- Predators of the Invasive Mussel Musculista senhousia (Mollusca: Mytilidae)1
Musculista senhousia (Benson in Cantor, 1842) is a soft sediment-dwelling mussel that has spread anthropogenically from its native Asia to North America, Australasia, and Europe. This byssal mat-forming species can become overwhelmingly dominant and have dramatic impacts within invaded ecosystems, but its invasion may meet "ecological resistance" from native predators. In Mission Bay, San Diego, California, three fish species and two shorebirds were found to prey upon the mussel. Experimental results suggest that predation can dramatically impact intertidal mussel populations and may account for observed seasonal declines in the species. Despite the creation of a byssal cocoon, which may afford the mussel some protection, several taxa worldwide have been found to be Musculista predators. In addition, in areas where the mussel is native, humans impact mussel populations by gathering it for animal feed or bait, or to remove it from commercial shellfisheries grounds.
Nonnative species are increasing in abundance throughout the world, and biological invasions now represent one of the most serious threats to the integrity of ecosystems (Vitousek et al. 1997, Mooney and Hobbs 2000). Exotic species can have a wide range of ecological interactions within invaded ecosystems, including competition with natives, alteration of the physical nature of habitats, or predation upon resident biota (Crooks and Khim 1999, Parker et al. 1999, Ruiz et al. 1999). Exotics also can be eaten by natives, which can provide food resources for resident biota as well as potentially limit the abundance of the invader. This control of exotics by the feeding activities of natives represents a potentially important form of "ecological resistance" to invasions, whereby the extent and impact of invaders can be limited (Elton 1958, Reusch 1998).
In the coastal embayments of San Diego, California, including San Diego Bay and Mission Bay, an abundant and conspicuous invader is the Japanese mussel, Musculista senhousia (Benson in Cantor, 1842). This mussel has successfully taken advantage of various synanthropic means of invasion in its spread around the world, including transport in ballast water, association with intentionally introduced oysters, and Lessepsian migration through the Suez Canal (Barash and Danin 1971, 1972, Carlton 1979, Crooks 1992). In addition to its native Asia, M. senhousia is now found in North America, Europe, Australia, and New Zealand (Carlton 1979, Willan 1987, Hoenselaar and Hoenselaar 1989). Musculista senhousia is a typical opportunistic species. It is small, with a maximum length of 3.5 cm, is short-lived, with a maximum longevity of approximately 2 yr, and has fast growth (Morton 1974, Tanaka and Kikuchi 1978, Crooks 1996). Musculista senhousia can achieve considerable densities in both intertidal and subtidal soft sediments (Kikuchi and Péres 1977, Crooks 1992). Typical abundances are [End Page 49] 5,000-10,000 m-2, but densities in excess of 150,000 m-2 have been reported in Mission Bay (Reusch and Williams 1998, Crooks and Soulé 1999, Dexter and Crooks 2000). Like other mytilid mussels, M. senhousia produces byssal threads. In its typical sandy or muddy habitat, the byssus is used to form a cocoon, which may protect the thin-shelled bivalve and stabilize the animal in the sediment (Morton 1974).
When the mussel occurs in high densities, individual cocoons can intertwine, forming a mat or carpet that contains shells, sediment, algae, and detritus (Morton 1974, Creese et al. 1997, Crooks 1998). These mats serve as biogenic habitat for a variety of small macrofauna, whose abundances within this structurally complex area are higher than in sediments without mats (Crooks 1998, Crooks and Khim 1999). Larger organisms such as surface-dwelling, suspension-feeding bivalves and eelgrass, however, can be inhibited by dense mats of mussels (Creese et al. 1997, Reusch and Williams 1998, Crooks 2001). In the intertidal habitats of Mission Bay, the mussel is typically seasonal, with highest abundances in the summer and fall (Crooks 1998).
Because of the important role of M. senhousia as a competitor and habitat modifier, it is of interest to identify potential mussel predators. There have been some studies to identify predators of exotics in marine systems (e.g., Carlton 1979, Carlton et al. 1990), but relatively few studies have employed experimental...