Population Characteristics of the Upper Infralittoral Sea Urchin <i>Arbacia lixula</i> (Linnaeus, 1758) in Eastern Mediterranean (Central Greece): An Indicator Species for Coastal Water Quality

The black sea urchin (<i>Arbacia lixula</i>, Linnaeus, 1758) is a non-edible marine echinoderm of high ecological importance with the potential to affect marine ecological communities. <i>A. lixula</i> were sampled monthly for one year from the supralittoral fringe at two locations in the Pagasitikos Gulf, in the north-western Aegean Sea. Morphometric characteristics exhibited significant spatiotemporal variation. The population in closer proximity to treated sewage effluent outflow exhibited significantly higher biometric relationships resulting in possible improved physiological conditions. Spatial distribution exhibited a clumped pattern of dispersion, consisting of predominantly six age classes. The dominant cohort was the four-year age class, comprising 31.2% of the total population. Significant negative allometric relationships were exhibited between all morphometric characteristics. The maximum approximate age of the total <i>A. lixula</i> population was estimated at 15.27 years. The von Bertalanffy growth equation for the entire population was estimated as: test diameter = <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mn>62.881</mn><mo>×</mo><mfenced><mrow><mn>1</mn><mo>−</mo><msup><mi>e</mi><mrow><mo>−</mo><mn>0.196</mn><mo>×</mo><mfenced><mrow><mi>A</mi><mi>g</mi><mi>e</mi><mo>+</mo><mn>1.147</mn></mrow></mfenced></mrow></msup></mrow></mfenced></mrow></semantics></math></inline-formula>. The gonadosomatic index indicated a seasonal cycle with a peak in late spring. The approximate age of sexual maturity was estimated at 4.45 years. We observed a significantly higher number of females than expected at the site in closer proximity to the treated sewage effluents (32% of total female number).