BIOTIC

BIOTIC Species Information for Mytilus edulis

Researched by

Lizzie Tyler

Data supplied by

University of Sheffield

Refereed by

This information is not refereed.

Reproduction/Life History

Reproductive type

Gonochoristic

Developmental mechanism

Planktotrophic

Reproductive Season

April to September

Reproductive Location

Water column

Reproductive frequency

Annual protracted

Regeneration potential

Life span

21-50 years

Age at reproductive maturity

1-2 years

Generation time

1-2 years

Fecundity

Egg/propagule size

75 µm diameter

Fertilization type

External

Larvae/Juveniles
Larval/Juvenile dispersal potential	>10km	Larval settlement period	See additional information
Duration of larval stage	1-6 months

Reproduction Preferences Additional Information

Life span
Longevity is dependant on locality and habitat. On the lower shore, few individuals probably survive more than 2-3 years due to intense predation, whereas high shore populations are composed of numerous year classes (Seed, 1969b). Specimens have been reported to reach 18-24 years of age (Thiesen, 1973). Mortality is size dependant and can be high, e.g. Dare (1976) reported annual mortalities of 74% in 25mm mussels and 98% in 50 mm mussels in Morecambe Bay, England.

Spawning
Spawning is protracted in many populations, with a peak of spawning in spring and summer. For example, in north east England, resting gonads begin to develop from October to November, gametogenesis occurring throughout winter so that gonads are ripe in early spring. A partial spawning in spring is followed by rapid gametogenesis, gonads ripening by early summer, resulting in a less intensive secondary spawning in summer to late August or September (Seed, 1969a). Mantle tissues store nutrient reserves between August and October, ready for gametogenesis in winter when food is scarce (Seed & Suchanek, 1992). Larvae spawned in spring can take advantage of the phytoplankton bloom. The secondary spawning, is opportunistic, depending on favourable environmental conditions and food availability. Gametogenesis and spawning varies with geographic location, e.g. southern populations often spawn before more northern populations (Seed & Suchanek, 1992). Reproductive strategies in Mytilus edulis probably vary depending on environmental conditions (Newell et al., 1982).

Fertilization
Fertilization is external. Fertilization can occur successfully between 5 -22°C and at salinities of 15 -40psu (Bayne, 1965; Lutz & Kennish, 1992). Fertilized eggs are 60-90µm in diameter (Lutz & Kennish, 1992).

Fecundity
Fecundity and reproductive effort increase with age and size, young mussels diverting energy to rapid growth rather than reproduction. Reproductive output is influenced by temperature, food availability and tidal exposure and can therefore vary from year to year. An individual female (ca 7mm) can produce 7-8 million eggs, while larger individuals may produce as many as 40 million eggs (Thompson, 1979).

Larval development
In optimal conditions larval development may be complete in less than 20 days but growth and metamorphosis in the plankton between spring and early summer, at ca. 10 °C, usually takes 1 month. However, it is not unusual for planktonic life to extend beyond 2 months in the absence of suitable substrata or optimal conditions (Bayne, 1965; Bayne, 1976a). Pediveligers can delay metamorphosis for up to 40 days at 10 °C (Lutz & Kennish, 1992) or for up to 6 months in some cases (Lane et al., 1985). The duration of the delay is mainly determined by temperature, with longer delays at low temperature (Strathmann, 1987). Larvae become less selective of substrata the longer metamorphosis is delayed.

Settlement
In many populations Mytilus edulis exhibits a two stage settlement, the pediveliger settling on filamentous substrates and then moving on to suitable adult substrata by bysso-pelagic drifting. However, McGrath et al. (1988) and King et al. (1990) found little evidence of bysso-pelagic drifting in populations in Norwegian fjords or the Baltic, and pediveligers settled directly into adult beds.
Pediveligers typically settle at ca. 260 µm (McGrath et al., 1988) but can delay metamorphosis until ca. 350 µm. Pediveligers can delay settlement for up to 7 weeks (Holt et al., 1998). Pediveligers test the substrata using their sensory foot. Settling pediveligers prefer discontinuities in the substrata (Chipperfield, 1953), and reportedly tend to avoid adults (Lane et al., 1985).
Primary settlement tends to occur on filamentous substrata, such as, bryozoans, hydroids, filamentous algae such as Polysiphonia sp., Corallina sp. and Mastocarpus sp., or the byssus threads of previously settled adults. Primary settlement may allow the pediveligers to avoid competition for food with adults or being inhaled by suspension feeding adults. Post-larvae may remain on their primary attachment until 1-2mm in size (sometimes larger), and many late post-larvae over-winter on algae, moving to adult substrata in spring, although many will leave the algae earlier due to winter storms or death of the algae (Seed & Suchanek, 1992). Newly settled mussels are termed 'spat'.

Dispersal
Dispersal is dependant on the duration of planktonic life. Maintenance of their position in the water column by active swimming ensures that larvae can be potentially dispersed over great distances by currents. In addition, post-larvae can become bysso-pelagic up to 2-2.5 mm in size, which may take ca. 2 months to achieve, during which time they may be transported significant distances by currents.

Recruitment
Recruitment is dependant on larval supply and settlement, together with larval and post-settlement mortality. Jørgensen (1981) estimated that larvae suffered a daily mortality of 13% in the Isefjord, Denmark. Lutz & Kennish (1992) suggested that larval mortality was approximately 99%. Larval mortality is probably due to adverse environmental conditions, especially temperature, inadequate food supply (fluctuations in phytoplankton populations), inhalation by suspension feeding adult mytillids, difficulty in finding suitable substrata and predation (Lutz & Kennish, 1992). First winter mortality in the Exe estuary averaged 68%, adults suffering 39% mortality after spawning and 24% due to bird predation (McGrorty, et al., 1990). Beukema (1992) reported recruitment failure in Mytilus edulis populations in the Wadden Sea after mild winters, which was thought to be due to a resultant increase in the number of small crabs or flatfish on the flats. Recruitment in many Mytilus sp. populations is sporadic, with unpredictable pulses of recruitment, possibly from the pool of young mussels on filamentous algae (Seed & Suchanek, 1992). Mytilus sp. is highly gregarious and final settlement often occurs around or in between individual mussels of established populations. Competition with surrounding adults may suppress growth of the young mussels settling within the mussel bed, due to competition for food and space, until larger mussels are lost (Seed & Suchanek, 1992).
Persistent mussels beds can be maintained by relatively low levels of recruitment. McGrorty et al., (1990) reported that adult populations were largely unaffected by large variations in spatfall between 1976-1983 in the Exe estuary.

Reproduction References

Hayward et al., 1996, Bayne, 1965, Lutz & Kennish, 1992, Bayne, 1983, Hrs-Brenko & Calabrese, 1969, Bayne, 1976b, Jørgensen, 1981, Chipperfield, 1953, Lane et al., 1985, McGrath et al., 1988, Sprung, 1984, Thorson, 1950, Seed, 1976, King et al., 1990, Seed & Suchanek, 1992, Mackie, 1984, Newell et al.,1982, Thompson, 1979, Thiesen, 1973, Dare, 1976, Holt et al., 1998, Strathman, 1987, McGrorty et al., 1990, Beukema, 1992, Widdows, 1991, Beaumont & Budd, 1982, Beaumont & Budd, 1984, Clay, 1967(d), Gosling, 1992(a), Gray et al., 1997, Seed, 1969a, Bayne, 1976a, Eckert, 2003, Heidi Tillin, unpub data, Julie Bremner, unpub data,