly endangered. Almost 300 species are found in the United States and Canada, and up to 40 species of hard-shell bottom dwellers can be found on a single stretch of a clean, swift river.

Freshwater mussels are also a paradoxical group: how can so many species persist side by side while feeding on the same foods – sediment, plankton, and other particles in the water column? Elsewhere in the wild, a few species would generally gain an advantage over others and eventually supplant them.

New genetic analysis of North American freshwater mussels by University of Michigan biologists concludes that a key to understanding the coexistence of many species of freshwater mussels lies in the distinctive ecology of their microscopic larvae short-lived, rather than in long-lived adults.

The female freshwater mussels of the Lampsilini tribe have developed a mind-boggling array of lures that use mimicry to attract nearby fish and “infect” them with mussel larvae, which then metamorphose onto the gills or fins. fish before dropping to the bottom of the river weeks later in the form of juvenile mussels. In biology, a tribe is a taxonomic rank higher than the genus but lower than the family and the subfamily.

The larvae of each species of lampiline mussel must infect a specific type of freshwater fish – bass, crappie, dart, sculpin or drum, for example – to complete their development. In the language of ecology, mussel larvae are obligate parasites, and the fish that shelter and disperse them are their hosts.

It is this competition for host fish that best explains the coexistence of several species of lampillin, according to the authors of an article published online Nov. 16 in the journal PeerJ.

The great diversity of fish available and the ability of mussel species to target specific host fish has led, over time, to infectious behaviors of specialized hosts and the evolution of new species through a process called radiation. adaptive, according to researchers at UM.

“This study provides an exciting new paradigm for the evolution of the rich but highly endangered freshwater mussel fauna of North America, and it highlights the cryptic role that their remarkably complicated and interesting larval life histories have. played in this process, “said the study manager. author Diarmaid O’Foighil, professor in the department of ecology and evolutionary biology at UM and curator at the Museum of Zoology. The main author of the study is UM doctoral student Trevor Hewitt, who carried out the work of his EEB thesis.

Adaptive radiation is a burst of evolution that takes advantage of an ecological opportunity to create several new species from a single parent species.

Adaptive radiation is widespread in nature and often involves a population of a given species moving to new habitats. Individuals define specific roles, or niches, in these habitats and gradually become different from the parent species and from each other.

Famous examples include Darwin’s finches, Hawaiian silver sword plants, cichlid fish from the Great Lakes of East Africa and rivers of South America, and anole lizards from the Caribbean.

But in all of these cases, the focus is on adult organisms and their adaptations to different habitats and new ways of life. The freshwater mussel study is one of the first to identify adaptive radiation focused on ecological niches in an organism’s larval stage, according to Hewitt.

“To our knowledge, this is also the first time that this important speciation model – adaptive radiation – has been proposed for a line of freshwater mussels,” Hewitt said. “In addition, we have constructed the first genomic lampshade phylogeny in order to place the diversity of host use and host infection strategies in a robust evolutionary context. “

The study involved collecting tissue samples from 54 species of lampiline mussels in the field (using a non-lethal biopsy technique) and specimens from museum collections. Genomic DNA was extracted from tissue samples and then sequenced. The genetic information was then used to build a family tree for the lampiline mussel, which biologists call a phylogeny.

Lampmussel mussels were selected for the study because of their great diversity of species, the availability of detailed background information on host-fish specificity and, more importantly, because Lampmussels are primarily specialized parasites. .

Information on modern lampillines, combined with information on evolutionary relationships preserved in their DNA, allowed researchers to test the prediction that the targeting preferences of mussel fish, if maintained over long periods of time, would lead to adaptive radiation centered on their larval stage and characterized by the evolution of specialized host infection behaviors.

The results supported this hypothesis and were consistent with the idea that Lampmussel mussels can legitimately be considered as an example of adaptive radiation.

In the wild, gravid female lampiline mussels use two main types of lures to attract fish: mantle lures and brood lures.

Some females wiggle a fleshy appendage called a mantle decoy that looks like a minnow, crayfish, worm, or other small creature that a larger fish might want to devour. When the predatory fish bites, it receives a mouthful of parasitic mussel larvae instead of an easy meal.

Other lampiline mussels depend on a brood lure: pregnant females produce a gelatinous mass filled with larvae shaped like insects, small fish or other prey, then release it to drift freely into the water. And others breed without decoys and simply release their larvae into the water, a method called on-the-fly release.

The new study found evidence for the early evolution of mantle decoys in lampiline mussels, with brood decoys and on-the-fly infection strategies occurring independently. The most common strategy was a mixed strategy in which mantle decoys are the predominant infection strategy, but gravid females also release simple, non-mimetic brood decoys at the end of the season.

Almost 70% of freshwater mussel species in the United States are in serious trouble, due to multiple factors, including water pollution, the construction of dams on rivers, and competition from non-native species. . Parasitic larval development makes these mussels even more vulnerable, as their continued existence is linked to the health of the host fish.

U.S. wildlife officials recently said 22 animals and one plant species were likely extinct and should be removed from the endangered species list. Eight of the 23 species are freshwater mussels and six of the eight are lampillines.

The other author of the PeerJ study is Amanda Haponski from the Department of Ecology and Evolutionary Biology at UM. Funding for the study was provided by UM’s Rackham Graduate School, UM’s Dr. Nancy Williams Walls Award for Field Research, Department of Ecology and Evolutionary Biology, and the John B. Burch Malacology Fund of the UM Museum of Zoology.

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