Taxonomy and Ecology of Sympatric Ampelisca Species (Crustacea, Amphipoda) From the Strait of Gibraltar to the Strait of Dover, North-Eastern Atlantic

The Ampeliscidae Kröyer, 1842 is amongst the most diverse amphipod families; it comprises four genera, Ampelisca being the richest with more than 200 species. The Ampelisca genus presents high morphological homogeneity and the identification of the species by ecologists remains difficult. Ampelisca are also characterized by a high degree of sympatry, a rare situation in amphipods, and in this study we report up to nine species coexisting at the same site. Recent benthic sampling and publications, namely on the Portuguese continental shelf and the English Channel, permit to revisit the available data on the taxonomy and propose an updated species identification key, as well as the distribution and ecology of the 40-recorded Ampelisca species along the North Eastern Atlantic coast, from the Strait of Gibraltar, in the South, to the Strait of Dover, in the North. The data allow discussing on the sympatry and syntopy of such diverse amphipod family with the co-occurrence of several species at various scales of observations, from the wider regional area, to the narrower local habitat. Two Ampelisca species were recorded exclusively on hard bottom, while the other tend to inhabit specific types of soft bottom, ranging from deep mud to shallow coarse sand and gravel, with a preference for continental shelf muddy and sandy habitats. A future sea water temperature increase scenario could modify the species geographical distribution and reproductive cycle, in this temperate North-eastern Atlantic province.


INTRODUCTION
The Ampeliscidae Kröyer, 1842 is one of the most diverse amphipod families, together with the Gammaridae Leach, 1814 and the Lysianassidae Lana, 1849. Ampeliscidae is composed of four genera comprising more than 300 species, with 9 species of Byblisoides, 27 species of Haploops, 75 species of Byblis and 203 species of Ampelisca recorded on the World Register of Marine Species [http://www.marinespecies.org/; accessed on 16th October 2020). The Ampeliscidae family accounts nowadays for 314 species, representing 3.0% of the described amphipod species, 10,320 in total (Horton et al., 2021)]. New species for this family continue to be annually described. Between 2000 and2020, 3 Byblisoides, 14 Byblis, 9 Haploops, and32 Ampelisca new to science were described, corresponding to 18% of the known species of this family.
Numerous data and reviews on the Ampeliscidae (see Bellan-Santini, 1982, 1983Dauvin and Bellan-Santini, 1985, 1986, 1996, 2000, 2002Bellan-Santini and Dauvin, 1988a,b, 1989, 1997Dauvin, 1996) permitted to differentiate the general distribution patterns of the genera, Byblisoides, Byblis and Haploops being mainly deep-and cold-water genera and Ampelisca more tropical and sub-tropical and shallow water genus. Nevertheless, dense Haploops populations have been reported in South Brittany shallow waters, Northern part of the Bay of Biscay (Rigolet et al., 2012(Rigolet et al., , 2014, while dense Ampelisca populations occurred in North Brittany at the entrance of the English Channel (Dauvin, 1988a(Dauvin, ,b,c,d, 1989. Given the Ampelisca high morphological homogeneity, the taxonomic identification of specimen to the species level remains difficult to ecologists, in spite of the existence of illustrated keys (Dauvin and Bellan-Santini, 1988). As a consequence, most ecological studies tend to report only the presence of Ampelisca sp. or Ampelisca spp., without detailing the species. This difficulty arises in part from poorly detailed descriptions accompanying the reporting of new Ampelisca species, most being known only from the type locality and represented by very few specimens. Other difficulties include the existence of cryptic species, such as Ampelisca brevicornis sensu lato and the discovery of several species for this large cosmopolitan species, i.e., A. cavicoxa Reid, 1951 andA. pectenata Reid, 1951 for the North-Eastern Atlantic (Kaim-Malka, 2000) or more recently the description of A. troncosoi Tato et al. (2012) from Galicia, Spain, previously miss-confused with A. heterodactylta (Schellenberg, 1925;Tato et al., 2012). Additionally, the high level of taxonomic expertise and the long time required to identify the Ampelisca to species level, especially for large sample collections, may not always be compatible with numerous ecological work. Apart from the high diversity of the Ampelisca, another particularity of this genus is to present a high degree of sympatry, not only at the regional and local scales, but also at the scale of the grab replicate sample (Dauvin et al., 1993). The Ampelisca species tend to inhabit distinct types of substratum. The genus is mostly present on soft bottom, from deep mud to shallow coarse sand and gravel (Bellan-Santini and Dauvin, 1988a,b;Bellan-Santini and Dauvin, 1989) and only very few species are found on hard bottom (i.e., A. rubella A. Costa, 1864 or A. lusitanica Bellan-Santini and Marques, 1986).
Recent benthic sampling campaigns on the Portuguese continental shelf (Martins et al., 2012(Martins et al., , 2013a(Martins et al., ,b, 2014Sampaio et al., 2016) and in the English Channel, i.e., in the Rade de Cherbourg (Baux et al., 2017;Andres et al., 2020) and in the Bay of Seine (Alizier, 2011), permit to revisit the available data on the taxonomy, distribution and ecology of the Ampelisca species along the North Eastern Atlantic, from the Strait of Gibraltar, in the South to the Strait of Dover, in the North.

Portuguese Continental Shelf
A total of 326 sites were visited during sampling campaigns with a 0.1 m² Smith McIntyre grab (one grab per site) conducted on the entire Portuguese continental shelf from Caminha on the Northwest, to Vila Real de Santo António on the Southeast (Martins et al., 2012(Martins et al., , 2013a(Martins et al., ,b, 2014Sampaio et al., 2016). The shallow and mid depth north-western shelf and areas located close to the major submarine canyons are characterized by coarser sediments with low fines and organic matter content, whereas the south-western and the deep north-western shelf are dominated by fine sands with moderate fines and organic matter content. The western part of the southern shelf is very heterogeneous while muds predominate off the major Portuguese rivers, the Tagus (Lisbon) and the Douro (Porto) on the west and the Guadiana (Vila Real Santo de António), on the south coast (see namely Cardoso et al., 2019).

English Channel
New data was obtained mainly from the Rade the Cherbourg and the Bay of Seine. The Rade de Cherbourg is Europe's largest roadstead, extending over a total area of 15 km², with a maximum depth ∼20 m, and a mean depth of ∼13 m. The macrofauna was sampled with a 0.1 m² Van Veen grab (three replicates per site) for different studies from 2012 to 2018 on the four sediment facies of the Rade and in surrounding bays in the North Cotentin for a total of 61 sites (Baux et al., 2017;Andres et al., 2020). The eastern part of the Bay of Seine (eastern part of the English Channel) and the lower part of the Seine estuary cover an area of ∼ 400 km², with a maximum depth of ∼20 m. Macrofauna was sampled in 2008-2009(Alizier, 2011) and in 2016-2017(Baux, 2018 for a total of about 100 sites with a 0.1 m² Van Veen grab (three to five replicates per site).
communities from the Strait of Gibraltar to the Dover Strait and at a large range of sediment types, from muddy to gravely sediment. A Jaccard similarity matrix among the samples was obtained and exploited by cluster analysis using the average clustering algorithm and by ordination analysis, using non-metric multidimensional scaling (nMDS). The Jaccard Similarity Coefficient is most appropriate to analyze our data (presence of species in geographical areas, Table 1), because it is precisely devoted to study the similarity between samples solely on the presence-absence of the species. Other presence-absence similarity coefficients could be used, but the Jaccard Coefficient is a classic choice, possibly one of the most used in Ecology. Moreover, the Jaccard similarity between two samples also has a very straightforward interpretation, representing the proportion of the common species to the two samples. An ordination analysis was also performed using principal components analysis (PCA), following a Hellinger transformation, this analysis allowing the joint plot of the samples (the geographical locations) and the associated species. All multivariate analysis were performed with PRIMER v.6 (Clarke and Gorley, 2006).

Taxonomy
A total of 40 species were recorded along the North-eastern Atlantic coast from the Strait of Gibraltar to the Strait of Dover (Appendix A in Supplementary Material). Since the publication of the Ampelisca taxonomic key from the northeastern Atlantic by Dauvin and Bellan-Santini (1988), the number of species known to this area has increased. Two species, A. declivitatis and A. macrocephala, were added to the list of recorded species in this area (see namely Dauvin and Bellan-Santini, 1996) and four new species were described for science (Ampelisca cavicoxa, A. parabyblisoides, A. sorbei, and A. troncosoi). The following taxonomic key includes these additions. A1 corresponds to the first pair of antennae, A2 to the second pairs and P7 to the pereiopod 7. The main morphological characters which served to the identification of Ampelisca species and used in the following key are indicated in

DISTRIBUTION
A total of 40 Ampelisca species are currently known from the study area, comprehending the North-eastern Atlantic coast between the Strait of Gibraltar, in Southern Spain, to the Strait of Dover, in the Eastern part of the English Channel (Figure 2). The presence of these 40 species were reported to eight geographical locations, indicated in Table 1, following the studies consulted and mentioned in Appendix A in Supplementary Material. The number of species varied from a maximum of 27 along the Portuguese coast, to a minimum of 7, in the eastern part of the English Channel. The second richest area was the southern part of the Bay of Biscay with 24 species, including eight deep-water species. There was a clear reduction of the number of species from the south to the north and undoubtedly a lack of data for two areas: the southern Atlantic coast of Spain and Galicia.
Five species were present in all areas: A. brevicornis, A. diadema, A. spinipes, A. tenuicornis, and A. typica. A. pectenata showed also a large distribution but was absent in Galicia and Southern Spain. Twelve species were recorded only in the southern part of the study area; most were Mediterranean and recorded in the Atlantic up to the southern coast of Portugal, in Algarve Bellan-Santini, 1991, 1993;Sampaio et al., 2016): A. anophthalma, A. calypsonis, A. dalmatina, A. latifrons, A. lusitanica, A. massiliensis, A. multispinosa, A. provincialis, A. pseudosarsi, A; pseudospinima, A. ruffoi and A. verga.
Ampelisca aequicornis was present only in the Bay of Biscay. The Southern species were distributed in the northern areas up to Galicia such as A. serraticaudata or the southern part of the Bay of Biscay such as A. heterodactyla, A. rubella and A. serraticaudata. Ampelisca gibba was absent in the English Channel probably due to the absence of a mud habitat in this megatidal sea due to high hydrodynamics and dominance of coarse sediments. Three species, A. armoricana, A. sarsi, A. spinimana were absent in the eastern part of the English Channel. Five species showed a limited spatial distribution, possibly due to lack of data or because they were recently described: A. caxicoxa, A. remora, A. sorbei, A. troncosoi and A. toulemonti.
Eight deep-water species were only reported in the bathyal part of the Bay of Biscay, possibly because of the deeper sampling solely performed in this study location: A. amblyops, A. anomala, A; declivitatis, A. eschrichtii, A. parabyblisoides, A. odontoplax, A. pusilla and A. uncinata.
Among the new observations in the study area, Ampelisca verga, a West African species originally described as a variety of A. aequicornis by Reid (1951, in Dauvin andBellan-Santini, 1985) off Dakar, Senegal, was recorded in the Algarve coast, southern Portugal, setting a new northern distribution limit for this species (Sampaio et al., 2016). Ampelisca toulemonti, described from one female coming from the Iroise Sea (Dauvin and Bellan-Santini, 1982), was recorded in the eastern part of the Rade de Cherbourg in the North Cotentin, setting the eastern most location of this species in the English Channel (Andres et al., 2020).
Among the 40 species (Appendix A in Supplementary Material), 17 were found only on the Continental Shelf, 16 were recorded both on the Continental Shelf and the Continental slope at depths up to 510 m. Seven species were strictly bathyal with a maximum sampling depth of 1,097 m.
A data matrix representing Table 1, 40 species × 8 locations, was analyzed to study the faunal resemblance among the samples representing the geographical locations.
The dendogram (Figure 3) showed high similarity of the fauna between the Iroise Sea and South Brittany, and between Southern Spain and Portugal. This cluster analysis showed two main groups, which were split at a similarity level of 35%, the southern group, including Southern Spain and Portugal, from the northern group, with the other locations. Within this group, at a level of 40% the South Eastern Bay of Biscay was separated, and at 50% Galicia was also separated from the other northern locations.
The nMDS (Figure 2) showed the opposition, along the horizontal axis, of the most northern and southern groups. The central locations (Bay of Biscay and Galicia) appeared in a transition position along this axis, between the most southern and northern. The vertical axis isolated the Bay of Biscay. A similar ordination solution was shown by the PCA analysis (Figure 4), with the succession along the horizontal axis, from left to right, of the most northern to the most southern locations, with the separation on the positive pole of axis 2 of the samples from Bay of Biscay. This analysis also depicted the species, as vectors. The southern group (South Spain and Portugal) was characterized by 11 species, most of them having a Mediterranean distribution. The Western and the Eastern English Channel, the Iroise Sea and South Brittany areas were characterized by seven species, largely distributed along the south/north gradient and present in the northern areas. The south eastern Bay of Biscay was characterized by 17 species, among which the deep-water Ampelisca species.

ECOLOGY Abundance
The denser populations of Ampelisca were registered in the northern part of the study area ( Table 2). Most of the species occurred rarely and were found in very low numbers of individuals, which, for some species, corresponded only to the specimens used for their description. With the exception of A. lusitanica and A. rubella, which occurred on hard-bottom, Ampelisca were mostly found on soft-bottom sediments, ranging from mud to gravel (Bellan-Santini and Dauvin, 1988bDauvin, , 1989). Most of the species recorded in the study area inhabited mainly muddy sand and sandy mud habitats and were abundant only in  shallow muddy fine sand sediment (Bellan-Santini and Dauvin, 1988aDauvin, , 1989Sampaio et al., 2016). The common species formed abundant populations in the English Channel. Ampelisca brevicornis reached 500 ind.m − ² in October 1982 in the Bay of Morlaix Bay, 403 ind.m² in July 1978 and 370 ind.m² in October 1979 in the Rance (Dauvin, 1988b). An abundant population was also reported in the subtidal fine sand of the Bay of Saint Brieux with 660 ind.m − ² (Le Mao, 2006).
Ampelisca tenuicornis formed very high abundance in the Rance, with 6,020 ind.m − ² in summer 1978, 3,870 ind.m − ² in summer 1979 and 2,830 ind.m − ² in summer 1980 (Dauvin, 1988a), and showed a peak of abundance at the end of September 1996 with 25,000 ind.m − ² (Desroy, 1998). In the Bay of Morlaix, its abundance reached 4,000 ind.m − ² in October 1977 (Dauvin, 1988a) and 17,500 ind.m − ² in October 1997 (Dauvin, unpublished data). Long-term study at a fine muddy-sand site to the east of the Isle of Wight on the south coast of England (depth 7-9 m) its abundance reached a maximum of about 40,000 ind.m − ² in late summer (Sheader, 1998). In the North Cotentin, the population reached 9,000 ind.m − ² in the North Cotentin (Andres et al., 2020), while its underpassed 1,000 ind.m − ² in the eastern part of the Bay of Seine (Alizier, 2011).

Sympatry in Ampelisca
Ampelisca populations often occur together, so that their distribution areas overlap or coincide. Several species can occur together in the same habitat and it is possible to identify up to eight species in 1974 in the same station in the Bay of Concarneau, South Brittany (Mesneguen, 1980) and nine species in two stations off the Portugal coast in 2007-2008 (Table 3).
In the western English Channel, Bay of Morlaix, nine species were recorded: A. armoricana, A. brevicornis, A. diadema, A. sarsi, A. spinipes, A. spinimana, A. pectenata, A. tenuicornis and A. typica (Dauvin et al., 1993). In 1977, on a subtidal fine sand community of this bay, three species were dominant A. armoricana, A. sarsi et A. tenuicornis (several thousands of individuals per m²) representing 90% of the abundance, 38% of the biomass and 50% of the secondary production of the community (Dauvin et al., 1993). The Ampelisca diversity (eight recorded species) and abundance were lower (< 100 ind.m −2 , cf. Table 3) in a muddy sand community of the Bay of Morlaix at the same period. The Ampelisca were shown to be very sensitive to hydrocarbon pollution and as a consequence to the Amoco Cadiz oil spill, which occurred in March 1978 in North Brittany (western English Channel), the species disappeared in the fine sand community (Dauvin, 1988a,b,c,d;Dauvin, 1989). Similar population collapse was also observed following the Aegean oil spill in the North of Galicia, Spain (Gomez Gesteira and Dauvin, 2005). In 1990, 12 years after the Amoco Cadiz oil spill, the abundances of the Ampelisca were at the same order of magnitude of those observed before the incident (Table 3). Ampelisca sarsi dominated the community, replacing A. armoricana which became the second most abundant species, while A. tenuicornis remained the third most abundant; the three other species showed lower abundances before and after the spill. The longterm survey of the colonization of Ampelisca illustrated the high resilience of these holobenthic species, without pelagic larvae, but with high capacity to reconstitute their populations (Dauvin et al., 1993).
In other northern sites, the Ampelisca also showed sympatric distribution; nevertheless, the abundances of the populations never surpassed 500 ind.m −2 ( Table 3).
On the 326 stations off the Portuguese continental shelf, Ampelisca were found in 221 stations (68% of the sampled stations) for a total of 19 species and the dominance of five species A. armoricana, A. brevicornis, A. spinimana, A. spinipes and A. tenuicornis. From two to nine species were found in 60% of the stations, while only one species was recorded in 40% of the stations. Nine stations accounted five species, nine other six species, four seven species, four eight species and finally nine species had been found in two stations ( Table 3). The total abundances of such sympatric populations were high and included between 1,240 and 1,920 ind. m −2 ( Table 3).

Taxonomy and Distribution
The North-eastern Atlantic Coast between the Strait of Gibraltar and the Strait of Dover registers 40 Ampelisca species, which is more than the total number of Ampelisca recorded for the Mediterranean Sea, 28 species (Bellan-Santini and Ruffo, 2003). The number of species reached was on the same order of magnitude along the Portuguese coast with 27 species where there is a mixture of Mediterranean, Atlantic and African faunas. The second richest area was the southern part of the Bay of Biscay with 24 species, including eight deep-water species, and 16 species habiting the continental shelf, number which remained lower than those of the Portuguese continental shelf. The biodiversity of Ampelisca showed a clear decrease from the south to the north in the studied area.
The Ampelisca taxonomy is well-established for this North-Atlantic area, and almost 50% of the species (17) had already been described by the end of the nineteenth century, with an extra 19 species during the second part of the twentieth century, while a single species, Ampelisca troncosoi Tato et al. (2012) was recently described.
Presently, new species of Ampelisca at the scale of the world are mainly described in the tropical zone of the Pacific Ocean and in the Indian Ocean (World Register of Marine Species, consulted on November 1st, 2020). In the early years of the twentieth century, the amphipod fauna of the European part of the North-Atlantic Ocean was amongst the better known worldwide, including the Ampelisca mainly due to the large number of new species descriptions by G.O. Sars, and E. Chevreux at the end of the nineteenth and the beginning of the twentieth century. Numerous new species were then described at the end of the twentieth century by D. Bellan-Santini and her co-authors, mainly for the Mediterranean Sea and surrounding areas (North Africa, Portugal and Spain).
Therefore, the discovery of new Ampelisca species for science in the study area is improbable. Nevertheless, genetic studies could be used to elucidate the existence of species hidden in "complex" species with a large geographical distribution such as Ampelisca brevicornis, for which sub-species could be morphologically distinguished (Kaim-Malka, 2000).

Changes in the Species Biology and Distribution in Relation to Climatic Changes
Climatic changes with an increase in sea water temperature along the North-eastern Atlantic coasts could affect both the distribution and the biology of the Ampelisca species.
Concerning the distribution, species tend to extend north their geographical reach, as observed for A. armoricana, A. sarsi, A. spinimana and A. toulemonti in the eastern part of the English Channel, and A. aequicornis and A. spinifer in the English Channel. Southern species presently known to occur along the North Atlantic coast of Africa could progress north and reach the south of Spain and the Portuguese coast, such as A. bidentata, A. ctenopus, A. hupferi, A. monoculata, A. palmata and A. senegalensis (Dauvin and Bellan-Santini, 1988). In both areas, the south of the Iberian Peninsula and the English Channel, encouragement should be given to identify the Ampelisca up to species level, in order to fully grasp such geographical changes in target species.
Concerning the biology, two reproductive cycles are known in the Ampelisca species (Bellan-Santini and Dauvin, 1988b). Univoltine cycles occurs namely in A. armoricana (Dauvin, 1988d) and A. sarsi (Dauvin, 1989). Females are ovigerous at the end of spring and release their young in summer, which reproduce only 1 year later, leading to a single generation and class per reproductive year. Other species present a bivoltine cycle such as A. tenuicornis (Dauvin, 1988a) and A. typica (Dauvin, 1988c). In these, the females are ovigerous at the end of winter-beginning of spring and release their young in spring, which reproduce at the end of summer, beginning of autumn, leading to two generations and one class per reproductive year. Some species are known to show both cycles, depending on the environmental conditions. This is the case of A. brevicornis, which showed both reproductive cycles in the English Channel depending on the sea water temperature, with a bivoltine cycle in years with warmer spring (Dauvin, 1988b), while being solely bivoltine in the Mediterranean Sea (Kaim-Malka, 1969). Sea water increase notably in spring could favor a bivoltine reproductive cycle in the future, which will change the secondary production of such amphipods. Rivas (1964) defined sympatric and syntopic species, which corresponded, respectively to "the reference to two or more related species which have the same or overlapping geographic distributions, regardless of whether or not they occupy the same macrohabitats (whether or not the species occurred together the same locality, " and "in reference to two or more related species which occupy the same macrohabitat. These species occur together in the same locality, are observably in close proximity, and could possibly interbreed." The large geographical and local distributions of Ampelisca species illustrate plainly these both concepts. With a high species richness at the scale of a region, such as along the Portuguese coast with the overlapping of 27 species, and the number of species occurring in the same benthic habitat, such as on Brittany and Portuguese soft-bottom habitats, with up to eight species in the same habitat in South Brittany and nine species in two stations from the Portuguese continental shelf ( Table 3).

Sympatry and Syntopy in Ampelisca
Co-occurrence, sympatry and syntopy of amphipods were not very common and mainly described to species living in the intertidal zone or very shallow waters such as the Haustoriidae, where five species cohabited on the New Hampshire intertidal zone (Croker, 1967), the Gammaridae with the case of five species of Gammarus coexisting in Danish brackish waters (Kolding and Fenchel, 1979), Pontoporeiidae with two Pontoporeia cooccurring in the Swedish Baltic Sea (Hill and Elmgren, 1987), Hyalidae where six phytal amphipod species of the genus Hyale occurring on the intertidal rocky shores of Coquirnbo, Chile (Lancelotti and Trucco, 1993), Talidridae with Talorchestia brito and of two age classes of Talitrus saltator co-existing along the French Atlantic coast (Fallaci et al., 1999) while eight species were collected in some Tunisian lagoons (Jelassi et al., 2015), and Ischyroceridae with three species of the genus Jassa co-occurring on a wide range of hard substrates in the Helgoland Island in the south of the North Sea (Jelassi et al., 2015).
In the Bay of Belfast, Parker (1984) examined the distribution of Ampelisca brevicornis, A. tenuicornis and A. typica. He showed that these species could live in the same biotope and had no sedimentary preferences in this region, contrary to Sheader (1977) who showed marked sedimentary preferences for two species A. brevicornis and A. tenuicornis in the sandy-muddy bottoms of north-eastern England, in the North Sea.
Few studies, such as that of Buhl-Jensen (1986) on the coasts of Norway had showed the coexistence in the same samples of two to four species of Ampelisca although one species, A. gibba presented high abundances in three of the 21 sampling stations. In the Bay of Concarneau (southern Brittany), Mesneguen (1980) recorded until eight species per station in 1974, but the abundance of the species remained lower than those observed in the Bay of Morlaix in the western entrance of the English Channel in 1977 ( Table 3). The Ampelisca Portuguese continental shelf fauna appeared particularly rich with 19 species recorded in the sampling campaigns in 2007-2008 covered the entire Portugal coast with a more dense number of stations in the North (Sampaio et al., 2016). This was in this location that nine Ampelisca species in a single grab (0.1 m²) were sampled, while between seven to nine species were found in 10 stations. This cooccurrence of Ampelisca species in a single grab per station was at our knowledge a record and was incomparable.
Along the North-America Pacific coast from the Baja California to the Bering Sea, large Ampelisca populations coexisted but only a single or a couple of species dominate the soft-bottom communities: A. macrocephala and A. eschriichti in the Bering Sea, A. agassizi and A. careyi off Vancouver Island (Canada), while in the Baja California only A. agassizi was present (Oliver et al., 1983). Schaffner and Boesch (1982) studied the spatial distribution of A. agassizi and A. vadorum in the palaeo-dunes of the continental shelf off New Jersey (USA), which reach 6 m high.
The simultaneous sampling of these two species was strongly linked to the exact position of the sample on the dunes: the deep depressions were almost exclusively populated by A. agassizi, reaching in this habitat abundances of 10,000 individuals per m². The shallower depressions and the sides of the dunes were populated by both species. The authors suggested that A. agassizi could be more capable to use nutrient resources.
In the end, if the coexistence of Ampelisca species seems to exist most often, the coexistence of large populations such as those observed in Morlaix Bay seems exceptional.

Perspectives
In conclusion, the following research recommendations for the European Ampelisca can be put forward: • Ampelisca species with a wide geographical distribution should be further studied using molecular tools in order to elucidate their taxonomy. Such studies could focus in the species A. brevicornis, A. diadema, A. tenuicornis, A. typica and A. spinipes; • Encourage ecologists to identify Ampelisca to species level in order to increase the overall knowledge on their distribution, including the geographical ranges; • Study in detail, and possibly experimentally, the reasons behind the important sympatry in Ampelisca and how it relates namely to resources competition; • Verify if trophic guilds, namely suspension feeders and surface deposit feeders, present distinct morphological characteristics, such as length of antennae, presence of setae, and number of setae, and if such adaptations could account for the coexistence of several species without trophic competition; • Conduct population biology studies at historical sites where the species were previously studied in order to examine the effects of climate change, namely increased temperature in coastal waters, on the life history traits of the Ampelisca.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

AUTHOR CONTRIBUTIONS
J-CD and VQ designed research. J-CD, LS, AR, and VQ performed research and analyzed data. J-CD and VQ wrote the paper. All authors contributed to the article and approved the submitted version.