First record of the invasive species Ianiropsis serricaudis Gurjanova, 1936 (Crustacea: Isopoda) in Arcachon Bay, Bay of Biscay (NE Atlantic)

The invasive isopod Ianiropsis serricaudis Gurjanova, 1936, originally described from the North-West Pacific Ocean, is herein reported for the first time on the French Atlantic coast. Its presence is probably due to oyster import activity, and the sampling of many brooding females in Arcachon Bay since 2013 suggests a well-established species. Its current European distribution is summarised and some morphological characters are discussed.


Introduction
Non-indigenous species are recorded regularly in European waters.The main pathways of introduction are shipping, aquaculture and via human infrastructure such as the Suez Canal.On the French coast, introductions are mainly due to aquaculture (51%) and shipping (39%) (Nunes et al. 2014).In the case of Arcachon Bay, the absence of a commercial port and the intensive oyster trading suggest the vector of introduction of non-indigenous species is aquaculture.This hypothesis is confirmed by the fact that all the introduced species reported from Arcachon Bay in recent years (Arcuatula senhousia (Benson, 1842), Ampithoe valida Smith, 1873, Aoroides Walker, 1898, Grandidierella japonica Stephensen, 1938, Melita nitida Smith, 1873, Paranthura japonica Richardson, 1909) are native to areas from where cupped oysters were imported to France: British Columbia and Japan.Furthermore, the oysters transfer among regions has caused the spread of numerous alien species in northern Europe (Goulletquer et al. 2002).
The present paper deals with the first record of the isopod Ianiropsis serricaudis Gurjanova, 1936 in Arcachon Bay, Bay of Biscay, which represents the first record on the French Atlantic coast.

Study area
Arcachon Bay (44º40′N; 1º12′W) is a 180 km² macrotidal coastal lagoon, connected to the Atlantic Ocean by a narrow channel and it receives freshwater inputs Leyre River in its south-eastern part (Figure 1).This lagoon is characterized by large intertidal flats (115 km²) covered by Zostera (Zosterella) noltei Hornemann seagrass bed ( 70

Results
A total of 294 specimens was collected: 102 males (14 specimens were terminal males) and 192 females including 63 brooding females.Specimens were found in polyhaline water, where salinity and temperature ranged between 27 to 35 PSU and 6 to 24 °C respectively (REPHY 2017).
According to Doti and Wilson (2010) and Hobbs et al. ( 2015), terminal males of Ianiropsis serricaudis can be distinguished from other Ianiropsis species by: (1) the length of antennal peduncle article 6 and 7 together as long as or longer than half the body length; (2) maxilliped palp projecting substantially, visible in front of head in dorsal view; (3) pereopod 1 dactylus with 2 claws; (4) pereopod 7 dactylus with 2 or 3 claws; (5) uropod protopod elongate and rectangular; (6) pleotelson lateral margin with 3 or 4 denticles (rarely more).Specimens from Arcachon Bay present head anterior margin in dorsal view linear (slightly convex between antennae), body length of terminal males about 2,7 mm.

Female (sexually dimorphic characters)
Length of antennal peduncle article 6 and 7 together smaller than half the body length; maxilliped palp not projecting, not visible in front of head in dorsal view; pereopod 1 as long as pereopod 2-7, carpus and propodus not particularly elongate.

Variability
Non-terminal males present (1) antennal peduncle article 6 and 7 together about a quarter of the body length; (2) maxilliped palp projecting slightly, not especially visible in front of head in dorsal view; (3) pereopod 1 as long as or slightly longer than those of pereopod 2 -7, carpus and propodus not particularly elongate.The main characters to distinguish terminal males of I. serricaudis from I. breviremis (G.O.Sars, 1883), the only Ianiropsis species previously known from European waters, according to Doti and Wilson (2010), are: (1) the head anterior margin in dorsal view broadly concave (versus rounded and projecting anteriorly between antennae in I. breviremis) and (2) the maxilliped palp projecting substantially, visible in front of head in dorsal view (versus not projecting substantially, not visible in dorsal view in I. breviremis).Specimens from Arcachon Bay present head anterior margin in dorsal view linear, slightly projecting anteriorly between antennae, as in original illustrations of I. serricaudis (Gurjanova 1936).This character is therefore doubtful and must be taken with caution for identification (Figure 4 A-C).According to Hobbs et al. (2015) and Marchini et al. (2016a), I. serricaudis has probably been occurring for many years in European waters, but previous confusion with Janira maculosa and I. breviremis could have delayed its reporting.The presence of many brooding females since 2013 in Arcachon Bay confirms a well-established population and therefore suggests that its presence could be prior to 2013.However, the absence of Janiridae in our collections before 2013 (presence of Janira maculosa in Blanchet (2004) and Salvo (2010) thesis, specimens used to estimate biomass) prevents confirming this hypothesis.The misidentification is probably due the fact that the avaible to Janiridae and Ianiropsis keys (Wilson and Wägele 1994;Doti and Wilson 2010) refer to terminal males only and are based on size of maxilliped palp and pereopod 1 carpus and propodus.These characters are dependent on the maturity of the specimens: non-terminal males can have (1) maxilliped palp not projecting substantially, (2) pereopod 1 carpus and propodus as long as or slightly longer than those of pereopods 2-7 (present observations).According to Doti and Wilson (2010), shape of males pleopod 1 is consistent across the species within the genus, so it can be used to distinguish Ianiropsis from Janira (Figure 4 D and E).

Discussion
Due to the absence of a commercial port and the significant oyster import activity, shellfish trade is probably the vector of I. serricaudis introduction to Arcachon Bay.This vector was suggested by Marchini et al. (2016a, b) to explain its introduction in Italy.But coastal vessel traffic, as mentioned by Hobbs et al. (2015) cannot be discarded.Due to its high invasive capability (Hobbs et al. 2015), I. serricaudis is probably present in many others areas of the French coast and European waters but has not yet been identified and documented.

Table 1 .
Records of Ianiropsis serricaudis in Arcachon Bay.N: number of individuals collected.Stations: see Figure 1.
Eclipse E400 microscope, and photographed with a Nikon DS-Fi2 camera.Drawings were carried out from pictures using Inkscape software (v.0.48).Body length (BL) was measured with NIS-Elements Analysis software from anterior margin of head to posterior end of telson.For SEM studies, specimens