Alluring larvae: Competition to attract fish drives species diversity
among freshwater mussels

Date:
November 16, 2021
Source:
University of Michigan
Summary:
North America's freshwater mussels are both impressively diverse
and highly imperiled. Nearly 300 species occur in the United States
and Canada, and up to 40 species of the hard-shelled bottom dwellers
can be found on a single stretch of a clean, swiftly flowing river.



FULL STORY ========================================================================== North America's freshwater mussels are both impressively diverse and
highly imperiled. Nearly 300 species occur in the United States and
Canada, and up to 40 species of the hard-shelled bottom dwellers can be
found on a single stretch of a clean, swiftly flowing river.


========================================================================== Freshwater mussels are also a paradoxical group: How can so many species persist side by side while feeding on the same foods -- sediments,
plankton and other particles in the water column? Elsewhere in nature,
a few species would typically gain an advantage over the others and
would eventually outcompete them.

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

Female freshwater mussels in the tribe Lampsilini have evolved a
mind-bending array of lures that use mimicry to attract nearby fish
and to "infect" them with mussel larvae, which then metamorphose on the
fish's gills or fins before dropping to the river bottom, weeks later,
as juvenile mussels. In biology, a tribe is a taxonomic rank above genus
but below family and subfamily.

The larvae of each lampsiline mussel species must infect a specific
type of freshwater fish -- bass, sunfish, darters, sculpins or drum,
for example -- to complete their development. In the language of ecology,
the mussel larvae are obligate parasites, and the fish that shelter and disperse them are their hosts.

It is this competition for host fishes that best explains the coexistence
of multiple lampsiline species, according to the authors of a paper
published online Nov. 16 in the journal PeerJ.



==========================================================================
The high diversity of available fishes and the ability of mussel species
to target specific fish hosts has led, over time, to specialized
host-infection behaviors and the evolution of new species through a
process called adaptive radiation, according to the U-M researchers.

"This study provides an exciting new paradigm for how much of North
America's rich, but highly endangered, freshwater mussel fauna has
evolved, and it highlights the cryptic role their remarkably complicated
and interesting larval life histories has played in that process,"
said study senior author Diarmaid O'Foighil, a professor in the U-M
Department of Ecology and Evolutionary Biology and a curator at the
Museum of Zoology. The study's lead author is U- M doctoral candidate
Trevor Hewitt, who did the work for his EEB dissertation.

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

Adaptive radiations are widespread in nature and often involve a
population of a given species that moves into new habitats. Individuals
stake out specific roles, or niches, in those habitats and gradually
become different from the parent species and from each other.

Famous examples include Darwin's finches, Hawaiian silversword plants,
the cichlid fishes of the East African Great Lakes and South American
rivers, and Caribbean anole lizards.



==========================================================================
But in all those 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 an adaptive radiation
centered on the ecological niches of an organism's larval stage, according
to Hewitt.

"To our knowledge, this is also the first time that this prominent model
of speciation -- adaptive radiation -- has been proposed for a freshwater mussel lineage," Hewitt said. "In addition, we constructed the first
genomic lampsiline phylogeny in order to place the diversity of host
use, and host infection strategies, into a robust evolutionary context."
The study involved collecting tissue samples from 54 lampsiline mussel
species in the field (using a nonlethal biopsy technique) and from
specimens in museum collections. Genomic DNA was extracted from the
tissue samples, then sequenced.

The genetic information was then used to construct a lampsiline mussel
family tree, which biologists call a phylogeny.

Lampsiline mussels were picked for the study because of their high species diversity, the availability of extensive background information about
host-fish specificity and -- most importantly -- because lampsilines
are predominantly specialist parasites.

The information about modern-day lampsilines, when combined with insights
about evolutionary relationships preserved in their DNA, enabled the researchers to test the prediction that the mussels' fish-targeting preferences, if maintained over long timescales, would lead to adaptive radiations centered on their larval stage and characterized by the
evolution of specialized host-infection behaviors.

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

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

Some females wave a fleshy appendage called a mantle lure that resembles
a minnow, a crayfish, a worm or some other small creature that a larger
fish might want to devour. When the predator fish takes a bite, it gets
a mouthful of parasitic mussel larvae instead of an easy meal.

Other lampsiline mussels rely on a brood lure: Pregnant females produce
a gelatinous, larvae-filled mass shaped like an insect, a small fish or
other prey item, then release it to drift freely in the water. And others reproduce without lures and simply release their larvae into the water,
a method called broadcast release.

The new study found evidence for the early evolution of mantle lures
in the lampsiline mussels, with brood lures and broadcast infection
strategies arising independently. The most common strategy was a mixed
one in which mantle lures are the predominant infection strategy, but
pregnant females also release simple, nonmimetic brood lures at the end
of the season.

Nearly 70% of U.S. freshwater mussel species are in serious trouble, due
to multiple factors that include water pollution, the damming of rivers
and competition from nonnative species. Parasitic larval development makes these mussels even more vulnerable, because their continued existence
is tied to the health of the host fish.

U.S. wildlife officials recently declared that 22 animals and one plant
species are 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 lampsilines.

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

========================================================================== Story Source: Materials provided by University_of_Michigan. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Trevor L. Hewitt, Amanda E. Haponski, Diarmaid
O'. Foighil. Evolution of
diverse host infection mechanisms delineates an adaptive radiation
of lampsiline freshwater mussels centered on their larval
ecology. PeerJ, 2021; 9: e12287 DOI: 10.7717/peerj.12287 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211116103156.htm

--- up 5 days, 2 hours, 55 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)