Bivalves can provide a wide range of ecosystem functions. For example, they can improve the water quality of the ecosystems due to their filter-feeding capacity to mitigate the effects of eutrophication (Valli et al. 1994, Ostroumov 2005, Coen et al. 2007, Lindahl and Kollberg 2009). Bivalves also promote the structures of ecosystems with their biomass, skeletons and burrowing activity (Dame 1996) and therefore contribute to the increase in habitat heterogeneity for fishs and invertebrates (Gutiérrez et al. 2003, Coen et al. 2007). They can also transfer chemical elements and nutrients from the water and seston to other trophic levels and to the sediments, linking benthic and pelagic ecosystems (Prins et al. 1998, Newell 2004). Moreover, many bivalve species play important economic roles through fisheries and aquaculture.

The smooth clam Callista chione (Linnaeus, 1758) is a venerid bivalve that inhabits coarse sand bottoms in the east Atlantic Ocean, from the UK down to Morocco, the Archipelago of Azores, Madeira and the Canary Islands, although it is more common in the Mediterranean Sea than elsewhere (Poppe and Goto 1993, Gofas 2013). The shell length can reach 100 mm and it is glossy and easily distinguished from any other European sea shell (Poppe and Goto 1993). The importance of C. chione is both ecological and economical. Baeta and Ramón (2013) reported that this bivalve was one of the main preys of the seastar species of the genus Astropecten in the western Mediterranean Sea, and they are also consumed by other invertebrates such as the gastropod Hexaplex trunculus (Morton et al. 2007) and the cephalopod Octopus vulgaris (Fiorito and Gherardi 1999). C. chione is commercially exploited and consumed in Portugal and the Mediterranean Sea (Gaspar et al. 2001, Tirado et al. 2002, Metaxatos 2004, Pubill et al. 2011, Baeta et al. 2014) where it is captured with small boats using different types of artisanal dredges. One of the main coastal shellfish areas where a major clam fishery targeted the smooth clam is the Maresme coast, located to the northeast of Barcelona (NW Mediterranean Sea). As reported by Baeta et al. (2014), the local administration commissioned a study of the smooth clam bed status in 2004 as a consequence of the decline in catches. The results prompted a temporary bed closure and an annual closing season in spring for the following four years. However, the population did not recover and the main causes of the huge reduction of the resource were overfishing and sand dredging (Baeta et al. 2014).

The reproductive system of bivalves consists of a pair of gonads, which in some species are fused in the middle (Galtsoff 1961). The gonads are made of branching tubules where gametes develop in the epithelial lining (Gosling 2003). The tubules join to form ducts leading into progressively larger ducts that terminate in a short gonoduct (Gosling 2003). Most of the bivalves are dioecious, that is, they have separate sexes. However, it is usually very difficult to distinguish the different gonads macroscopically, so the use of microscopic techniques is essential to differentiate sexes (Tirado et al. 2002). The reproductive cycle in bivalves involves gonadal development, ripeness of the gametes and spawning, but there is an enormous variation in the timing and duration of each stage both within and between species (Gosling 2003). Many exogenous (mainly temperature) and endogenous factors are involved in the reproduction processes. The role of these factors in the initiation and duration of the gametogenesis differs from their role in the timing and synchronization of spawning, a much briefer event (Seed and Suchanek 1992). The study of the reproductive cycle in bivalves is essential to understand the biology of the species and to develop an appropriate fisheries management. For example, size at first maturity is one of the parameters commonly evaluated to assess the management of fish and shellfish stocks (Walker 1992, Williams and Babcock 2005, Camacho-Mondragón et al. 2012).

The aim of this study was to investigate the reproductive cycle of C. chione in the Maresme coast (NW Mediterranean Sea) and to provide the first estimation of size at first maturity in a Mediterranean population. For the purpose we used histological techniques to study the reproductive stage, sex ratios and quantitative measurements to determine oocyte diameters. We also described the spermatozoa using scanning electron microscopy. Our research reports the first case of hermaphroditism in this species.

The bivalve C. chione is able to reproduce the whole year round with a spawning peak in spring. The gonadal development of this species had been previously studied through histological analysis on two occasions in the Mediterranean Sea (Valli et al. 1994, Tirado et al. 2002). In those studies, C. chione was also found to be capable of reproducing the whole year round. Tirado et al. (2002) observed three spawning peaks in southern Spain, finding the main emission of gonads in spring, as observed in our study. Valli et al. (1994) observed that the spawning period occurred in spring and summer in the Gulf of Trieste, with individuals increasingly spawning until a major peak in August. The gametogenic cycle of C. chione described in Portugal reported three spawning peaks and the release of the gametes lasted from January to October. Water temperature and nutrient supply are known to directly affect the physiological status of bivalves, regulating gametogenic cycles and spawning, among other physiological processes (Wilson and Hodgkin 1967, Hilbish and Zimmerman 1988, Gosling 2003, Morgan et al. 2013). In agreement with these studies, we found a very strong relationship between the amount of ripe individuals and the water temperature. Therefore, we hypothesize that the differences and similarities found between the gonadal cycle reported in the present study and the previous ones are most likely caused by temperature and food supply inter-site variations, though data on food supply are needed to support this hypothesis.

The sex ratio of C. chione was 1:1, as previously described in other populations (Tirado et al. 2002, Metaxatos 2004, Moura et al. 2008). Our study reports the first cases of hermaphroditism for C. chione. In the Veneridae, hermaphrodite individuals have been found in two different species, Ruditapes philippinarum (Delgado and Pérez-Camacho 2007, Lee et al. 2013), with frequency values similar to our findings (Drummond et al. 2006), and Megapitaria squalida (Romo Piñera et al. 2009). The high hermaphrodite frequency (between 21.8% and 23.5%) for M. squalida was related to the low population density resulting from overfishing (Romo Piñera et al. 2009). The smooth clam population studied here was also affected by overfishing, but the frequency of hermaphrodites was much lower than what reported by Romo Piñera et al. (2009) for M. squalida. Therefore, we do not think that overfishing would cause hermaphroditism in C. chione, as hermaphroditic individuals have occasionally been found in bivalve species strictly considered gonochoric.

There are some discrepancies in the size of C. chione oocytes: whereas Metaxatos (2004) reported oocyte sizes ranging from 54.5 to 130 µm and Pérez-Larruscain et al. (2011) described a mean oocyte diameter of 89.5 µm under laboratory conditions, Moura et al. (2008) reported a ripe oocyte size of only 36 µm diameter. Our results agree with the studies done in the Mediterranean Sea, i.e. Metaxatos (2004) and Pérez-Larruscain et al. (2011), finding bigger sizes which are in the size range of other venerid species (Mouëza et al. 1999, Chung 2007, Popović et al. 2013). The differences observed in the size of oocytes from Atlantic and Mediterranean smooth clam populations were related by Moura et al. (2008) to the tissue sample state (preserved or fixed) and also to genetic differences. The size of the sperm cells and acrosome agree with those described by Nicotra and Zappata (1991) in C. chione and are in the size range of other Veneridae species (Gwo et al. 2002).

Most of the C. chione dissected in the present study had developed sexual structures, in contrast to previous studies, including the smallest bivalves analysed in both periods (18.8 mm and 20.6 mm length in 2005-2006 and 2014, respectively). Moura et al. (2008) reported that individuals might start spawning at 30 mm length, and the size at first maturity was estimated at a shell length of 50.81 mm on the southwestern coast of Portugal. In a later work, the same authors reported that C. chione needed more than four years to reach sexual maturation (Moura et al. 2009). In contrast, Metaxatos (2004) studied smears of C. chione gonads and reported that 25% of one-year-old smooth clams could be sexed, but gonadal maturation for the whole population began at the age of two years old. The author also reported that gonads were discerned in C. chione individuals larger than 12 mm. Size at first maturity found in the Mediterranean for C. chione in this study is much smaller than that reported by Moura et al. (2008) in the NW Atlantic. There is no agreement on the best method for estimating the size at first maturity in bivalves, as there are differences in the method of observation of the gonads and/or the period of the reproductive cycle sampled. Some authors analyse histological sections of samples collected in the time period when adult specimens are ripe (Moura et al. 2008, Delgado et al. 2013), which requires previous knowledge of the reproductive cycle of the species. Other authors estimated the SL₅₀ using samples collected monthly throughout one year, using two different methods: microscopic analysis of histological sections of the gonads (Camacho-Mondragón et al. 2012) or microscopic examination of smears of the sexual products (Derbali et al. 2009). Despite these differences, all methods are considered appropriate and commonly used for the SL₅₀ estimation.

It is a common fisheries management practice to impose a minimum legal catch size greater than the size at first maturity, in order to allow the individuals to participate in spawning for some time before being fished (Williams and Babcock 2005). In this study we report a decrease in the SL₅₀ for a C. chione population, which is to our knowledge the first mention in an overexploited bivalve population. We cannot assure that the nearly 9 mm SL₅₀ decrease observed between 2005-2006 and 2014 was not affected by the different methodology used here to determine the value in each period, but the results are beyond the particular value and strongly suggest a downward trend. From a management point of view, this information is important to promote the periodical revision of this parameter as changes in the life history traits of the species occur over time. Changes in size and age at first maturity in many fish stocks often occurred after the stock abundances declined sharply. For example, after stock collapses in the early 1990s, both size and age at first maturity declined sharply for the cod (Gadus morhua) stocks in the northwest Atlantic (Saborido-Rey and Junquera 1998, Hutchings 2005). In mollusks, including gastropods and bivalves, differences in size at first maturity have been reported between populations of the same species (Hooker and Creese 1995, Cruz et al. 2000). In the gastropod Haliotis iris, such differences might be related to differences in growth rates between populations (Hooker and Creese 1995). Furthermore, McShane and Naylor (1995) found differences in size at maturity and growth rate of the gastropod H. iris at small spatial scales between headlands and bays separated by as little as 200 m. Cruz et al. (2000) reported that the differences in age and size at first maturity of the bivalve Argopecten ventricosus from two populations might be a consequence of both internal triggering mechanisms and environmental conditions. Changes in the biology of the species may also represent a plastic response to changes in the environment, such as compensatory density-dependence in growth and maturation, or a direct response of maturation to increased body condition or to temperature. In this study no environmental information was available (chlorophyll a, seston concentration), apart from surface sea water temperature, to investigate the possible causes for the reduction in SL₅₀, but our results reinforce the need to revise this tool periodically to design management strategies.