Unionoida

Unionida is a monophyletic order of freshwater mussels, aquatic bivalve molluscs. The order includes most of the larger freshwater mussels, including the freshwater pearl mussels. The most common families are the Unionidae and the Margaritiferidae. All have in common a larval stage that is temporarily parasitic on fish, nacreous shells, high in organic matter, that may crack upon drying out, and siphons too short to permit the animal to live deeply buried in sediment. The shells of these mussels are variable in shape, but usually equivalve and elongate. They have solid, nacreous valves with a pearly interior, radial sculpture, and an entire pallial line. Families, genera, and species in the order Unionida are found on six continents, where they are restricted exclusively to freshwater rivers, streams, creeks and some lakes. There are approximately 900 species worldwide. Around 300 species of these freshwater mussels are endemic to North America.Unlike other bivalve orders, Unionida has no marine species, although one species (Glebula rotundata) tolerates brackish water. This widespread trait and its global distribution suggests the group has inhabited freshwater throughout its geologic history. Unionida burrow into the substrate in clean, fast flowing freshwater water rivers, streams and creeks, with their posterior margins exposed. They pump water through the incurrent aperture, obtaining oxygen and filtering food from the water column. Freshwater mussels are some of the longest-living invertebrates in existence. These clams have, like all bivalve mollusks, a shell consisting of two parts that are hinged together, which can be closed to protect the animal's soft body within. Like all mollusks, the freshwater mussels have a muscular 'foot', which enables the mussel to move slowly and bury itself within the bottom substrate of its freshwater habitat. Unionida have a unique and complex life cycle involving parasitic larvae. This larval form used to be described as 'parasitic worms' on the fish host, however, the larvae are not 'worms' and do not harm fish under normal circumstances. Most of these freshwater mussel species have separate sexes (although some species, such as Elliptio complanata, are known to be hermaphroditic). The sperm is ejected from the mantle cavity through the male's excurrent aperture and taken into the female's mantle cavity through the incurrent aperture. Fertilised eggs move from the gonads to the gills (marsupia) where they further ripen and metamorph into glochidia, the first larval stage. Mature glochidia are released by the female and then attach to the gills, fins or skin of a host fish. Typically, the freshwater mussel larvae (glochidia) have hooks, which enable the individual to attach itself to fish. Some freshwater mussels release their glochidia in mucilaginous packets called conglutinates. The conglutinate has a sticky filament that allows it to adhere to the substrate so it is not washed away. There is also an even more specialized way of dispersal known as a super-conglutinate. The super-conglutinate resembles an aquatic fly larva or a fish egg, complete with a dark area that looks like an eyespot, and it is appetizing to fish. When a fish consumes it, it breaks up, releasing the glochidia. Mussels that produce conglutinates and super-conglutinates are often gill parasites, the glochidia attaching to the fish gills to continue their development into juveniles. A cyst is quickly formed around the glochidia, and they stay on the fish for several weeks or months before they fall off as juvenile freshwater mussels which then bury themselves in the sediment. This unique life cycle allows Unionida freshwater mussels to move upstream with the fish host species. Many of these freshwater mussel species face conservation issues due to habitat degradation and in some cases due to over-exploitation for the freshwater pearl industry, and for the nacre of their shells, which was used in button manufacturing. Of the North American Unionida about 70% are either extinct (21 species), endangered (77 species), threatened (43 species) or are listed as species of special concern (72 species). These bivalve mollusks were heavily exploited for freshwater pearls, and for their nacre which was used in the button manufacturing industry in the late 19th and early 20th centuries. The effects of heavy fishing for freshwater mussels in North America in for use in manufacturing buttons put many of these species close to extinction. The 'pearl rush' in North America occurred in the mid to late 1800s as people could easily find freshwater mussels in rivers and streams by 'pollywogging' for mussels, some of which had freshwater pearls which they could sell for a significant price. The art of 'pollywogging' involves shuffling one's feet in the mud feeling around for freshwater mussels. Because this was relatively easy to do, and an easy way to make money from freshwater selling pearls, this period has been euphemistically called the 'pearl rush', and some historians have compared it to the gold rush in California. A formal freshwater mussel fishing industry was established in the mid-1850s to take advantage of this natural resource. The 'pearl rush' to find freshwater pearls became so intense in some rivers that literally millions of freshwater mussels were killed in a few years. In some rivers and streams entire freshwater mussel beds were completely eliminated. Although the negative impact of the 'pearl rush' on freshwater mussel populations was significant, in the cold light of history it was relatively minor compared to the over fishing that took place just a few years later with the 'pearl' button industry.