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

38 39 Magellania venosa, the largest recent brachiopod (Davidson 1886, McCammon 1973), 40 occurs in clusters and banks in population densities of up to 416 ind m-2 in the fjord Comau, 41 northern Chilean fjord region. Below 15 m depths, it co-occurs with the mytillid Aulacomya 42 atra and dominates the benthic community below 20 m. To determine the question why 43 Magellania venosa is a successful competitor, the brachiopod's in situ growth rate was stud-44 ied and its overall growth performance was compared with that of other brachiopods and 45 mussels. The length growth was measured between Februay 2011 and March 2012 after 46 mechanical tagging and Calcein staining. Settlement and juvenile growth were determined 47 from recruitment tiles installed in 2009 and from subsequent photocensus. Growth of 48 Magellania venosa is best described by the general von Bertalanffy growth function, with a 49 maximum shell length (L ∞) of 71.53 mm and a Brody growth constant (K) of 0.336 yr-1. The 50 overall growth performance (OGP index = 5.1) is the highest recorded for a rynchonelliform 51 brachiopod and in the range of that for Mytilus chilensis (4.8 ‒ 5.27), but lower than that of 52 Aulacomya atra (5.74). Maximal individual production (P Ind) is 0.29 g AFDM ind-1 yr-1 at 53 42 mm shell length and annual production ranges from 1.28 – 89.25 g AFDM yr-1 m-2 (1 ‒ 54 57% of that of Aulacomya atra in the respective fjord). The high shell growth rate of Magel-55 lania venosa, together with its high overall growth performance may explain the locally high 56 population density of this brachiopod in the fjord Comau. However, the production per bio-57 mass of the population (̅ ⁄-ratio) is low (0.535) and Magellania venosa may play a minor 58 role in the food chain. Settling dynamics indicates that Magellania venosa is a pioneer spe-59 cies with low juvenile mortality. The brachiopod-bivalve coexistence suggests that neither the 60 presence of potential brachiopod predators nor space competitors (i.e. mytiilids) affect the 61 survival of the brachiopod population. 62 63 64