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size, high fecundity, small egg size, and semelparity. Nevertheless, other variable
demographic features such as lifespan, and age at maturity were difficult to determine
because these features have not been documented. The life-history periodic strategy is
distinctive of species inhabiting predictable and seasonal environments (Winemiller
and Rose, 1992; McCann and Shuter, 1997).
Two large species (Pontaster tenuispinus Düben and Koren and Psilaster
andromeda (Verrill)) with large eggs, low fecundity and low G.I. were found at ~
650-850 m depth. The reasons why species with these features are found at this
specific depth are still unknown and further studies are required in order to understand
the factors that determine their distribution. The water mass structure at this depth has
been defined by Rice et al. (1991), Van Weering et al. (1998) and New and Smythe-
Wright (2001) as the boundary between Eastern North Atlantic Water (ENAW) and
Mediterranean Overflow Water (MOW) at ~800 m. It is possible that the reproductive
features of these species are influenced in a specific way by the hydrodynamic
characteristics of this particular zone, as sharp density differences are known to focus
internal wave energy on the continental slope.
From 700 to 1100 m depth only species with small adult size, large egg size
with low fecundity and low Gonad Index were found. Howell et al. (2002) defined
this zone as the upper bathyal zone, and it is characterized by a rapid succession of
species with species displaying narrow total adult bathymetric ranges. The 1100 m
boundary of this zone was associated with changes in currents. The main currents
around the Porcupine Seabight flow anticlockwise as part of the European Slope
Current, which flows northwards (McMahon et al., 1995; New and Smythe-Wright,
2001) (Fig. 6). Above 1000 m the European Slope Current is thought to have seasonal
variations in bathymetric range, mean speed, and direction, showing a complete
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