Scientific background

Recent findings and predictions strongly suggest that global climate change affects marine biodiversity and ecosystem functioning (Hughes 2000, Walther et al. 2002, Root et al. 2003, Kendall et al. 2004). The distribution ranges of marine species are often confined by certain climate regimes (Gaston 2003). For a number of species within the northern hemisphere, an increase of water temperatures was followed by northward shifts of their distribution ranges (Barry et al. 1995, Southward et al. 1995, Parmeson & Yohe 2003, Walther et al. 2002, Root et al. 2003). Expanding species may encounter new environmental conditions, e.g. with respect to competition for resources or predator pressure. Indirect effects of temperature on biological interactions may only effect a small and local change in the biodiversity and ecosystem functioning. Small environmental perturbations, however, may also cause major shifts, even from one ecological state to another (Scheffer et al. 2001, Scheffer & Carpenter 2003).

When environmental conditions change (e.g. when species move northward or when they remain at the same place under changing circumstances), a transformation in behaviour, physiology or morphology during early development might incur a selective advantage. For numerous marine species, propagation must be timed to match reproduction with the most optimal environmental conditions for the first vulnerable life stages (Sastry 1975, Todd & Doyle 1981, Olive 1992). Global warming may create a mismatch between the timing of propagation and that of food availability and/or presence of main predators (Cushing 1990). For example, rising seawater temperatures appear to affect the timing of spawning of the marine bivalve Macoma balthica relative to that of the spring phytoplankton bloom and the settlement of juvenile shrimps on the tidal flats, decreasing food availability during the pelagic stage and increasing predation pressure after settlement (Philippart et al. 2003).

For broadcast spawners, for example, spawning must be also timed to ensure synchroneous release of sperm and eggs (Levitan & Petersen 1995, Yund 2000). Most broadcast spawners undergo repeated, but synchronous, spawning ('iteropartiy'), whilst only few release their gametes all at once ('semelparity'). Under variable conditions, semelparity increases the probability that any successful offspring will be produced at all (Murphy 1968, Roff 2002). Because the spawning strategy was found to be related to latitude (Jenkins et al. 2001, O'Riordan et al. 2004), rising seawater temperatures may locally affect the success rate of a particular spawning strategy and subsequently affect a shift from semelparity to iteroparity within a species or within a guild.

New environmental conditions may not only affect the timing and frequency of propagation and settlement, but also the morphology of the larvae and settlers. Shell shape, for example, can be structured by the degree of wave-exposure (Coe & Fox 1942, Steffani & Branch 2003), the overcrowding posing physical compression on individuals (Seed 1968), food availability (Brown et al. 1976, Seed 1968, Steffani & Branch 2003) and the nature of substrate (Orton 1936). Morphological flexibility of feeding structures has been proved to exist in the larvae of distantly related phyla, such as molluscs and echinoderms (Boidron-Metairon 1988, Hart & Strathmann 1994, Strathmann et al. 1993).

The aim of this project is to further and fuller understand the relationship between the recruitment processes and local environmental conditions. Latitudinal patterns in timing and frequency of propagation and settlement, and the morphology of larvae and settlers (e.g. structural body size and relative size of feeding structures) may indicate how organisms respond to variations in seasonality (e.g. temperature, food availability and predation pressure). Propagation is, in very many marine organisms, the culmination of a long process of gametogenic development, and it is therefore important to understand how environmental conditions control the entire reproductive cycle (Olive 1995). Understanding factors that contribute to recruitment success of marine organisms is necessary to better resolve questions with regard to potential effects of global warming on marine ecosystems and may contribute to the investigation of the increase in biological diversity form polar to equatorial regions (Willig et al. 2003).

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