Comparison of Talitrus saltator (Amphipoda, Talitridae) biology, dynamics, and secondary production in Atlantic (Portugal) and Mediterranean (Italy and Tunisia) populations

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Abstract

Talitrus saltator biology, population dynamics, and reproduction were studied more or less simultaneously at three sand beaches: Lavos, on the western coast of Portugal; Collelungo, on the Italian coast of the Thyrrenian Sea; and Zouara, on the northern coast of Tunisia. The species exhibited a consistent pattern of aggregated distribution. Densities were higher at Lavos than at Collelungo and Zouara. Reproduction took place from early March to late September at Lavos and Collelungo, and from late February to early November at Zouara. The average sex ratio was favourable to males at Lavos and Collelungo, and to females at Zouara. Based on data from Lavos, the population abundance was positively correlated with temperature, while the percentage of juveniles in the population was positively correlated with temperature and sediment moisture. Adult individuals from the Atlantic population were larger than the Mediterranean ones, while newborn individuals from the Mediterranean were slightly larger than Atlantic ones. Life span was estimated at 7–11 months at Lavos, 6–9 months at Collelungo, and 6–8 months at Zouara. Cohorts born at the beginning of the reproductive period tend to have shorter lives than the ones born later in the season, with longer life spans occurring in cohorts that crossed the winter to breed in the next year. The minimum period necessary for sexual differentiation after birth was estimated at ±4 weeks at Lavos, ±3 weeks at Collelungo, and ±4.5 weeks at Zouara, for males, and ±6 weeks at Lavos, and ±5 weeks at Collelungo and Zouara, for females. The period necessary for female's sexual maturation after being born was estimated at ±10 weeks at Lavos, and ±8 weeks at Collelungo and Zouara. At the studied sites, T. saltator appeared as semiannual species, with iteroparous females appearing to produce at least two broods per year, and exhibited a bivoltine life cycle. Growth production (P) was estimated at 0.74 g m−2 yr−1 ash-free dry weight (AFDW; 17.7 kJ m−2 yr−1) at Lavos, 0.12 g m−2 yr−1 AFDW (2.8 kJ m−2 yr−1) at Collelungo, and 0.61 g m−2 yr−1 AFDW (14.3 kJ m−2 yr−1) at Zouara. Elimination production (E) was estimated at 1.40 g m−2 yr−1 AFDW (33.5 kJ m−2 yr−1) at Lavos, 0.20 g m−2 yr−1 AFDW (4.8 kJ m−2 yr−1) at Collelungo, and 1.11 g m−2 yr−1 AFDW (26.6 kJ m−2 yr−1) at Zouara. The average annual biomass (B̄) (standing stock) was estimated at 0.13 g m−2 at Lavos, 0.014 g m−2 at Collelungo, and 0.084 g m−2 at Zouara, resulting in P/B̄ ratios of 5.7 at Lavos, 8.2 at Collelungo, and 7.3 at Zouara, and E/B̄ ratios of 10.8 at Lavos, 14.4 at Collelungo, and 13.1 at Zouara. The present results, combined with information from literature, revealed a geographic variation in T. saltator populations with regard to their morphological characteristics, growth rates, life spans, and life cycles.

Introduction

The littoral zone of sandy beaches constitutes a harsh environment, and the animal communities living at the interface between land and sea have evolved ecophysiologically (e.g. Morrit & Spicer, 1982) and behaviourally to cope with the environmental constraints of this changing habitat. However, in recent times, human impact has severely threatened the sand littoral ecosystems, stressing the need to investigate the extent to which organisms can adapt to environmental and human-induced changes, foreseeing a sustainable use of beach environments. In fact, biological responses to environmental changes are complex, since different phenotypes may result from modification of gene expression, and the genome may also change through natural selection (Hoffmann & Parsons, 1991). It is, therefore, important to evaluate the adaptation of local populations to such changes, and the degree of natural variation between and within populations appears as a useful indicator (Hazlett, 1988).

Talitrid amphipods constitute one of the predominant arthropod groups in sandy beach fauna, exhibiting a dynamic equilibrium with environment. Owing to their ecological importance, talitrids have been studied worldwide from different points of view. For example, a large number of articles have been published on talitrid behaviour, behavioural plasticity, and genetic determination of different behaviours (e.g. Gerard, Vancassel, & Laffort, 1993, Scapini, Lagar, & Mezzeti, 1993, Scapini, 1997), while factors influencing their spatial distribution and oriented movements in sandy beaches have also been studied to a great extent (e.g. Borgioli et al., 1999, Scapini, Buiatti, & Fallaci, 1989, Scapini, Buiatti, De Matthaeis, & Mattoccia, 1995, Scapini, Porri, Borgioli, & Martelli, 1999, Scapini, & Fasinella, 1990, Scapini, & Quochi, 1992). With regard to biodiversity, along the European coasts, talitrid populations have been compared genetically to assess inter- and intra-specific variations (De Matthaeis, Cobolli, Mattocia, & Scapini, 1995). Finally, in terms of applied research, a number of articles have been published on trace metals (Cu, Zn, Fe, Cd, Pb, Mn, and Ni) concentrations and bioaccumulation by talitrids, and on their role in biomonitoring (e.g. Fialkowski, Rainbow, Fialkowski, & Smith, 2000, Weeks, 1992, Rainbow, Moore, & Watson, 1989).

Population size, reproductive strategies, and potential for dispersal between habitats are usually considered to reflect evolutionary fitness. Likewise, it may be assumed that different beaches offer different food resources, and that local populations would vary with regard to feeding habits (see, for example, Pennings, Carefoot, Zimmer, Danko, & Ziegler, 2000), efficiency in using the available energy, productivity, and reproductive capacity. Nevertheless, there are no comprehensive studies of strand-line talitrids in this respect, and the contribution of this group to the energy of beach ecosystems is still poorly understood. In fact, there are few studies on the reproductive biology and population dynamics of talitrids (Williams, 1978, Van Senus, 1988, Marsden, 1991a, Williamson, 1951a, Palluault, 1954, Hartog, 1963, Persson, 1999, Jones and Wigham, 1993), and even less on secondary production (e.g. Van Senus, & McLachlan, 1986, Cardoso & Veloso, 1996).

Talitrids comprise species such as the sandhopper Talitrus saltator, widely distributed along the Mediterranean and European sandy coasts, and generally abundant where it occurs. Williams (1978) provided an elegant approach to studies on the annual pattern of reproduction of this species in northern Europe (British Isles). The underlying objective of the present study was to investigate the adaptation of T. saltator to life in sandy beaches, and simultaneously to provide data necessary for the development of a population dynamics model (Anastácio, Gonçalves, Pardal, & Marques, 2003). For this purpose, a comparative study of Atlantic and Mediterranean populations was carried out, focusing on their biology, population dynamics, and productivity.

Section snippets

Study sites

To allow comparisons between Atlantic and Mediterranean populations, three different sets of data were provided by sampling campaigns carried out at three sandy beaches: Lavos, on the western coast of Portugal; Collelungo, on the Italian coast of the Thyrrenian Sea; and Zouara, on the northern coast of Tunisia (Fig. 1).

The Lavos sandy beach, located on the central region of Portugal, is approximately 25 km long, between the mouth of the Mondego River and the Ervedeira coastal lagoon, close to

Spatial distribution and abundance

Talitrus saltator exhibited a consistent pattern of aggregated distribution at the three study sites, the spatial distribution always being higher than 1 (Elliot, 1977). Despite seasonal variations, densities were consistently higher at Lavos beach in the Atlantic, than at Collelungo and Zouara in the Mediterranean (Fig. 3).

Despite differences in density, the pattern of seasonal variation appears to be very similar at Lavos and Collelungo: the lowest densities in late winter, a rapid increase

Spatial distribution and population abundance

Talitrus saltator showed a pattern of aggregated distribution at all the three sites. Talitrids are usually associated with freshly deposited wrack (Marsden, 1991b), which was confirmed with regard to T. saltator. This patchiness is evident at small spatial scales, e.g. of the order of 0.25 m−2, but becomes diluted at larger scales, since no significant correlation was found between population density and drift material biomass when account was taken of the whole set of replicates collected at

Acknowledgements

The authors are indebted to all colleagues who assisted in field and laboratory works. A special thanks goes to Maria Gabriel Fontes for her precious help in preparing the figures. This work was carried out in the scope of the research project ‘Bases for the integrated sustainable management of Mediterranean sensitive coastal ecosystems’, funded by the European Union (IC18-CT98-0270) and by the project POCTI/CTA/36553/00 from FCT. The research complies with the current laws in the countries

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