Growth phenotypes mediated by environmental and endogenous cues in Mytilus galloprovincialis

Gills are the main organ for food and oxygen uptake, one of the favourite sites for parasite infestation and key for the development of differential growth rates in mussels. Growth and growth patterns might be compromised by stressful conditions in the field, as well as reproduction, which might in turn be affected by endogenous differences in growth rate. We hypothesized that (i) the response to parasitic infection is different in the growing phenotypes (fast and slow growing individuals) and that (ii) different gametogenic developments appear in the different growth phenotypes based on their differential ability to acquire, process and store energy. Two sets of 300 mussels were placed in two cages anchored to the breakwater: low and high intertidal. Caging lasted for 12 months and samples were taken after 6 (May) and 12 months (November) to measure the standard metabolic rate, gill surface areas, histological alterations, parasitic prevalence and gametogenic stages. The slow-growing phenotype is caused by underdeveloped gill and digestive gland tissues, not by parasitic infection of these organs. Their lower energy intake options provide them lower energy remnants, allocating the energy primarily into gonadal development and spawning earlier in the season. Fast growing mussels are better equipped, and even seasonal-, habitat- or parasite-induced high levels of atrophy in their digestive gland do not prevent them on having more energy available for growth, allowing them to cope with higher parasitic loads and developing more somatic growth as well as gonadal growth.