An Update of Amphipoda Checklist for the English Channel

: An updated checklist for 2022 of amphipods from the English Channel (La Manche) is given for eight geographical zones. This revision brings the inventory of 1999 up to date with recent data from the Normano-Breton Gulf and other studies focused on non-indigenous fauna, as well as recent data from benthic and suprabenthic studies, mainly in the eastern part of the English Channel (EC). The total number of species in the entire EC is 269. The number of species is much higher in the western basin (WEC) than it is in the eastern basin (EEC) or in the central part of the EC. The amphipod species listed here are distributed between the eight zones as follows: French WEC: 201 species; English WEC: 194; Normano-Breton Gulf: 224; Bay of Seine: 172; Wight: 97; French EEC: 149; English EEC: 64; and Central EC: 61. Of these species, 180 are present in both basins of the EC, while 78 are present only in the western basin and 11 are present only in the eastern basin. The low number of amphipods (<100) recorded in three of the eight zones is probably due to the lack of observations in these parts of the EC. Among the 269 amphipod species recorded with conﬁdence in the EC, 24 are new to the EC since 1999, 12 are non-indigenous species, and 44 are observed only in one of the eight zones, mainly in the three zones of the western basin of the EC.


Introduction
More than 20 years ago, a checklist of amphipods recorded in the English Channel (EC) was published by [1]. It was based on data from [2,3], inventories of amphipods drawn up by the marine stations of Roscoff [4], Plymouth [5], and Wimereux [6], as well as numerous articles focused on amphipods or the marine fauna in several parts of the EC [7][8][9][10][11][12][13]. Moreover, Ref. [14] gives a fully updated list of marine invertebrates, including amphipods, from the Normano-Breton Gulf. Newly developed methods such as experimental plate immersion or scraping of dikes and pontoons have been used to study the sessile and motile species that colonize hard substrates in coastal environments, mainly in Normandy harbours and marinas [9,12]. This has led to a significant increase in our knowledge of these particular environments and has allowed us to detect the presence of several non-indigenous species (NIS) of amphipods. Suprabenthic sledge sampling near the sea bottom in the EC has also facilitated the collection of mobile fauna [7,8].
Moreover, diverse human activities in the EC have led to impacts on the macrobenthic communities. As a consequence, several surveys have been carried out in the Bay of Seine and the eastern part of the EC related to granulate extraction, the dredging and deposition of spoil, and the future implementation of offshore wind farms [15].
Over the past two decades, additional data have become available to revisit and redefine the list of amphipods recorded in the EC. In the present study, amphipod species checklists are given for eight geographical zones of the EC. Previously, Dauvin (1999) [1] provided amphipod checklists for only five zones, where available data were more or less complete due to the existence of Marine Station Inventories, i.e., Roscoff for the western EC along the French coast, Plymouth for the western EC along the British coast, Dinard for the Normano-Breton Gulf, Luc-sur-Mer for the Bay of Seine, and Wimereux for the eastern part
Six species are added for the English side of the Western EC: Caprella mutica Schurin, 1935, from Plymouth harbour [17]; this species was also present in the Normano-Breton Gulf, the Bay of Seine, and along the English coast, Wight, and the eastern EC, confirming that C. mutica is a well-established species in the EC [12,17,47,84]. The study of [18] has led to the addition of five new species to the English side of the Western EC: three Amphilochidae: Amphilochoides boeckii G.O. Sars, 1895, Amphilochoides serratipes (Norman, 1869), and Paramphilochoides intermedius (Scott, 1896); one Maeridae, Maera loveni (Bruzelius, 1859); and one Photidae, Photis pollex Walker, 1895. All these species are recorded around the coast of the British Isles [3].
For the eastern part of the EC, three species were added to the checklist of [1]: two NIS Ptilohyale littoralis (Stimpson, 1853) [10,58] and Caprella mutica Schurin, 1935 [17], and Echinogrammarus berilloni (Catta, 1878), sampled in the upper part of the Le Tréport harbour (unpublished data). The checklist of [1] includes 255 species of amphipods in the EC (i.e., 240 Gammaridea, four Hyperiidea, and eleven Caprellidea). Among the 255 species reported, Anonyx sp. (Kröyer, 1838) and Lysianassa sp. (H. Milne Edwards, 1830) are probably new species, but in the absence of new records or descriptions, they are not considered in the current checklist.
Furthermore, in the 1999 checklist, seven species were reported as doubtful in the EC (Table 3). Thus, only one species, Odius carinatus (Spence Bate, 1862), listed as doubtful in the 1999 checklist in the EC has been confirmed as present in the EC from the first checklist of EC Amphipoda [14]. Table 3. Species reported as doubtful in the English Channel in [1]. In fact, Ref. Dauvin (1999) [1] gives 232 as the number of confirmed species of Gammaridea in place of 240, and the total number of amphipod species as 247 in place of 255.

Species
One species, Gammarus insensibilis, has been reported outside the five main zones inventoried by [1]; it has been recorded in the English Channel from the Portland zone (Wight) by Lincoln (1979). This species has been reported in the Normano-Breton Gulf by [14] and is also found in the Bay of Seine. Furthermore, two species, Protohyale (Protohyale) grimaldii (Chevreux, 1891) (= Hyale grimaldii) and Jassa slatteryi Conlan, 1990, show a large geographical distribution extended to the north and south of the EC. In 1999, both species were reported as probably present in the EC but were not included in the 1999 checklist (see Gouilleux, 2017, for distribution of J. slatteryi along the French coast of the north-eastern Atlantic). J. slatteryi was recognized as present in the Normano-Breton Gulf during a survey of the colonization of panels placed in Dielette harbour (unpublished data).

Non-Indigenous Species
Twelve NIS have been reported in the EC, including eight that were recently recorded:  (Stephensen, 1932). Therefore, they could be recorded in the following years in the EC [3,43,55,74,90].

Pattern Distribution of Amphipod Fauna in the English Channel
The 269 species listed here belong to 57 families, with 21 of them containing only a single species and one family alone accounting for 16 species (Figure 2). Only eight families comprise more than 10 species (Figure 2). A total of 44 species (16%) are recorded in only one of the eight delimited zones, with 21 being found in two zones and 29 in three zones. Most of the species recorded in one, two, or three zones are found only in the three zones of the western basin of the EC. A total of 14 species are recorded in all eight zones of the EC, with 43 other species in seven out of the eight zones; most of these species are absent in the Central Part of the EC (15 species), the Wight zone (18 species), and on the English side of the eastern EC (10 species): i.e., in the three zones with the lowest number of reported species.
reported in 1965, Monocorophium sextonae (Crawford, 1937) reported in 1976 in the Bay of Seine, and Platorchestia platensis (Krøyer, 1845) reported in the same zone at the beginning of the 1990s in Normandy [89].

Pattern Distribution of Amphipod Fauna in the English Channel
The 269 species listed here belong to 57 families, with 21 of them containing only a single species and one family alone accounting for 16 species (Figure 2). Only eight families comprise more than 10 species (Figure 2) [14]. This increase is also observed in harbours of the North-Cotentin and the Bay of Seine (addition of 42 species), where additional studies have been carried out in relation to numerous surveys linked to anthropic activities. For the French side of the eastern EC, the number of additional species reaches 13, while the number of species in the western part of the EC remains similar between the 1999 and 2022 checklists.
When applied to the eight zones and the 269 species recorded in the EC, Hierarchical Cluster Analysis (HCA) shows that we can separate the Central EC and the English side of the Eastern EC from the six other zones at a level of similarity of 50% (Figure 3). At a level of 40%, Wight is separated from the five other zones, while, at a level of 20%, we can distinguish the eastern basin of the EC (Bay of Seine and French side of the eastern EC) from the three zones of the western basin of the EC.
When applied to the eight zones and the 269 species recorded in the EC, Hierarchical Cluster Analysis (HCA) shows that we can separate the Central EC and the English side of the Eastern EC from the six other zones at a level of similarity of 50% (Figure 3). At a level of 40%, Wight is separated from the five other zones, while, at a level of 20%, we can distinguish the eastern basin of the EC (Bay of Seine and French side of the eastern EC) from the three zones of the western basin of the EC.

Discussion
The number of amphipod species collected in the English Channel was 269. This census significantly increases the historical number of known amphipods of the English Channel; 162 species were listed for the English Channel in the Fauna of the Amphipods of France [2] out of a total of 320 species known at the beginning of the 20th century for the whole of France including fresh waters. The Fauna of the Amphipods of the British Isles [3] listed 186 species of Gammaridea in the English Channel out of a total of 271 species for the whole of the British Isles, including 135 for the Normano-Breton Gulf and 161 for the Plymouth region on the English side of the western EC, slightly less than the number given in the inventory of Plymouth amphipods: 179 [5]. Reference [4] lists 145 species for Roscoff, while Bertrand and Retière counted 138 species for the Normano-

Discussion
The number of amphipod species collected in the English Channel was 269. This census significantly increases the historical number of known amphipods of the English Channel; 162 species were listed for the English Channel in the Fauna of the Amphipods of France [2] out of a total of 320 species known at the beginning of the 20th century for the whole of France including fresh waters. The Fauna of the Amphipods of the British Isles [3] listed 186 species of Gammaridea in the English Channel out of a total of 271 species for the whole of the British Isles, including 135 for the Normano-Breton Gulf and 161 for the Plymouth region on the English side of the western EC, slightly less than the number given in the inventory of Plymouth amphipods: 179 [5]. Reference [4] lists 145 species for Roscoff, while Bertrand and Retière counted 138 species for the Normano-Breton Gulf, and, finally, Glaçon drew up a checklist of 92 amphipods for the fauna of Wimereux (see [1]).
Among the 12 non-indigenous species recorded in the EC, eight had been reported since 1999 and had been mainly discovered in marinas, harbours, and shallow waters during samplings devoted to the knowledge of the fauna living in hard-bottom habitats. The pathways of their recent introductions were attributed to oyster transfers and ship traffics on both sides of the English Channel [12,17,88].
The three zones of the western basin of the EC account for more species than the eastern basin of the EC ( Table 2). Out of a total of 269 species, 180 (67%) have been recorded in both basins of the EC and therefore display a wide geographical distribution, while 78 species have been reported only in the western basin and 11 species solely in the eastern basin. Even if fewer surveys have been carried out in the eastern compared to the western English Channel, there appears to be a west-to-east impoverishment of the amphipod fauna of the EC-as observed for other zoological groups such as polychaetes and mollusc [91]-except in the Bay of Seine, where there has been a great increase in the number of species over the last two decades, probably due to the increase in collection in this zone (130 in 1999 as against 173 in 2022). This impoverishment appears to be in response to a climatic gradient from Atlantic Ocean waters, with low annual thermohaline variability in the west to waters of much more continental character and high annual thermohaline variability in the east [92]. Moreover, it is clear that the poorest zones in terms of species diversity are those with the fewest studies, such as in the central part of the EC. However, many physical, biological, and physiological factors such as temperature, current and sediment patterns, depth, and competition and interactions between species could also influenced the presence of amphipods in the eight zones of the English Channel.
Unless there is a significant climate change with a high increase of sea-water temperature allowing the spread of warm-amphipod species into the EC, the number of amphipod species present in the English Channel is expected to remain in the same order of magnitude as estimated for the British Isles as a whole (about 290-300 species). Nevertheless, in the future, we expect to see an increase in the numbers of species reported in the three zones poorest in amphipods because new studies will cover the central part of the EC as well as two zones on the English side of the eastern EC. It is also likely that new records could be concerned with species recently described or reported in neighbouring zones, mainly along the French coast of the Bay of Biscay [93][94][95] and around the coasts of Ireland (see [29,33,34,37,41,42,45,53,63,64,66,68,69,71,75,[77][78][79]81,82]), i.e., approximately 20 species (Table 2). Particular attention to amphipod identification by engineers from consulting offices and researchers from university laboratories on amphipod collections from new studies should lead to the discovery of these potential species.
Funding: This research received no external funding.

Institutional Review Board Statement: Not applicable.
Data Availability Statement: Not applicable.