Diamysis lagunaris Ariani & Wittmann 2000

Diamysis lagunaris Ariani & Wittmann, 2000 Figs 5, 6, 7 Material examined. North Sea Canal • 11 ♀♀ ad. with BL 5.7–7.0 mm, 2 ♂♂ ad., 5.7–6.0 mm, 3 subad., 12 imm. (damaged, in vial), 1 ♂ ad., 6.2 mm (most thoracopods broken, on slide); Spaarnwoude, loc. code NVO-SW; 52.4367°N, 4.7057°E; 16 Oct. 2023; T. van Haaren leg.; canal-km 9.5, shallow (<1 m), sandy bottom, pond net; sample code 2023 EASC 00024 • 2 ♀♀ ad., 9.2–10.2 mm, 1 ♂ ad., 7.2 mm, 1 imm., 3 juv. (well-preserved, in vial); 23 Apr. 2024; T. van Haaren and J. I. Knetsch leg.; sample code 2024 EASD 00029; remaining sampling data as for preceding • 1 ♂ ad., 6.1 mm (most thoracopods broken, in vial); Buitenhuizen, Assendelft, loc. code NVO-AD; 52.4363°N, 4.7178°E; 10 Oct. 2023; T. van Haaren leg.; canal-km 10, shallow (<1 m), sandy bottom, pond net; sample code 2023 EASC 00025 • 2 ♂♂ ad., 7.0– 7.3 mm, 1 ♀, 9.3 mm, carrying 27 damaged nauplioid larvae (adults well-preserved, on slides); 22 Apr. 2024; T. van Haaren and J. I. Knetsch leg.; sample code 2024 EASD 00030; remaining sampling data as for preceding • 1 spec. with head missing; Kleine N’IJplas west side, loc. code KNIJP 2 - Ow, 52.4179°N, 4.8632°E; 18 Apr. 2024; T. van Haaren leg.; canal-km 19.5, shallow (<1 m), pond net; sample code 2024 EASD 00033 • 1 spec.; Kleine N’IJplas south side, loc. code KNIJP-Oz, 52.41711°N, 4.86044°E; sample code 2024 EASD 00028; remaining sampling data as for preceding. Diagnosis. Modified from Ariani and Wittmann (2000) in order to include the large-sized specimens of the NSC (size comparison given below). Forward directed disto-mesial lobe (arrow in Fig. 6 B) above inner flagellum of male antennular trunk less than half as long as basal width of flagellum. Antennula with well-developed appendix masculina (Fig. 6 B). Cornea occupies 50–70 % of the length of eyestalk (Fig. 5 E). Eyestalks with strongly developed fenestra paracornealis (arrow in Fig. 5 E), even if not always well visible in poorly pigmented stalks. Carapace (Fig. 6 A) without fringes in both sexes. Distal segment of maxillary palp with 5–25 denticles (Fig. 6 C). Basal segment of thoracic exopods 1–8 with outer corner mostly spiniform (Fig. 6 E), though less frequently rounded in exopods 5–8. Carpopropodus of thoracic endopods 3–8 with 3–2 (4), 2–3, 2–3, 2–3, 2, and 2–3 segments (Fig. 6 D, E); tarsus comparatively slender with slender styliform claw. Carpopropodus of endopod 3 (Fig. 6 D) with basal segment 2.9–3.7 times as long as its maximum width. Basal segment not longer than remaining segments combined in carpopropodites 3 with more than two (Fig. 6 D) segments. Penes (Fig. 6 F) with all or with most setae smooth, with few barbed setae, if any. Exopod of male pleopod 4 is 2 - segmented (Fig. 6 H), basal segment distally with a smooth seta, occasionally accompanied by a small, barbed seta. Scutellum paracaudale (Fig. 5 D) well-rounded to biconvex with rounded to acute edge. Statoliths composed of vaterite. Telson (Fig. 5 F) trapezoid to subtriangular, length 1.1–1.5 times maximum width; maximum width 1.8–2.7 times that at apex. Each lateral margin all along with 6–16 spines decreasing in length from basis to about half telson length and from there increasing in length distally. Terminal margin of telson with pair of large disto-lateral spines flanking a deep cleft penetrating 11–19 % telson length; cleft with distinctly convex lateral margins forming an angle of <90 °; margins all along armed with laminae. Telson with totals of 14–34 spines and 9–26 laminae. Note. Body length of adults is 6–10 mm in females (n = 14) and 6–7 mm in males (n = 6) in brackish waters of the NSC at 52°N in the North Sea versus 5–7 mm in females (n = 1236) and 4–6 mm in males (n = 617) at the type locality in the mixoeuhaline to weakly metahaline lagoon Lago di Caprolace at 41°N in the Tyrrhenian Sea (Ariani and Wittmann 2000). On the average larger adult sizes in colder climates is a normal feature in Mysidae (Wittmann 1984). Fenestra paracornealis with 1–5 free ommatidia in series along the margin of the cornea but not integrated in the cornea (arrow in Fig. 5 E). Figs 5, 6 show the essential details of the present specimens for determination at species level. In addition, a key to the currently acknowledged 14 species plus two non-nominotypical subspecies of the genus Diamysis is given below. Distribution. Type locality of D. lagunaris is the Lago di Caprolace at the Lazio coast, Tyrrhenian Sea. Other published Mediterranean records cover Aegean, Sardinian and Ligurian seas, the Strait of Messina, Gulf of Lion, and the Baleares (Ariani and Wittmann 2000; Wittmann and Ariani 2012). Published NE-Atlantic records extend from the Gulf of Cádiz to the south and west coasts of Portugal (as D. bahirensis in Drake et al. 1997; Cunha et al. 2000; San Vicente and Munilla 2000; as D. lagunaris in Wittmann and Ariani 2012). The present records extend the known Atlantic distribution northward to the Dutch coast of the North Sea (Fig. 7). Potential native versus introduced status of populations is considered in the ‘ Discussion’. The normal salinity range is 14–49 psu. Only two positive samples were previously known from oligohaline waters (S = 2–3 psu), namely from different stations in the Rhône Delta on the Mediterranean coast of France (Wittmann and Ariani 2012). The records in the re-naturalized sites of the NSC were made before (10–16 Oct. 2023) as well as after (22–23 Apr. 2024) 5 Jan. 2024 when the near-surface salinity was <2 psu. This makes it plausible but not definite that the mysids were exposed to and had survived oligohaline conditions in winter. Among the currently acknowledged 14 species of Diamysis, ten species are Mediterranean endemics (among these, D. cymodoceae is also found in the Marmora Sea and the southern Black Sea; D. lagunaris also found in the NE-Atlantic) as well as one each from the Pontian, Ponto-Caspian, Caspian, and W-Indian Ocean plus Red Sea.