Population structure, growth and production of a recent brachiopod from the Chilean fjord region

Magellania venosa, the largest recent brachiopod, occurs in clusters and banks in high abundances of up to 416 ind m-2 in the fjord Comau, northern Chilean fjord region. At some stations, it numerically dominates the subtidal benthic community below 15 m depth, competing for space with the mytillid Aulacomya atra. To determine the question why Magellania venosa is a successful competitor, the brachiopod’s in situ growth rate was studied and its overall growth performance was compared with that of other brachiopods and mussels. The length growth was measured between Februay 2011 and March 2012 after mechanical tagging and Calcein staining. Settlement and juvenile growth were determined from recruitment tiles installed in 2009 and from subsequent photocensus. Growth of Magellania venosa is best described by the general von Bertalanffy growth function, with a maximum shell length (L∞) of 71.53 mm and a Brody growth constant (K) of 0.336 yr-1. The growth rate is the highest recorded for a rynchonelliform brachiopod. Maximal individual somatic production (PInd) is 0.29 g AFDM ind-1 yr-1 at 42 mm shell length and annual production ranges from 1.28 – 89.25 g AFDM yr-1 m-2. The high shell growth rate of Magellania venosa, together with its high overall growth performance may explain the high abundances of this brachiopod in the fjord Comau. However, the somatic production per biomass of the population ( -ratio) is low (0.535) and Magellania venosa may play a minor role in the food chain. Settling behaviour indicates that Magellania venosa is a pioneer species with low juvenile mortality. The brachiopod-bivalve coexistence suggests that neither the presence of potential brachiopod predators nor space competitors (i.e. mytiilids) affect the survival of the brachiopod population.