biology zoology insects pearl mussels

Pearl Mussels in Peril

Is there any help for pearl mussels?

We hear and read so much about plants and animal species being threatened by extinction, but not all species make headlines, like for instance the freshwater pearl mussel with the beautiful name Margaritifera margaritifera. Not only oysters and other marine molluscs produce pearls: the freshwater pearl mussel of the holarctic region on both sides of the Atlantic has for centuries yielded valuable wild pearls, too. But the times when these bog mussels were literally plastering the beds of fast flowing streams of areas poor in nutrients and calcium are nearly over – and that despite the fact that this animal can probably reach an age of 200 or even more years.

Scientists are witnessing the gradual disappearance of this magnificent member of the bivalve family and are seemingly unable to do anything. Although the freshwater pearl mussel can reach a far greater age than almost any other animal and a single individual may produce 200 million eggs in a lifetime, the path to adulthood is tortuous for the baby clam. Once a year females discharge their eggs from their gonads into their gills. Here the approximately 2-4 million eggs manage to get fertilized either by sperm from other individuals (acting as fathers) that the gills capture or, if other mussels are nowhere near, by semen from the mother, which under such conditions can turn into a hermaphrodite.

Fertilized eggs develop into 0.07 mm large (or better minuscule) glochidia-larvae, which the mother individual will release into the river water in late summer. In order to develop further, the larvae must be inhaled by a fish into its gill chamber. Here the lucky ones that get there, attach themselves with clasping-valves (which function like the jaws of a gin trap) to the gills of the fish and start living a parasitic life for a month or so. Glochidia which are not fortunate enough to find a host fish perish. However, fish is not fish and even if a glochidium larva has made it into a minnow, eel, bull-head or rainbow trout, it will still die within a few hours after attachment, for these fishes are able to reject the parasite. Only salmon (which is either absent or terribly rare nowadays from central European rivers) and brown trout will nurture the parasite. If, however, parasitized repeatedly, the immune system of even these hosts, can produce antibodies, eliminating them too as potential hosts.

To increase the chances of being taken up by a fish, another freshwater mussel by the name of Lampsilis sp. with a similar life cycle develops a fleshy fish-like growth protruding from its shell. Bigger host fish are tricked by the dummy and attracted to this lure of the gravid clam. Snapping at the dummy they end up getting a mouthful of glochidia instead of a small fish. Greed rarely pays. But what causes the present over-ageing of the freshwater pearl mussel populations is not just a shortage of suitable host fish for their larvae, there is also pollution and the fact that baby mussels require 10 years to reach sexual maturity and until then they are extremely susceptible to all kinds of pollution. And what about the pearl? The pearl, actually a calcareous concretion of concentric layers of calcium carbonate (CaCO3) around a tiny irritant can take many years to be formed, given the small amount of calcium in the mussel’s habitat. It’s not worth killing the last remaining methusalems of the freshwater pearl mussel for; cultured pearls from oysters are also iridescent and quite beautiful – and may even come in different colour varieties.

© Dr V.B. Meyer-Rochow and, 2018.
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