Bioacoustics of Gammaropsis longipropodi

Description. Holotype Female. Body about 2.7 mm long. Head ( Fig. 4 A) rostrum minute; anterior cephalic lobe subtriangular, slightly produced forward; eyes circular, moderate; inferior antennal sinus not deep. Antenna 1 ( Fig. 4 B) setose with feebly plumose setae posteriorly, with peduncular articles 1–3 in length ratio of 1:1.2:0.9; peduncular article 1 stout, with a short plumose seta on anterior margin and two pairs of plumose setae at posterodistal corner; peduncular articles 2–3 slender; flagellum 6-articulate, articles with setae distally, distal article with aesthetasc. Antenna 2 ( Fig. 4 C, D) as long as antenna 1, also setose with feebly plumose setae posteriorly; gland cone not developed; with peduncular articles 3–5 in length ratio of 1: 2.1:2 peduncular articles 4–5 slender and slightly curved, with plumose setae anterodistally; flagellum 7-articulate, articles with setae distally, article 6 with a pair of dentate setae distally, article 7 reduced. Upper lip ( Fig. 4 E) subhexagonal, slightly concave and covered with minute setae apically. Lower lip ( Fig. 4 F) inner lobe ovoid, with minute setae on apical and medial margins; outer lobe swollen apically with minute setae, mandibular process produced. Left mandible ( Fig. 4 I) with 4-dentate incisor and 4-dentate lacinia mobilis; accessory setal row with four dentate setae; molar triturative with a plumose seta; palp robust, 3-articulate; article 1 shortest; article 2 concave with 14 setae medially, and with two setae on lateral surface; article 3 slightly swollen, 0.7x length of article 2, with two and three setae on lateral surface, with six serrate setae on half of medial margin and five serrate setae on distal margin. Maxilla 1 ( Fig. 4 G) outer plate with ten dentate setae apically; palp bi-articulate; article 1 short, obliquely truncated distally; article 2 slightly dilated distally, with a row of four serrate setae on distal surface and six stout setae apically. Maxilla 2 ( Fig. 4 H) inner plate with five plumose setae along medial margin, apex with several simple setae marginally and serrate setae submarginally; outer plate larger than inner plate, lined with several marginal and submarginal simple setae on apex, with three feebly plumose setae at laterodistal corner. Maxilliped ( Fig. 4 J) inner plate slightly expanded laterodistally, with three stout and four plumose setae on apex, with a stout seta on medial margin subdistally and eight plumose setae on surface; outer plate elongate semicircular, not exceeding distal end of palp article 2, with eight stout setae those gradually increasing from distal half to apex of medial margin and with five pairs of facial setae bearing round apices; palp 4-articulate, slender; article 1 obliquely truncated distally; article 2 not expanded, setose medially; article 3 slightly dilated, with setae distally; article 4 falcate, slightly shorter than article 3, with a elongate seta apically. Gnathopod 1 ( Fig. 5 A, B) coxa expanded forward and slightly diminished ventrally, obliquely truncated proximally, ventral margin round and minutely crenulate, lined with ten setae; basis not slender, width almost steady in distal 3/4, with weak anterior lobe, with one abnormal and three serrate setae on anterior margin, with nine serrate and 13 short setae on posterior margin and a group of three setae at posterodistal corner; ischium with moderate anterior lobe, lined with ten setae on mediodistal and laterodistal margins successively; merus subrectangular, longer than ischium, produced anterodistally, angulate at distal 2/3 of posterior margin with four setae, with two rows of three and eight setae on lateral surface subdistally; carpus trapezoidal, 0.6x length of basis, anterior margin slightly convex with three submarginal setae and with two groups of two and four setae at anterodistal corner, posterior margin weakly crenulate and expanded downward with four pairs of serrate setae, with two oblique rows of setae on marginal surface laterally and setae on medial surface; propodus elongate triangular, gradually diminished distally, as long as carpus, anterior margin convex with two pairs of setae and with a group of six setae at anterodistal corner, posterior margin lined with four groups of three setae submarginally, with stout defining setae located at centre, distal 2/3 of posterior margin finely pectinate and with five setae, palm indistinct with a small seta near hinge; dactylus falcate, stout, 0.6x length of propodus, with pectinate inner margin, with developed distal teeth bearing three setae, with a feebly plumose seta. Gnathopod 2 ( Fig. 5 C, D) coxa enlarged, subrectangular, obliquely truncated proximally, ventral margin convex and minutely crenulate, lined with 13 setae; basis stout, shorter than coxa, with six minute setae on anterior margin and three long setae on medial surface proximally, with developed distal lobe possessing oblique stridulation ridges composed of 26 engraved slots on lateral surface and its apex reaching distal end of ischium, with three long plumose and 13 short setae on posterior margin and one plumose and three simple setae at posterodistal corner; ischium with small anterior lobe, with a seta on posterior margin and a group of three setae at posterodistal corner; merus subrectangular, smoothly diminished distally in width, setose distally; carpus convex anteriorly, with a row of eight setae along distal margin, with developed carpal lobe posteriorly bearing setae on convex apex; propodus 1.5x length of carpus, convex and densely setose anteriorly, slightly expanded posteriorly, palmer margin distinct and concave with small seta, densely setose submarginally, defined by a stout seta; dactylus falcate, stout, 0.6x length of propodus, pectinate on anterior margin, with developed distal teeth bearing three setae, with a feebly plumose seta. Pereopod 3 ( Fig. 5 E, F) coxa subrectangular, slightly dilated anteroventrally, with a long seta at anteroventral corner and lined with eight short setae on ventral margin, with stridulation ridges composed of 33 oblique engraved slots on ventral margin submarginally; basis stout, 0.9x length of coxa, lined with eight minute setae and with two simple and four serrate setae proximally, with a pair of unequal plumose setae on anterior margin distally, posterior margin slightly expanded and lined with four serrate and nine simple setae, with seven plumose setae distally; ischium with moderate anterior lobe, with three plumose setae on posterior margin centrally and a group of three long setae at posterodistal corner; merus 0.6x length of basis, anterior margin expanded and produced distally, densely setose with plumose setae marginally and submarginally, posterior margin straight with one simple and five plumose setae, with a pair of setae at posterodistal corner; carpus rectangular, 0.6x length of merus, with a pair of long setae at anterodistal corner, with two groups of unequal setae on posterior margin; propodus 1.3x length of carpus, not slender and slightly diminished in width distally, with two long setae on anterior margin and a group of four setae at anterodistal corner, with two minute setae on posterior margin and three setae at posterodistal corner; dactylus falcate, 0.6x length of propodus. Pereopod 4 ( Fig. 6 A, B) coxa subrectangular, slightly curved distally, lined with eight setae on ventral margin; basis expanded, slightly larger than that of pereopod 3, anterior margin weakly angulate centrally and lined with seven single and one paired setae, with three serrate setae proximally and two plumose setae distally, posterior margin with six long serrate setae at centre, distal 2/3 of posterior margin forming slightly swollen lobe lined with eleven plumose setae; ischium with moderate anterior lobe, with a pair of simple and plumose setae at posterodistal corner; merus 0.5x length of basis, anterior margin expanded, anterodistal corner weakly produced with a group of six setae, posterior margin straight with a simple and a plumose seta proximally, posterodistal corner with a group of four setae; carpus subrectangular, 0.6x length of merus, convex anteriorly, with a long seta at anterodistal corner, with a pair of setae on posterior margin, posterodistal corner slightly oblique with a pair of setae; propodus not slender, 1.4x length of carpus, gradually diminished in width distally, with a single and a pair of setae on anterior margin and with a group of three setae at anterodistal corner, with two minute setae on posterior margin and with a seta at posterodistal corner; dactylus falcate, 0.5x length of propodus. Pereopod 5 ( Fig. 6 C–E) shorter than pereopod 4, with length ratio of ischium, merus and carpus 1:1.5: 2; coxa bilobate, anterior lobe enlarged, circular, expanded downward, lined with ten setae on ventral margin, posterior lobe smaller and about 1/4 of anterior lobe, with a minute seta ventrally; basis subcircular, slightly expanded back and forth, lined with twelve minute setae on anterior margin, with 17 plumose setae on medial surface, with five minute setae on posterior margin; ischium quadrate, broader than long, with a seta at anterodistal corner, with moderate posterior lobe; merus rectangular, anterior margin slightly convex, with a group of three setae at anterodistal corner and a seta at posterodistal corner; carpus rectangular, slightly diminished than merus in width, anterior margin convex, with a pair of unequal setae at anterodistal corner, with a minute seta on posterior margin and a group of one serrate and three unequal setae at posterodistal corner; propodus not slender, width steady, as long as carpus, with one stout and two simple setae at anterodistal corner, with a seta on posterior margin and a stout seta at posterodistal corner; dactylus short, reduced, slightly sinuous posteriorly, with two pairs of setae on anterior margin distally and a lateral seta on posterior margin distally, unguis slightly curved distally. Pereopod 6 ( Fig. 6 F–H) slender, longer than pereopod 5; coxa bilobate, not deep, anterior lobe with one plumose and three simple setae, posterior lobe round with two minute setae ventrally; basis subovoid, anterior margin swollen and slightly dilated distally with seven minute setae proximally, posterior margin slightly expanded proximally and lined with eight minute setae; ischium subquadrate, broader than long, with a pair of setae at anterodistal corner, with moderate posterior lobe; merus, carpus and propodus rectangular, width steady, with length ratio of 1:1.3:1.4; merus with a seta at anterodistal and posterodistal corners, respectively; carpus with a pair of stout and simple seta at anterodistal corner and a group of three setae at posterodistal corner; propodus with a pair of unequal setae at anterodistal corner and with a group of six setae at posterodistal corner; dactylus falcate, 0.4x length of propodus, slightly wrinkly anteriorly, with a seta and a denticle on anterior margin distally and a long plumose seta on posterior margin proximally, unguis developed. Pereopod 7 ( Fig. 7 A, B) slightly longer and slender than pereopod 6; coxa unilobate, semiovoid, with one plumose and five minute setae anteriorly and one minute seta posteroventrally; basis subovoid, smaller than that of pereopod 6, anterior margin slightly convex, lined with eleven simple setae and with five plumose setae distally, posterior margin more extended, weakly angulate proximally, lined with six setae, with a pair of setae at posterodistal corner, with one simple and two plumose setae on medial surface; ischium subrectangular, with a seta at anterodistal corner, with moderate posterior lobe; merus, carpus and propodus rectangular, width steady, with length ratio of 1:1.1:1.3; merus with a long seta at posterodistal corner; carpus with a group of three setae at posterodistal corner; propodus with two setae on anterior margin, a pair of stout setae at anterodistal corner and a group of seven setae at posterodistal corner; dactylus falcate, 0.5x length of propodus, slightly wrinkly anteriorly, with a seta and a denticle on anterior margin distally and a long plumose seta on posterior margin proximally, unguis developed. Pleonal epimera ( Fig. 4 K) epimeron 1 round ventrally with a plumose seta submarginally; epimeron 2 subrectangular, slightly extended backward, round ventrally with two plumose setae submarginally; epimeron 3 produced backward at posteroventral corner, ventral margin with one plumose and three short setae submarginally. Pleopod 1 ( Fig. 7 G) peduncle subquadrate, short, with twelve plumose setae marginally and submarginally on distal half of lateral margin, with a pair of retinaculae at mediodistal corner; inner ramus longer than outer ramus, both rami 10-articulate and articles with a pair of plumose setae distally. Pleopod 2 ( Fig. 7 H) peduncle subquadrate, short, with four plumose setae submarginally and two simple setae distally on lateral margin, with a pair of retinaculae at mediodistal corner; inner and outer rami 9-articulate and 10- articulate, respectively, articles with a pair of plumose setae distally. Pleopod 3 ( Fig. 7 I) shortest; peduncle with one simple and three plumose setae submarginally and three setae distally on lateral margin, with a pair of retinaculae at mediodistal corner; inner and outer rami 8-articulate and 9- articulate, respectively, articles with a pair of plumose setae distally. Uropod 1 ( Fig. 7 C) peduncle longer than rami, with one medial and twelve lateral setae dorsodistally; inner ramus with three dorsomedial and four apical setae; outer ramus slightly shorter than inner ramus, with nine dorsolateral and two apical setae. Uropod 2 ( Fig. 7 D) shorter than uropod 1; peduncle with five lateral setae dorsally; inner ramus longer than outer ramus, with four dorsal and three apical setae; outer ramus with four dorsal and three apical setae. Uropod 3 ( Fig. 7 E) biramus; peduncle 0.9x length as long as that of uropod, gradually convergence distally; inner ramus very short, triangular, with a seta apically; outer ramus bi-articulate; proximal article 0.5x length of peduncle, with two dorsal and one pair of distal setae, distal article very short, with a simple and a stout seta apically. Telson ( Fig. 7 F) fleshy, triangular in dorsal view, both outer sides with a tubercle and a short seta. Male. Unknown.

Remarks. This new species is closely similar to Photis ariakensis ( Hirayama, 1984 ) . However, Photis stridulus sp. nov. can be easily distinguished from P. ariakensis by the following features: 1) the carpus on gnathopod 1 of P. stridulus sp. nov. is slightly longer than that of P. ariakensis ; 2) the palm on gnathopod 1 of P. stridulus sp. nov. is short but obviously distinguishable (vs. indistinct in P. ariakensis ); 3) the oblique stridulation ridge on the lateral surface of the basis on gnathopod 2 is present (vs. absent in P. ariakensis ); 4), the palm of gnathopod 2 is quite different between the two species: the posterior margin of the propodus in P. stridulus sp. nov. is longer than that of P. ariakensis and the palmer margin is more excavate in P. stridulus sp. nov. ; 5) the posterior margins of bases on pereopods 6 and 7 are more expanded proximally in P. stridulus sp. nov. ; and 6) the posteroventral corner of pleonal epimera 3 is more produced in P. stridulus sp. nov. (vs. oblique in P. ariakensis ) ( Hirayama, 1984 ). Myers (2009) has mentioned that genus Photis exhibits no or very little sexual dimorphism, different from other corophiidean amphipods ( Myers, 2009 ). However, the anterodistal lobe and stridulation ridges of the basis on gnathopod 2 as well as the ventral stridulation ridges on coxa 3 are dimorphic sexually in several species ( Conlan, 1983 ). In general, the anterodistal lobe of the basis on gnathopod 2 in females is absent or weaker than that in males. However, several species have developed anterodistal lobe of basis on gnathopod 2 in females, including Photis baeckmannae Gurjanova, 1951 , P. bifurcata Barnard, 1962 , P. californica Stout, 1913 , P. coeca Barnard, 1962 , P. conchicola Alderman, 1936 , P. fischmanni Gurjanova, 1951 , P. hawaiensis J.L. Barnard, 1955 , and P. spinicarpa Shoemaker, 1942 ( Stout, 1913 ; Alderman, 1936 ; Gurjanova, 1951 ; Barnard, 1955 , 1962 ; Conlan, 1983 ). Among them, only one species, P. fischmanni , has stridulation ridges of the basis on gnathopod 2 and coxa 3 simultaneously in females. This new species, P. stridulus sp. nov. also has anterodistal lobe and stridulation ridges of the basis on gnathopod 2 and ventral stridulation ridges on coxa 3 in female. However, P. stridulate is readily discriminated from P. fischmanni based on the following features: 1) the degree of setation of the appendages is weaker than that of P. fischmanni ; 2) the eyes are not occupying most regions of the anterior cephalic lobe (vs. occupying most regions in P. fischmanni ); and 3) the carpus and propodus on gnathopod 1 are slender than those of P. fischmanni (vs. more expanded posteriorly in P. fischmanni ) ( Gurjanova, 1938 ; Conlan, 1983 ).