Submarine canyons are known to be hotspots for biodiversity (Gili et al. 2000, De Leo et al. 2010). Their geomorphology shapes the sediment transport patterns on the continental margin, enhancing productivity as they play the role of particulate and organic matter traps (Granata et al. 1999). Previous studies on such structures have shown that particles from shallow coastal adjacent areas can be transported into the greater depths of the canyon axis, where they sink to the bottom due to reduced horizontal advection and higher residence time (Ahumada-Sempoal et al. 2015). This phenomenon fertilizes the sea bottom and benefits benthic communities.

The western Mediterranean continental margin is cut by several submarine canyons, some of which have been thoroughly studied from a fisheries management perspective. These canyons are important fishing grounds for many fish and crustacean species of both ecological and commercial interest, which also use these areas as spawning, nursery or recruitment zones (Sardà et al. 2009, Fernandez-Arcaya et al. 2013). Among these species, decapod crustaceans are the most valuable group for the local fisheries industry, ranking the highest in selling prices. For instance, the deep-sea blue and red shrimp Aristeus antennatus, targeted by the bottom-trawling fleet in the submarine canyon areas, can account for up to 50% of the fishermen’s associations annual revenue and is subject to a local management plan in the area of Palamós, north of Blanes (BOE 2018).

Although most decapod crustacean species in the northwestern Mediterranean Sea have been widely studied in their adult form (Maynou et al. 1996, Company and Sardà 2000, Company et al. 2003), little is still known about their larval abundance and distribution, and even less in the case of deep-sea species due to the difficulty of sampling in offshore locations. This information is essential to assess the species dispersal and connectivity patterns. In the case of A. antennatus, only five larval stages have been described using morphological methods and confirmed by analysing molecular markers: the first protozoea (PZ I), subsequent to the hatched naupliar phase, protozoea II (PZ II), protozoea III (PZ III), mysis I (M I) and mysis II (M II) (Heldt 1955, Torres et al. 2014a, Carreton et al. 2019). As illustrated in a recent study, the PZ I of A. antennatus is morphologically very similar to that of Gennadas elegans, a small mesopelagic shrimp of no commercial interest, and the correct identification of this stage calls for a combination of morphological and molecular methods (Carreton et al. 2019, Carreton et al. unpubl data). First, these PZ I are distinguished morphologically from the rest of Dendrobranchiata PZ I by their smooth carapace, third maxilliped present, and antennule and antenna of similar length (dos Santos and Lindley 2001). They are then analysed through barcoding to differentiate whether they belong to A. antennatus or G. elegans. Although further stages can be clearly identified using only morphological characters, this difficulty with the PZI larvae is a drawback in decapod larval ecology studies in the northwestern Mediterranean Sea, where both species are an important part of the plankton community. In this area, previous studies on zooplankton have mainly focused on brachyuran crab larvae (Abelló 1989, Abelló and Guerao 1999) and others have given a broader view of the decapod larval community (Fusté 1987). It was only recently that a study tackled the larval distribution of decapod crustaceans in commercial fishing grounds, showing an occurrence of 25% of A. antennatus larvae in the Balearic Sea with an average abundance of 0.1 ind./1000 m³ (Torres et al. 2013). However, to our knowledge no studies have been reported in submarine canyon areas of the Mediterranean, despite their critical role in the reproduction of these species (Sardà et al. 2009). The recurrent meanders and eddies that affect submarine canyons produce a highly fluctuating hydrodynamic circulation pattern and transport (Ahumada-Sempoal et al. 2013). Settlement of decapod larvae has also been proved to respond to hydrodynamic conditions (Criales et al. 2013, 2015), so it is particularly interesting to acquire knowledge of larval distribution in these areas.

Blanes submarine canyon incises the continental margin of the northwestern Mediterranean Sea, from 41°N to 41.8°N and from 2.7°E to 3.2°E, off Blanes port. It has a north-south orientation and its width increases with depth until it reaches a maximum of 23 km at 2600 m depth (Canals et al. 2013). The continental shelf in this area of the eastern Iberian Peninsula is relatively narrow, with the upper canyon located only 4 km from the coastline. It is a deep and narrow canyon with a smooth eastern (upstream) wall and a sharp and abrupt western (downstream) wall. Water circulation in the area is generally cyclonic along the continental slope, as a result of the combination of the Atlantic Water arriving through the Gibraltar Strait and the Northern Current coming from north to south from the Gulf of Lions (Flexas et al. 2008). The authors suggest that the speed of the Northern Current starts to increase by the end of summer, entering the canyon, and reaches its maximum in the winter months, when the mixed layer becomes thicker. The interaction of the predominant SW flow with the bottom topography of the canyons may produce transient local mesoscale eddies and anticyclonic gyres over the continental shelf, thus causing “inversions" of the circulation pattern in the area northeastward (Rubio et al. 2005, Ahumada-Sempoal et al. 2013). Given the particularities of the geomorphology in the submarine canyon, hydrodynamics is likely to influence the larval distribution and abundance of particles and pelagic life forms such as eggs and larvae by modifying the retention rates and transport dynamics (Ahumada-Sempoal et al. 2013). The anticyclonic gyres produced over the canyon head may force them to aggregate over the shelf break around 150 m depth (Ahumada-Sempoal et al. 2015).

The present study aimed to assess the composition, distribution and abundance of the decapod larval community in Blanes submarine canyon under summer thermal stratification conditions. In particular, we expected to find the larvae of the deep-sea blue and red shrimp Aristeus antennatus, as the area is one of its main nursery grounds in the northwestern Mediterranean Sea. The linkage of the community structure with hydrological conditions is discussed.

This study offers initial information on the larval distribution of A. antennatus in the northwestern Mediterranean Sea. The decapod crustacean larval communities described differ from those of previous studies in the northwestern Mediterranean Sea (Fusté 1987, Torres et al. 2014a), mainly due to the predominance of larvae of the deep-sea blue and red shrimp Aristeus antennatus in our study. Our results support the hypothesis that the Blanes submarine canyon is an important spawning area for A. antennatus, because its first protozoeal stage, which occurs shortly after spawning, was found to be the dominant taxon in the decapod larval community at the end of the reproductive period of the species, i.e. late September. In addition, the occurrence of one PZ I larva of the species below the upper 70 m where they have been reported in earlier studies (e.g. Heldt 1955, Carbonell et al. 2010, Carreton et al. 2019) shows the importance of performing more stratified samplings to further evaluate the vertical distribution pattern of these larvae. These findings also support the model proposed by Carbonell et al. (2010), which considers that the first protozoeal stage performs an ontogenetic migration through the water column to surface layers, where the subsequent larval stages will develop.

The larvae of the family Sergestidae have been cited as a predominant taxon in the plankton of offshore waters of the northwestern Mediterranean throughout the year (Torres et al. 2014a, Simão et al. 2014). Some genera of this family, particularly Eusergestes arcticus, are accidentally caught in the nets of bottom trawlers in the area (Gorelli et al. 2016). The presence of protozoeal stages of these species in the submarine canyon indicates proximity to the spawning grounds. The mud shrimp Alpheus glaber is very common in Mediterranean waters, and ovigerous females of the species can be found in March and August (Zariquiey-Álvarez 1968). Its larvae have been previously found in the northwestern Mediterranean (Fusté 1987, Torres et al. 2014a), although in lower numbers than those obtained in the present study. The two Polycheles typhlops larvae from this study were identified through molecular methods, described, and included in a phylogenetic analysis (Torres et al. 2014b).

The highest diversity and species richness found in the submarine canyon head supports the idea that submarine canyons are key structures for many species, which aggregate in these areas for spawning (Sardà et al. 2009, Fernandez-Arcaya et al. 2013). The upstream and downstream walls of the canyon host significantly different communities of decapod larvae. Aside from the influence of the local circulation pattern, this distribution could be explained by a patchy distribution of zooplankton but, especially in the case of the early zoeal stages, it also provides information on the habitat preferences of the species during the spawning period. A paired study of decapod adult and larva distribution is desirable to account for dispersal capacity and habitat preferences of the different life stages of the species.

Modelling exercises of particle dispersal carried out in previous studies (Ahumada-Sempoal et al. 2015) have suggested that particles released upstream (on the eastern wall of the canyon) show greater retention rates over the canyon head than over its surroundings. In these theoretical experiments, retention of around 40% of the particles takes place at depths shallower than 100 m when the particles are released above the shelf break at 150 m depth. In our study, considering the water circulation described for Blanes canyon (Flexas et al. 2008), the general abundance values were higher at the stations that follow the main path of the Northern Current: B12, B11, B8, B19, B16 and B14.

The distribution pattern of the larvae was similar for early and late larval stages, but densities were higher overall for the former, probably due to their high mortality. A similar distribution pattern throughout larval stages indicates a good larval retention in the area. In addition, the circulation pattern in the canyon probably favoured the retention of larvae from adult individuals inhabiting the area as opposed to an import of larvae from nearby areas. The vertical distribution of larvae follows the depth distribution of summer chlorophyll a in the northwestern Mediterranean, with a deep chlorophyll maximum located above 100 m depth according to in situ historical data from the oceanographic buoy of the OOCS located at Blanes canyon head (Bahamon et al. 2011). A study including more stratified (MOCNESS) samples and thorough in situ oceanographic data needs to be conducted to further explore the influence of water mass circulation in decapod larva distribution.

We expected relatively high densities of decapod larvae in our area due to summer stratification conditions and the role of the canyon as a retention zone (Ahumada-Sempoal et al. 2015), but the overall larval density values obtained for our study were much lower than those of previous studies in nearby areas (Table 5). These values may be related to the sampling period being at the end of September, when most decapod species have ended their reproduction period and larvae have already settled and/or recruited to the adult habitats. The relatively low total density of crustacean larvae may also be related to the presence of a transient anticyclonic gyre, as reported by the oceanographic reanalysis data. According to Ahumada-Sempoal et al. (2015), this gyre affects the area during the summer months and is formed over the slope as a consequence of a counter-current flowing northeastwards, parallel to the coast (NE direction), opposed to the southward offshore current generated by northern winds. Meanders and anticyclonic eddies are recurrent in Blanes canyon, producing high fluctuations in the three-dimensional circulation patterns within the canyon (Flexas et al. 2008). Transient anticyclonic gyres of this sort may form over the canyon head, altering the general distribution pattern of particles and living organisms such as eggs and larvae (Ahumada-Sempoal et al. 2015).

Larval ecology remains one of the main knowledge gaps in marine science, and additional studies are still needed before we can move on to further analysis of the communities. In coastal species, biophysical models have helped understand the influence of oceanic currents and tides in the transport and settlement of decapod larvae (e.g. Pires et al. 2013, Criales et al. 2015). In the case of deep-sea species larvae, once evidence of larval presence has been reported, a reasonable next step is to explore the combined potential of stratified field samplings, oceanographic data and biophysical modelling to explain their distribution.

The decapod larval community of Blanes canyon at the end of September is dominated by three taxa: the deep-sea blue and red shrimp Aristeus antennatus, the family Sergestidae and the mud shrimp Alpheus glaber. Decapod larval communities are richer and more diverse at the canyon head than on the upstream and downstream walls and in the canyon axis.