| Reproduction Preferences Additional Information | Reproduction: Reproduction in barnacles is discussed in detail by Rainbow (1984), Barnes (1989), Klepal (1990), Barnes (1992), Anderson (1994) and the references therein. Key points follow. - Semibalanus balanoides is an obligate cross-fertilising hermaphrodite.
- The barnacle penis is substantially longer than the body and is capable of searching an area around the adult to find a receptive 'functional female'.
- Copulation takes place in the UK from November to early December and although an individual 'functional male' may inseminate a single 'functional female' up to 6-8 times (dispensing all its seminal fluid), insemination by more than one functional male is required to successfully fertilise all the eggs. Up to 6 concurrent penetrations may occur (Rainbow, 1984; Anderson, 1994).
- After copulation the penis degenerates and is re-grown during summer ready for the following November. Penis and gonad development in the population is highly synchronous, and probably controlled by light and temperature regime since gonad maturation is inhibited by 15 °C or greater and a light period greater than 12h/day (Barnes, 1992).
- Fertilised embryos are held in two egg sacs and incubated in the mantle cavity over winter, during which the barnacle does not moult (anecdysis).
- Nauplii larvae are released from the barnacle between February and April, in synchronisation with the spring algal bloom. Hatching takes place later in the north and east of Britain.
- Synchronisation with the spring algal bloom is enabled by the release of a hatching substance, which is secreted by adult barnacles following ingestion of phytoplankton (Barnes 1957; Crisp 1956; reviewed by Clare, 1995). Hatching substance is released into the mantle cavity by the adult and has been identified as an eicosanoid, which may function by stimulating the release of embryonic dopamine (Clare, 1995). In response, the nauplii twitch repeatedly until they break free of the egg membrane and are released. The hatching factor is probably a complex mixture of hydroxy fatty acids, analogous to sex pheromones in insects (see Clare, 1995).
- 'Spawning' of nauplii in response to the spring phytoplankton bloom ensures that larvae grow and develop under optimum conditions when food supply is at its highest and have time to develop and lay down food reserves prior to settlement.
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Nauplii larvae are planktotrophic and develop in the surface waters for about two months. They pass through six nauplii stages before eventually developing into a cyprid larva. Cyprid larvae are specialised for settlement (see general biology). Peak settlement occurs in April to May in the west and May to June in the east and north of Britain.
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Semibalanus balanoides produces one brood per year of 5000 -10,000 eggs/ brood in mature adults but varies with age and location e.g. at Port Erin, Isle of Man fecundities of 2500-4000 eggs/ brood (max. 13,000) were reported while 400-8000 eggs / 1.5mg oven dried body weight were recorded in Scotland (Barnes, 1989).
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Reproduction may be affected by temperature, latitude, light, feeding, age, size, crowding, seaweed cover and pollution. High shore Semibalanus balanoides breed first and low shore specimens last (up to 12 days difference)(Barnes, 1989). Fertilization is prevented by temperatures above 10 °C and continuous light. Differences in breeding times with latitude are probably mediated by temperature and day length, e.g. in Spitzbergen fertilization occurs 2-3 months earlier than in the UK. Increased crowding or seaweed cover may decrease feeding and reduce fecundity.
- Barnacles grow rapidly in the first season after settlement. Newly metamorphosed larvae are very squat and only form the adult shape at 3 mm. Semibalanus balanoides may become sexually mature in the first year after settlement although this is often delayed until 2 years of age (Anderson, 1994).
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Life span of Semibalanus balanoides varies with the position on the shore. Barnacles low on the shore typically die in their third year, whereas those from near the mean level of high water neaps may live for five or six years.
Recruitment: Settlement and subsequent recruitment is highly variable. - Jenkins et al. (2000) reported variation in settlement and recruitment at all spatial scales studied (10s, 1000s of metres and 100s of km) in Sweden, the Isle of Man, southwest Ireland and southwest England and between 2 years, 1997 and 1998. Substantial variation in settlement and recruitment occurred between sites, but was not consistent between the two years studied. Variation in settlement explained 29 -99% of variation in recruitment across all sites, although not all variation in recruitment was explained by settlement at all sites. They also observed significant variation between replicate samples within sites in 1997. Recruitment was lower in southwest England than southwest Ireland even with similar settlement due to variation in post settlement mortality.
- Settlement density may also be influenced by onshore or offshore winds, resulting in irregular and sharp peaks of settlement, e.g. north Yorkshire or north west Scotland coasts (Kendall et al., 1985). Settlement density may be directly related to orientation of the shore to the prevailing winds. Settlement was enhanced by onshore winds in the Isle of Man (Hawkins & Hartnoll, 1982) but offshore winds and calm seas in Anglesey (Rainbow, 1984). Hawkins & Hartnoll, (1982) and Jenkins et al. (2000) suggested that failure to recruit in any one year is probably less likely when progeny are produced locally and disperse over short distances, whereas where dispersal is wide the chance of larvae encountering adult habitat is subject to varying hydrographic conditions, especially in offshore islands where isolation may exacerbate loss of larvae due to offshore transport.
- In poor years settlement occurred mainly in the later part of the season suggesting either that early larvae failed or were lost (Kendall et al., 1985), or that the phytoplankton bloom, and so release and development of larvae, was late.
- Macroalgae canopies inhibit cyprid settlement and sweeping of algal fronds or bulldozing by grazing limpets may cause high post-settlement mortality, up 82-97% under Fucus serratus canopy (Jenkins et al., 1999). Fucus serratus was found to inhibit settlement more than Fucus spiralis (which has a less dense canopy) and Ascophyllum nodosum (which floats upright in the water column). However, the long term survival of spat reaching >6mm under the canopy was enhanced, especially high on the shore due to reduced risk of desiccation under the canopy (Jenkins et al., 1999).
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The cyprids are capable of settling above their usual zone on the shore but their upper limit (below Chthamalus montagui) is maintained by their lower tolerance to temperature and desiccation when compared to chthamalids. Mortality in early life is highly variable, e.g. Kendall et al. (1985) noted that under highly desiccating conditions 70% of a single days input of barnacle spat to the upper shore died within 24 hrs, but overall, in 48 hrs in 1978 mortality was 13% however, in 1980, when intertidal was exposed to 27 °C, 48hr survival was reduced to 30%.
- Long term monitoring of intertidal barnacle populations in southwest England demonstrated a correlation between the relative abundance of Semibalanus balanoides to Chthamalus spp. and the planktonic ecosystem and sea temperatures over a 40 year period (1954-1987) (Southward, 1991; Southward et al., 1995). Semibalanus balanoides increased in abundance in cooler years and Chthamalus spp. in warmer years, possibly due to the increased survival of Semibalanus balanoides spat at lower temperatures and reduced desiccation (Kendall et al. 1985). At increased temperatures Chthamalus spp. are likely to produce more and earlier broods of larvae, and compete more effectively with Semibalanus balanoides which will suffer increased mortality at high to mid shore (Southward et al., 1995).
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