First insights into population parameters and spatial distribution of the two morphotypes of Mactra stultorum (brownish and whitish) (Linnaeus, 1758) (Mollusca: Bivalvia) along the southwestern Spanish coast

ABSTRACT

The aim of this study was to address an analysis of spatial distribution, reproduction, population parameters and secondary production of the natural beds of the well-defined morphotypes of the clam Mactra stultorum (brownish and whitish variants) along the SW Spanish coast. The main findings showed that the presence and the frequencies of both morphotypes increased near the estuaries of rivers, particularly the Guadalquivir River. However, subtle differences in habitat preferences have been revealed and the brownish variant showed a wider and deeper distribution. High similarity in the gonadal development was recorded for both variants. It started in January, continued into late winter and early spring and ended with a spawning period from April to August. An energy storage-utilization cycle, coupled to the reproductive and resting periods and driven by seawater temperature variations and food availability, was also observed. The abovementioned period of gamete emissions resulted in a recruitment pattern with a one-time settlement period per year. Von Bertalanffy growth functions provided an asymptotic length of 46.7 mm (whitish) and 50 mm (brownish) and growth constants of 0.55 (whitish) and 0.50 year⁻¹ (brownish). The mean annual biomass ($\overline{B}$) ranged between 0.062 (whitish) and 0.076 g AFDW m⁻² year⁻¹ (brownish) (AFDW: ash-free dry mass). Individual somatic production showed the highest value at 32 and 34 mm shell length, and annual production (P) ranged between 0.075 and 0.113 g AFDW m⁻² year⁻¹, resulting in renewal rate (P/$\overline{B}$) values between 1.21 and 1.37 for the whitish and brownish variants, respectively. These results provide basic knowledge about the reproduction and population dynamics of this species along the Atlantic coast, particularly in the sandflats near the estuary of the Guadalquivir River. They could also shed additional light upon taxonomic differentiation and clarify the ecological role of both variants when compared with two bivalve sympatric species of the intertidal and subtidal areas.

KEYWORDS:

• Mactra stultorum
• reproduction
• growth
• production
• distribution
• Gulf of Cádiz
• SE

Acknowledgements

We would like to thank our colleagues at IEO (Centre of Cadiz), especially Ana Juárez, Verónica Duque, Esperanza García, Carlos Farias and Rafael Donato for collaborating in the biological sampling and technical support. We would also like to thank the owners and the crew of the ship (PITI-II) that was used in the sea surveys for their support in sampling. We also thank the ranger team of the Doñana National Park, the team of Doñana Biological Station (EBD-CSIC) and Lesley Crewdson for the English revision of this manuscript.

Disclosure statement

No potential conflict of interest was reported by the author(s).

The present study was supported by the project FEMP_AND_04 (‘Programa plurianual en el ámbito de la recopilación de datos de la comunidad autónoma de Andalucía: Análisis de la pesquería de coquina en el caladero del Golfo de Cádiz’) co-financed by the European Maritime and Fisheries Fund (FEMP); and the project VENUS (‘Estudio integral de los bancos naturales de moluscos bivalvos en el Golfo de Cádiz para su gestión sostenible y la conservación de sus hábitats asociados’) (0139_VENUS_5_E; INTERREG-POCTEP), co-financed by the European Regional Development Fund (FEDER, Interreg V-A España-Portugal (POCTEP) 2014–2020 program).