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Rocky Mountain Ridged Mussel (Gonidea Angulata)

Biology

General

Compared with some other North American freshwater mussels, relatively little is known about the biology of this taxon. This may be in part due to its rather isolated taxonomic position. Taylor (1988), for example, thought it a likely relative of the Korean genus described in Yoo and Habe (1962). Rosenberg et al. (1994) and Lydeard et al. (1996) have also emphasized its unique taxonomic position in North America. The species has not been artificially cultivated in the laboratory or raised for more than a short period of its life span.

 

Life cycle and reproduction

Freshwater mussels have a complex life cycle, which includes a short parasitic stage attached to a fish host. The life of a freshwater mussel can be partitioned into five distinct stages:

a larva (called glochidium) developing in the gill of a female mussel,

a short planktonic glochidial stage expelled from the female mussel,

a brief (2-3 weeks) to long (2-4 months) parasitic (commensalistic) glochidium attached to the gills or fins of a living host fish,

a free-living benthic juvenile, and

the benthic adult mussel.

Generally, reproduction occurs when the male releases sperm into the water column, which is siphoned into the female to fertilize the eggs (Figure 6). Reproduction may be triggered by increasing water temperatures and day length. Development and retention of larvae (0.1 – 0.4 mm in size) within the female may last from 1 to 4 months (i.e. tachytictic females) to 6-10 months (i.e. bradytictic females).

Figure 6.  Generalized life cycle of a freshwater mussel. Numbers refer to the five life stages listed above.

 

Figure 6.  Generalized life cycle of a freshwater mussel. Numbers refer to the five life stages listed above.

Timing for reproduction, gravidity, egg and sperm release, and larval development are not specifically known for G. angulata. Collections have been made of gravid individuals in April through July but not in August through October, implying a tachytictic habit. Glochidial morphology of this species is unknown. Fertilization in G. angulata has not been described; however, it is likely that fertilization of the eggs is internal, as described above, as female unionids receive sperm during the process of filter feeding, at which time glochidia that are being brooded in the marsupial compartment of the gills are fertilized. It is unlikely that female G. angulata eject their larva in bound mucilaginous tubes called conglutinates. If fertilization is external, crude counts suggest that gravid females may release in excess of 10,000 eggs (compared to most unionids which produce hundreds of thousands to millions of glochidia) into the water column. There is no evidence of self-fertilization in this taxon, as judged from the literature and specimens examined by one of the authors (TJF).

Glochidia are typically released from the female in spring and early summer (April to July). The larvae drift passively with the plankton until they contact a suitable host teleost. Timing is critical for these larvae, as they cannot survive for long outside the female or without a host fish. Some mussels may depend on only a single fish species as a host, whereas others can parasitize many different fish species. The attachment of glochidia usually does not cause problems for the host fish (Cunjak and McGladdery 1991). If glochidia find a host fish, they attach to the gills or fins and remain attached for one to six weeks (or more or less) while transforming into a juvenile mussel. As juveniles, they drop off the fish and begin a free-living benthic life.

Likely, gonadal development takes place from the late winter to late spring, with egg and sperm release in the early summer to early fall. Only a few mature, gravid specimens have been noted. These tended to be large forms perhaps 10 or more years old. It is likely that reproduction is annual for mature individuals, so that under ideal circumstances the species is a yearly iteroparous breeder. It is probable that this species is tachytictic; the subfamily Ableminae is characterized as having a short breeding season. Other life cycle patterns are also possible, though less likely (see Dillon 2000). Strong seasonality in most of the streams in which this taxon occurs makes it unlikely that reproduction occurs in more than one period per year. Hence, the species appears to be univoltine and iteroparous. See Bauer and Wächtler (2001), McMahon and Bogan (2001) and Dillon (2000) for information on reproductive strategies in this group.

 

Survival

Glochidial survival in freshwater mussels is typically low, ranging between 10 and 18,000 individuals per billion glochidia that survive to the 1-2 year stage in the few taxa for which estimates have been made (Jansen et al., 2001). Adults of this species seem to be comparatively short-lived, perhaps 20-30 years from growth ring counts, versus the 120 years or more for the sympatric Margaritifera falcata.

 

Physiology

Relatively little specific information is available for this taxon. Gonidea angulata seems to be one of a group of taxa perhaps less tolerant of siltation, nutrient enhancement, substrate shift, and of low flow régimes than are many eastern North American taxa. The taxon appears to be somewhat stenothermal, favouring cold-water oligotrophic habitats, and was so classed by Frest (1999) and Frest and Johannes (2001). However, it has shown some tolerance of increasing nutrient loads in the middle Snake River and elsewhere. In the Okanogan and Snake Rivers, it has survived decimation of native salmonid populations, which has been implicated in reducing the populations of the conspecific Margaritiopsis falcata. Dense populations, on the other hand, have been observed only in relatively pristine to moderately degraded streams.

Feeding may be limited under temperature extremes, during periods of relatively high sediment influx, or to avoid pollution or other habitat condition surges. The taxon commonly occurs with other cold-water stenothermal or stenotopic molluscs, such as Margaritifera falcata; Anodonta californiensis; Juga spp.; Fisherola or Lanx spp. and Fluminicola spp. All these taxa are regarded as sensitive molluscs by Frest and Johannes (1995b).

Vannote and Minshall (1982) reported that G. angulata was well adapted to aggrading rivers due to its well-formed distal inhalant and exhalant siphons and to its ability to bury most, if not all, of its shell in sediments without affecting feeding. Furthermore, G. angulata has a strongly angular, wedge-shaped shell, and the foot is commonly positioned at a right angle to the shell ridge providing for a very strong anchor.

 

Movements/dispersal

Unionaceans are dispersed primarily during the glochidial stage, first by currents and then by fish; hence, distribution and ecology are limited by current and fish host(s). Thus, all available habitat need not be occupied, and range may vary in size, depending upon the number and distribution potential of the fish host(s). The larval glochidia of G. angulata will disperse via fish movements (both upstream and downstream). Due in part to its relatively wide historical range, it is presumed that the host fish were (are) represented by more than one species and perhaps more than one family.

After leaving the fish host, the transformed glochidia, now called juveniles, typically drop to the substrate. The substrate on which they land and the water column in which the substrate occurs must be suitable for survival (i.e., low turbidity, suitable substrate for embedding, and the presence of other habitat attributes required for survival).

Repeat visits to some U.S. Okanogan sites seem to indicate little or no seasonal or life stage movement of post-glochidial forms within this taxon, even in a relatively shallow stream. Adult specimens in the Little Granite Reservoir showed little sign of movement even as the reservoir was dewatered, effectively dying in place. As with many large freshwater mussels, juveniles and young specimens seem more active than individuals older than 5 years, which appear to be effectively sessile unless disturbed. Displaced individuals have been observed to reorient and rebury themselves. The taxon does not appear to migrate seasonally or during the breeding season, either in river, lakes or streams though definitive data are lacking. It appears unable to readily colonize new habitats, such as borrow pits, highway lakes, or dredged river sections, unlike the regionally more common Anodonta oregonensis.

Densities of adult G. angulata may vary considerably depending on the suitability and availability of habitat. For example, as much as 10 m or more distance separates individual G. angulata (1/25 m2) in the Lower Granite Reservoir, Washington, and densities of approximately 16/m2 were observed in 1988 and 1991 in the Okanogan River, Washington (T.J. Frest, unpublished data). In the Salmon River Canyon, Idaho densities of G. angulata ranged from 5.5 – 183/ m2 depending on the composition of the substrate (Vannote and Minshall 1982). The density of G. angulata is surely correlated to the distribution and availability of suitable habitat and it is presumed that densities of mussels will be lower in areas with poorer quality habitat than in areas with better quality habitat.

 

Nutrition and interspecific interactions

Gonidea angulata is an obligate filter feeder, as are most unionids. Most unionaceans filter organic debris from the water column; some may directly absorb certain nutrients, especially in the younger stages. As a result of their feeding technique, the volume of water filtered by an adult G. angulata is significant, which leaves them especially vulnerable to dissolved pollutants, some of which are involuntarily sequestered. For details on unionid feeding, see McMahon and Bogan (2001) and Dillon (2000).

 

Behaviour/adaptability

No information is available.