Mapleleaf mussel (Quadrula quadrula) COSEWIC assessment and status report: chapter 6

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Biology

General

Freshwater mussels in the family Unionidae have a complex life cycle that includes a larval stage called a glochdium (pl. glochidia) that is an obligate parasite, most commonly on a fish host. One species (Simpsonaias ambigua) has been reported using a mudpuppy as host (Howard 1951) and another (Strophitus undulatus) to be able to develop to maturity without any host (Lefevre and Curtis 1910).

The life history of Quadrula quadrula reflects the general pattern of unionid mussels. During the spawning period, the male releases sperm into the water through the excurrent siphon. Sperm are carried by the water current, taken in by the female through the incurrent siphon and gills filter the sperm out of the water. In the female, ova are released and held in a specialized area of the gill called the marsupium (pl. marsupia). The sperm that has been filtered from the water is carried into the marsupium where the ova are fertilized. Fertilized ova develop into glochidia and are brooded in the marsupium. There are 2 patterns of brooding and glochidial release: long term (bradytictic) and short term (tachytictic); and the pattern of release may be related to temperature (Watters and O’Dee 1998). Quadrula quadrula is considered to be a tachytictic species that spawns early in the season, and broods glochidia for a short time before release (Parmalee and Bogan 1998). Fully developed glochidia have 2 valves that may be rounded to ovate, axe-head shaped, or triangular in shape, and may or may not have hooks for host attachment, depending on the species. Quadrula quadrula glochidia are ovate, lack hooks and measure approximately 80μm in length and height (Clarke 1981). Glochidia may be released singly or in packets called conglutinates or even super conglutinates that may be metres in length. Conglutinates and super conglutinates may have markings that mimic potential prey items of potential fish hosts. Quadrula quadrula releases glochidia in small, lanceolate, white-coloured conglutinates. The female mantle is modified to act as a lure for potential fish hosts in some species. The mantle of Q. quadrula is not modified in any visible way. The number of potential fish hosts varies from few to many depending on the mussel species. Howard and Anson (1922) identified the flathead catfish (Pylodictis olivaris) and Schwebach et al. (2002) identified the channel catfish (Ictalurus punctatus) as hosts for Q. quadrula glochidia which attach to the gills. Other mussel species’ glochidia attach to the fins or skin, with the site depending on the mussel species and adaptations of the glochidia for attachment. If glochidia do not attach to an appropriate host they will die. In rare cases, some species have been reported as not requiring a host for life cycle completion. Glochidia attach to the host and become encysted. Glochidia obtain nourishment from the host allowing them to transform to juveniles which are recognized by development of the foot. The amount of time for transformation is variable both within and among mussel species and may be temperature and host dependent. Once transformed, the juvenile mussel breaks out of the capsule and drops to the substrate and will begin to grow and develop as a free-living mussel. The percentage of glochidia successfully reaching maturity is extremely low due to high mortalities associated with reaching a suitable host, dropping onto suitable habitat and the sensitivity of juvenile stages to environmental conditions.

Life cycle and reproduction

Quadrula quadrula is dioecious. Males and females cannot be distinguished based on external morphology.

Quadrula quadrula is considered to be a short-term brooder (tachytictic). Reports on the length of the brooding season appear to vary with the location ranging from May to August in the United States (Parmalee and Bogan 1998) and late spring to early summer in Canada (Clarke 1981) and may be temperature dependent. In Manitoba, Q. quadrula brooding glochidia have not been observed, and ova have never been observed in the marsupium later than mid-June (Carney 2003a, personal observation). This would seem to suggest that in Manitoba at least there may not be successful reproduction every year and that there is a very limited time during which glochidia are available for host infestation.

Quadrula quadrula releases glochidia in lanceolate conglutinates that are approximately 10 mm in length and 3 mm in width at the broad end. The conglutinate is typically white and may have sparse randomly distributed dark “dots”. Presumably the fish host is infested when it bites the conglutinate releasing the glochidia, which can then attach to the gills. There are no data on the fecundity of Q. quadrula. Haag and Staton (2003) investigated reproductive traits of several mussel species including the congeners Quadrula asperata and Quadrula pustulosa collected from sites in Mississippi and Alabama. For those 2 species they report that fecundity increases with shell length for both species. There were differences in maximum fecundity, and fecundity as a function of age, for these 2 species. Quadrula asperata had a maximum fecundity approaching 30,000 developing ova in the gills whereas Q. pustulosa had a maximum fecundity of approximately 50,000 ova. Haag and Staton (2003) also report differences in age at which reproductive capacity is maximized for these 2 species. Maximum fecundity for Q. asperata is reached around 15 years of age while Q. pustulosa reaches maximum fecundity around 30 years of age (Haag and Staton 2003). Fecundity for both species declines following those maxima. These data would suggest that Q. quadrula likely reaches sexual maturity around the same age as Q. asperata and Q. pustulosa, somewhere between 3 and 10 years. The differences reported for these 2 congeners make it difficult to predict maximum fecundity and age after which fecundity declines for Q. quadrula. It seems likely that climatic conditions in Canada could extend time to maturity and would result in fecundity values that are much less than those reported for Q. asperata and Q. pustulosa from individuals collected from the southern U.S.

Howard and Anson (1922) identified the flathead catfish (Pylodictus olivaris) as a suitable host. This species is considered to be not native in Canada (Scott and Crossman, 1973) although 4 specimens have been caught in Canadian waters of Lake Erie (Gagnon, pers. comm.; Crossman and Leach 1979). There are no documented specimens of this species recorded from Manitoba. The flathead catfish was the only known host for Quadrula quadrula until Schwebach et al. (2002) reported successful transformation of Q.quadrula glochidia on channel catfish (Ictalurus punctatus), a species common in both Manitoba and Ontario (Scott and Crossman 1973; Stewart and Watkinson 2004). Transformation took 51-68 days with water temperature starting at 13° C and ending at 20° C (Schwebach et al. 2002). It seems reasonable to assume that channel catfish would serve as a suitable host for Canadian Q. quadrula populations. Other Quadrula species also have been recorded as having catfish species as suitable hosts (Fuller 1974). It is worth noting that the brown bullhead (Ameiurusnebulosus) has a range that is closest, and almost identical, to that of Q. quadrula (Lee et al. 1980) and it seems likely that this catfish species would be a suitable host, although this has yet to be demonstrated.

Quadrula quadrula is a long-lived species. Carney (2003a, b; personal observation) using shell thin sections (Clark 1980; Neves and Moyer 1988) reported individuals from the Assiniboine River in Manitoba up to 64 years with an average age of 22.1 (n = 47). Age data specific to Ontario populations are not available but are likely to be consistent with those reported from Manitoba.

Predation and parasitism

Predators of mussels include fishes, mammals and birds. Baker (1918) identified freshwater drum (Aplodinotus grunniens), lake sturgeon (Ascipenser fulvescens), pumpkinseed (Lepomis gibbosus) and suckers (Catostomus spp.) as fish predators of mussels. Muskrat (Ondatra zibethicus) predation can be an important factor that may limit some mussel populations (Tyrrell and Hornbach 1998) and have been reported to prey upon Quadrula quadrula (Nakato et al. 2005). Muskrat have been reported to eat as many as 37,000 mussels annually in a northern Alberta lake (Convey et al. 1989; Hanson et al. 1989) and may represent a serious threat to endangered species and populations (Neves and Odum 1989). Larval chironomids feeding on mussel tissue can result in the loss of up to 50% of the gill (Gordon et al. 1978), affecting both respiration and reproduction, and may represent either parasitism or predation depending on definitions used.

Mussels are parasitized by helminths (Digenea) and mites which may have a detrimental effect on the infected mussel. Mussels can be infected by both adult digeneans and by larval digeneans that include the sporocyst or metacercaria stages. Parasitism by sporocysts can result in sterilization of the infected mussel (Esch and Fernandez 1993). An extensive survey of mussels in Manitoba that included examination for parasites did not show parasitism of Quadrula quadrula by sporocysts (Carney 2003a). A single individual Q. quadrula was infected with metacercariae but this stage has not been demonstrated to cause sterility and there was no indication of sterility in that infected individual. A variety of adult helminths also can parasitize mussels. Aspidogaster conchicola in the pericardial sac and kidney, and Cotylogaster occidentalis in the gut, both have been reported from Q. quadrula (Hendrix et al. 1985). Carney (2003a) examined Q. quadrula from the Assiniboine River in Manitoba and reported approximately 60% infected with A. conchicola and 6% infected with C. occidentalis. Mean intensity of infection ranged from 1-71 (mean = 17) for infections by A. conchicola and mean intensity of 1-2 for infections by C. occidentalis. Infection by A. conchicola may cause damage to the renal epithelium (Williams 1942; Michelson 1970).

Adult mites belonging to the family Unionicolidae are common symbionts in the mantle cavity of mussels and also in the mantle tissue as parasitic larvae, and there may be close evolutionary ties between the two groups that possibly represent examples of coevolution, in some cases (Vidrine, 1996). Carney (2003a) reported no Quadrula quadrula collected from the Assiniboine River in Manitoba to be infested with mites. To date there are no data pertaining to parasitism of Q. quadrula from Ontario. It does seem likely that what has been reported from Manitoba would also be valid for populations in Ontario, given that the hosts and parasites are present in both regions (Carney 2003a; Hendrix et al. 1985).

Physiology

No studies specific to the physiology of Quadrula quadrula are known. As molluscs they would require a supply of calcium sufficient to meet their needs to make shell as they grow. Juveniles of other unionid species are known to be sensitive to ammonia at levels below current U.S. Environmental Protection Agency water quality criteria (Augspurger et al. 2003). It seems likely this sensitivity would extend to Q. quadrula. Quadrula quadrula must be able to survive a wide range of temperatures. In winter the rivers in Manitoba would be ice covered with water temperature at or near 0º C while in summer water temperatures can reach 27º C.

Dispersal/migration

Adult freshwater mussels are largely sessile, typically moving no further than 100 m, and usually much less than that. Therefore the ability of the adult stage to disperse or migrate is highly constrained. Larval mussels can be transported great distances while attached to the fish host. This is the dispersal stage and the limit to dispersal is how far the fish host moves. Mussels that use highly vagile hosts, or that use many host species, should be capable of greater dispersal than mussels that use hosts that do not travel very far or mussels that use a single host species. Therefore it is the host that determines the dispersal of the mussel that may introduce the mussel into new habitats, enhance gene flow between and among populations and bring new individuals into populations that may be in peril. The known hosts of Quadrula quadrula are catfish species that are known to travel great distances over short time periods (Stewart and Watkinson 2004). This would suggest Q. quadrula is capable of being dispersed over large distances. Berg et al. (1998) used allozyme variation to show there were high levels of gene flow among distant populations within the Ohio and Mississippi drainages. Populations in Ontario and Manitoba each lie within drainages that are isolated from each other and have been separated from the Ohio-Mississippi drainages for thousands of years; rescue from these sources is not possible. American populations within the Red River drainage in North Dakota and Minnesota have been subject to the same population declines as in Manitoba (Hart 1995; Ceas 2001; Carney 2003b) and as such do not provide a source population from which Manitoba populations could be replenished. A similar situation occurs in the American waters of the Lake Erie drainage. As a consequence it is not reasonable to assume that depauperate Canadian Q. quadrula populations could be naturally replenished from more robust American populations, since those American populations are in the same state as the Canadian populations.

Interspecific interactions

There has been extensive debate regarding the food and source of nutrition of unionid mussels. Much of this debate has centered on the fact that there are problems in separating what is ingested and what is actually assimilated. As unionids filter water they feed by sorting food particles in the mantle cavity before ingestion and in the stomach after ingestion (Nicholls and Garling 2000). Although the feeding and nutrition of Quadrula quadrula has not been specifically addressed, Raikow and Hamilton (2001) and Christian and Smith (2004) demonstrated that a variety of mussel species were using similar food resources. The following discussion assumes that what has been reported for other mussel species can be extended to Q. quadrula.

Available information indicates adult mussels feed upon fine particulate organic matter suspended in the water column (Tanklersley 1996; Ward 1996). However, it is unclear regarding which components of this suspended material are actually used for nutrition (Gatenby et al. 1993; Nicholls and Garling 2000). Allen (1914) reported diatoms and algae in the gut and proposed that algae, protozoa, bacteria and organic material were the primary sources of nutrition (Allen 1921). Ingested materials are not necessarily digested but may survive passage through the digestive tract (Churchill and Lewis 1924; Miura and Yamashiro 1990). Imlay and Paige (1972) suggested that unionids derive their nutrition from feeding on bacteria and protozoa. Nicholls and Garling (2000) studied the diet of 7 mussel species using stable isotope analysis, gut contents and biochemical analyses and determined there was positive selection of food items in the diet based on the concentration of algae and diatoms in the gut. Although algae and diatoms were selectively ingested and concentrated in the gut, the main source of nutrition was bacteria. However, algae were still a necessary part of the diet by providing essential nutrients (Nicholls and Garling 2000).

There is also uncertainty regarding the degree to which mussels feed upon detritus and organic matter within the substrate. Juvenile mussels may use cilia on their foot to feed from the substrate, a method called pedal feeding. Gatenby et al. (1993) suggested juveniles fed on algae and silt. Yeager et al. (1993) and Yeager and Cherry (1994) have shown that juvenile mussels may feed primarily on bacteria from within the substrate but the importance of this as a source of nutrition for adults is less clear. Adult mussels are capable of filtering suspended particles ranging in size from 0.9 – 250 µm (Silverman et al. 1997; Nicholls and Gatenby 2000) suggesting suspended organic materials are important in the diet and nutrition of mussels. However, Raikow and Hamilton (2001) used stable isotope ratios to determine diets of 12 mussel species and suggested their diet was comprised of 80% deposited material and 20% suspended material.

If the preceding data apply to Quadrula quadrula the following conclusions regarding feeding may be drawn. Feeding on interstitial material may be the primary source of nutrition for juveniles. Adults may also be feeding from the substrate as well as upon organic materials suspended in the water column. Bacteria appear to be the primary source of nutrition with algae supplying many essential elements in the diet. Clearly there is much more research required to clarify the food habits and nutrition of unionid mussels.

Adaptability

The range of suitable habitats suggests this species is quite adaptable. Glochidia have been successfully transformed in an experimental situation using channel catfish as host. Informal accounts of these mussels being successfully transplanted by commercial interests suggest this species can be transported and transplanted.