Sphaeriidae

Definition

Sphaeriidae (also known as pea clams or fingernail clams) is a family of small, freshwater bivalves in the order Sphaeriida and consist of 10 genera with 154 species. They are both hermaphrodites and ovoviviparous, giving birth to young that resemble miniature versions of adults [1].

Habitat

In North America, native species of sphaeriid have a broad distribution often ranging from the Atlantic to Pacific coast. Introduced species, such as Corbicula fluminea which originates from southeast Asia, also exhibit widespread distribution [2]. Many of these species occur in ephemeral ponds, small, variable flow streams, and profundal regions of lakes [3]. The preference of sphaeriids for these regions of low water flow, high silt, and large organic loads may reflect their lifestyle of sediment detritus feeding [4]. Species diversity in the genus Pisidium has also been shown to increase with decreasing particle size, indicating substrate preferences among sphaeriids possibly linked to their sediment feeding mechanisms (Kilgour and Mackie, 1998). Many sphaeriids are tolerant to air exposure which they achieve through unique emersion adaptations (Byne and McMahon, 1994).

Physiology & Life History

Sphaeriids display large variations in life history strategies on both an intra- and interspecific level (Holopainen and Hanski, 1986). They exhibit traits commonly associated with unstable habitats (short life spans, early maturity, small adult size, and increased energetic allocation to reproduction) as well as traits associated with stable habitats (slow growth rates, low fecundity, and release of large, fully developed young) (Sibly and Calow, 1986). All species exhibit viviparous reproduction with embryos developing in specialized chambers from outgrowths of the inner demibranch gill filaments. The internal gestation of young lead to sphaeriids having the largest young out of any freshwater bivalves with newborns measuring 0.6 to 4.15 mm and having a biomass of 0.6 to 4.6% that of adults (Mackie, 1984) (Burky, 1983). Seasonal water availability fluctuations and hypoxia associated with their habitats have also influenced the development of developed juveniles as they are more susceptible to environmental stress. This means that sphaeriids dedicate more energy into fewer offspring than those produced by other freshwater bivalves (Burky, 1983). This initial head start in development in juveniles also leads to faster maturation to a reproduction before the start of seasonal stress. The term “bet hedging” has been used to describe the mixture of r- and K-selective traits allowing for increased fitness in environments with periodic, predicable stress (Stearns, 1980).

References

1. Allaby, Michael. A Dictionary of Zoology, Oxford University Press, Incorporated, 2020. ProQuest Ebook Central, https://ebookcentral-proquest-com.gate.lib.buffalo.edu/lib/buffalo/detail.action?docID=6230105.
2. McMahon, Robert. (2000). Invasive characteristics of the freshwater bivalve Corbicula fulminea. Nonindigenous Freshwater Organisms: Vectors, Bilogy, and Impacts (2000), pp 315-343.
3. Burky, A. J. 1983. Physiological ecology of freshwater bivalves. in: Russell-Hunter, W. D., Ed. The Mollusca. Vol. 6 : Ecology. Academic Press, New York, pp. 281 – 327.
4. Lopez, G. R., Holopainen, I. J. 1987. Interstitial suspension-feeding by Pisidium sp. (Pisididae: Bivalvia): a new guild in the lentic benthos? American Malacological Bulletin 5:21 – 30..