Apocorophium acutum (Chevreux, 1908)

Gouillieux, Benoit, Blanchet, Hugues & Gonzalez, Patrice, 2022, Redescription of Apocorophium acutum (Crustacea, Amphipoda, Corophiidae) with material from type locality and key of world Apocorophium species, ZooKeys 1106, pp. 101-119 : 101

Apocorophium acutum (Chevreux, 1908)

Corophium acutum Chevreux 1908: 75 (original description), fig. 6.-1910: 271, h.- Chevreux and Fage 1925: 366-367, figs 359, 374.- Poisson and Legueux 1926: 320-325, figs 5,6 (Banyuls form).- Schellenberg 1928: 672-673.- Shoemaker 1934: 26-27.- Crawford 1937: 624-625.- Ruffo 1938: 147-148.- Salfi 1939: 31-62, figs 1-7.- Shoemaker 1947: 59, fig. 9.- Hurley 1954: 439-442, figs 2, 3 (in part).- Barnard 1969: 42.- Bousfield 1973: 205, fig. LXIV.- Lincoln 1979: 534, fig. 256 (in part).- Dickinson et al. 1980: 12.- Myers 1982 (in Ruffo ed.): male 188-190, fig. 126.- Barnard and Karaman 1991: 185.

Apocorophium acutum Bousfield and Hoover 1997: 123-125, fig. 35.- Lowry and Stoddart 2003: 88.- Ren 2006: 296-297, fig. 117.- Bano and Kazmi 2012: 113.- Hossain and Hughes 2016: 376-381, figs 2-5.

Corophium acutum Not Corophium acutum Poisson and Legueux 1926: 320-325, figs 5, 6 (Caen form).- Myers 1982 (in Ruffo ed.): female 188-190, fig. 126.

Apocorophium acutum Doubtful Apocorophium acutum Jung and Kim 2007: 247-250, fig. 1.- Demicheli and Verdi: 2018, 1169-1173, figs 1-3.

Material examined.

Algeria • 6 brooding females and 1 juvenile; Bône (type locality); 4 May 1900; MNHN-IU-2013-19982; campaign MELITA, St. 677 "Melita II", on concrete blocks removed from the harbor • 1 female, same data as for preceding; MNHN-IU-2016-3401; dissected brooding specimen .

France • many males and females; Arcachon Bay / station “bouée 13"; 44°38'07.20"N, 001°14'06.60"W; 2 m depth; 20 September 2014; Benoit Gouillieux leg.; mussels, hand-collected on submerged part of a navigation buoy; MNHN-IU-2016-3426 and MNHN-IU-2016-3427 • many males and females; Arcachon Bay / station " Arcachon harbor"; 44°39'36.53"N, 001°09'06.59"W; 0.5 m depth; 20 February 2020; Benoit Gouillieux leg.; on floating pontoons in harbor • 414 specimens; Arcachon Bay / station “blockhaus”; 44°34'00.40"N, 001°14'15.14"W; 5 m depth; between May 2018 and April 2019; Benoit Gouillieux leg.; with Hydrozoa Amphisbetia operculata (Linnaeus, 1758) • many males and females; Trébeurden / harbor; 48°46'12.21"N, 003°35'09.71"W; 0.5 m depth; 1 February 2020; Gabin Droual leg.; on floating pontoons (Port); MNHN-IU-2016-3392 .

Description

(Figs 2 - 5 ). Based on adults females, Chevreux collection, MNHN-IU-2016-3401 and MNHN-IU-2013-19982, Bône, Algeria, 4 May 1900, campaign MELITA, St. 677 "Melita II", type locality.

Head. Head with rostrum pointed distally, triangular in dorsal view, reaching lateral ridge of head. Eyes visible in alcohol. Antenna 1 weakly setose; peduncular article 1 rectangular, ventral margin with three robust setae, dorsomedial margin with two robust setae; length ratio of peduncular articles 1-3 = 1.00: 0.72: 0.31; flagellum 5-articulate, shorter than peduncle, articles 2-4 with a small aesthetascs ventrodistally. Antenna 2 peduncular article 3 wider than long, with a pair of mediodistal robust setae; peduncular article 4 with three solitary robust setae; peduncular article 5 with a robust seta medially and a small process mediodistally; flagellum 3-articulate, distal article tiny with two robust setae. Lower lip inner lobe subovate, coalescent proximally, rounded apically; mandibular process small and blunt; both lobes covered with patch of pubescence medially. Right mandible well developed, incisor process and lacinia mobilis produced inward, bluntly tridentate; accessory setal row with three curved, finely pectinate blades, followed by tuft of pappose setae and a brush-like seta; molar well developed, massive, truncate; palp biarticulate, proximal segment shorter than distal, with 1 finely plumose seta apically, distal segment slender, with pubescence medially and long plumose seta apically. Left mandible similar, except for molar process which presents 2 additional blades. Maxilla 1 outer plate armed with seven setal-teeth apically; palp biarticulate, proximal segment short, distal one extending beyond end of outer lobe, with row of seven distal setae. Maxilla 2 inner plate with longitudinal row of pinnate setae on inner and distal margins; outer plate extending beyond end of inner one, with row of pinnate setae on distal margin. Maxilliped inner plate slender and elongate, basal portion with row of about nine plumose setae, inner margin with four and two pinnate setae; outer plate not reaching distal end of palp article 2, basal portion with row of about ten plumose setae, inner margin densely setose; palp 4-articulate, article 2 elongate, about three times as long as wide, inner margin densely setose, outer margin with one plumose seta distally, article 3 with rounded distal corner, distal article small, 0.24 × article 3, with apical setae.

Pereon. Gnathopod 1 subchelate; coxa ventral margin with three long plumose setae, anterior margin with two simple setae; basis anterior margin unarmed, posterodistal corner with unequal setae; ischium quadrate, with long pinnate setae posterodistally; merus short, with long pinnate setae posterodistally; carpus slightly narrowing distally, anterior margin with one median and two distal simple setae, posterior margin with two rows of pinnate setae; propodus 0.9 × carpus, posterior margin slightly convex, medial portion with pectinate setae, palm transverse, slightly convex, edge laminar and transversally striated, limited posteriorly by two robust setae; dactylus falcate. Gnathopod 2 simple; coxa small, with one long simple seta anteriorly; basis subrectangular, anterodistal and posterodistal corners with a simple seta; ischium flat, depressed, posterodistal corner with a simple seta; merus convexly curved posteriorly, with two rows of long pinnate setae along posterior margin and medial portion; carpus isosceles triangle in shape, strongly widening distally, with two small simple and few long pinnate setae posterodistally; propodus weakly narrowing distally, 1.6 × carpus, proximal third of medial portion with oblique row of pinnate setae, anterior and posterior margins sparsely setose, posterodistal corner with simple and plumose setae; dactylus short, flexor margin with two teeth and simple setae.

Pereopod 3 coxa small; basis weakly expanded medially, anterior margin with two setules, posterior margin bare, posterodistal corner with cluster of simple setae; merus anterodistal corner with cluster of simple setae, posterior margin with one medial and one distal simple seta; carpus small, subtriangular, with setae on anterior margin; propodus about twice as long as carpus, posterior margin and anterodistal corner with simple setae; dactylus simple, subequal in length to carpus and propodus length together. Pereopod 4 similar to pereopod 3, except basis anterior margin slightly more setose. Pereopod 5 coxa depressed, much wider than long, narrowing distally; basis slightly widened medially, anterior margin weakly setose, posterior margin with one setule; merus widening distally, antero and posterodistal corners with simple setae; carpus short, with two oblique rows of three proximal and four distal robust setae respectively; propodus about four times as long as wide, weakly setose; dactylus short. Pereopod 6 similar to pereopod 5, but about 1.3 × longer; basis more subrectangular, with a row of setules and four plumose setae. Pereopod 7 elongate, much longer than either pereopod 5 or 6; basis elongate-ovate, moderately expanded anteriorly, densely setose along both margins with long plumose setae; ischium to propodus linear and rectangular; length ratio of articles 2-7 = 1.00: 0.26: 0.52: 0.5: 0.56: 0.16.

Pleon. Epimera 1-2 subovate, ventral margins rounded, with long plumose setae; epimeron 3 subrectangular, distinctly longer than epimera 1-2, ventral margin bare, hind margin with many long simple setae. Urosomites 1-3 fused, without notch laterally; uropod 1 arising mainly ventrally. Uropod 1 peduncle rectangular, about 2.2 × outer ramus, ventrodistal process present, triangular, blunt, lateral margin with row of robust setae, proximal ones short and setae like, medial margin bare except a small distal robust seta; outer ramus slightly shorter than inner, lateral margin with six robust setae, medial margin bare, three subdistal robust setae, the middle one the longest; inner ramus slightly curved medially, lateral margin with four robust setae, medial margin bare, three subdistal robust setae, the middle one the longest. Uropod 2 peduncle longer than rami, without ventrodistal process, outer margin with setae on distal half; rami with one distal robust seta; outer ramus slightly shorter than inner with simple setae marginally. Uropod 3 uniramous, peduncle short, broad, with three simple setae on outer margin; ramus subelliptical, narrowing distally, margins with unequal simple setae. Telson fleshy, thickened, grooved centrally, subtriangular, broadest in middle.

Male (sexually dimorphic characters, based on specimens from Trébeurden, France, 1 February 2020, MNHN-IU-2016-3392).

Antenna 1 and 2 (Figs 2D , 5A, D ) with longer and more numerous setae; antenna 2 peduncular article 3 with a single or a pair of smaller robust setae; peduncular article 4 with 1-3 ventromedial small robust setae, with two ventrodistal processes; peduncular article 5 without robust seta, with ventroproximal and ventrodistal process, size of process function to maturity of the specimen.

Variability (based on specimens from Bône, Arcachon Bay, and Trébeurden; same data as material examined).

Head with rostrum reaching or not lateral ridge of head; antenna 1 peduncular article 1 with one or two dorsomedial and two to four ventral robust setae, sometimes no left right symmetry; antenna 1 flagellum with five or six articles; female antenna 2 peduncular article 4 with two to four robust setae, and few times in larger specimens a distal pair; male antenna 2 peduncular article 4 with one to three medioventral robust setae, ventrodistal process subequal or the upper one slightly shorter; maxilliped article 2 outer margin with one to three plumose setae distally; gnathopod 1 coxa ventral margin with one to three long plumose setae; gnathopod 2 dactylus with generally two teeth on flexor margin for adult specimens (specimen with BL <2.2 mm mainly juveniles with only one tooth, 1.7% of adults specimens examined with three teeth), sometimes no left right symmetry, an adult male specimen (BL = 2.46 mm) with two teeth on the left and a single tooth on the right gnathopod 2; pereopod 3 and 4 dactylus reaching between proximal to distal end of carpus; pereopod 5 and 6 carpus with clusters of three or four proximal and three to five distal robust setae; urosome with or without small lateral depression which looks like a vestigial notch; uropod 1 peduncle with five to nine robust setae along outer margin, sometimes with one to three proximal simple setae, rami with three to five robust setae on outer margin; uropod 3 peduncle shorter to subequal in length to ramus, variously expanded, with or without setae dorsally; telson more or less acute, dorsodistal robust setae tooth-like, mostly not observed.

Ecological data.

Thirty-eight brooding females were examined; BL ranged from 2.49 to 4.53 mm; fecundity ranged between 4 and 37 eggs/marsupium; mean fecundity of 9 eggs/marsupium. Eggs were ovoid, with mean major and minor diameters of 0.323 ± 0.055 mm and 0.266 ± 0.037 mm, respectively (x̄ ± s; n = 351). These females were collected from two locations in Arcachon bay. In both locations, there were moderate but significant positive correlation between body length and number of eggs per marsupium ( Spearman’s ρ =0.69 and 0.55, for locations “bouée 13" and "Arcachon harbor", respectively). There was no significant correlation between the size of eggs and the number of eggs in the marsupium of females (Spearman correlation tests, p -values>0.05).

Field data collected in Arcachon Bay showed that gender features could be distinguished from a body length of c. 1.7 mm. Smaller female displayed smooth oostegites, while larger female displayed ramified oostegites with or without eggs (Fig. 6 ; Kruskal-Wallis test followed by post-hoc Dunn tests, p -values <0.05). Mature female with ramified oostegites or bearing eggs reached higher size than male (Fig. 6 ; Kruskal-Wallis test followed by post-hoc Dunn tests, p -values <0.05).

Figures

Figure 2. Apocorophium acutum (Chevreux, 1908). (A-C) brooding female, BL = 3.19 mm, MNHN-IU- 2013 - 19982, Bone (D) male, BL = 2.32 mm, MNHN-IU- 2016 - 3392, Trebeurden (E-K) female, BL = 3.55 mm, MNHN-IU- 2016 - 3401, Bone. A head and antenna 1, dorsal view B left antenna 1, outer view C right female antenna 2, inner view D right male antenna 2, inner view E maxilla 2 F left mandible G right mandible H maxilliped I maxilla 1 J upper lip K lower lip. Scale bars: 0.1 mm.

Figure 5. Apocorophium acutum (Chevreux, 1908), specimens from Arcachon Bay, station " bouee 13 ", France, 20 / 09 / 2014 A male specimen, lateral view B female head, dorsal view C female antenna 1 and 2, inner view D male antenna 1 and 2, inner view E mandibles F female pleotelson, dorsolateral view showing vestigial notch (dotted edge) G female, telson, dorsal view. Scale bars: 0.5 mm (A); 0.250 mm (B-D); 0.05 mm (E); 0.1 mm (F, G).

Figure 6. Frequency distribution of body size (in mm) for juvenile, male, female with smooth oostegites (F. smooth), female with ramified oostegites but no eggs (F. ramified), and female with eggs (F. eggs) of Apocorophium acutum (Chevreux, 1908) in Arcachon bay, station " blockhaus ".