Ampelisca mississippiana, Soliman, Yousria & Wicksten, Mary, 2007

Soliman, Yousria & Wicksten, Mary, 2007, Ampelisca mississippiana: a new species (Crustacea: Amphipoda: Gammaridea) from the Mississippi Canyon (Northern Gulf of Mexico), Zootaxa 1389, pp. 45-54 : 46-53

Ampelisca mississippiana , sp. nov.

( Figures 1–5 )

Material examined. Holotype: Adult female; TL 5.6 mm. Northern Gulf of Mexico, head of the Mississippi Canyon (28 32' 30.7" N, 89 49' 43.9" W), 480 m, muddy bottom (approximately 73% clay, 25.5% silt and 1.5% sand), 17 June 2000, R.V. Gyre. DGoMB station MT1, USNM.

Paratypes: 10 females. Mississippi Canyon (28 32' 6.6" N, 89 49' 32.2" W), 498 m, 2 June 2001, R.V. Gyre. DGoMB sta. MT1, USNM. 15 females. Mississippi Canyon, 28 33' 12.1" N, 89 49' 18.5" W, 13 August 2002, 476 m, R.V. Gyre, USNM. 16 females. Same location, date and depth as holotype. TCWC cat. No. 2­ 9119.

Other material: Approximately 300 individuals, including 3 males, same collecting locations, under study at Texas A&M University.

Etymology. The name of the species refers to the Mississippi Canyon, where the specimens are found in abundance.

Description. Female TL 5.6 mm. Body smooth and rather colorless. Head broad, longer than deep, subequal in length to first 3 body segments, projecting anterodorsally above antenna 1. Distal margin concave. Lower front edge oblique, convexly curved posteriorly. Anteroventral margin with short setae. Corneal lenses absent, ganglia of eyes easily visible under cuticle. Antenna 1 very short and slender, shorter than antenna 2 peduncle, article 1 robust. Peduncular article 2 long, about 1.5x length of article 1. Article 2 slimmer than article 1. Flagellum with 7­9 articles. Antenna 2 as long as body or longer depending on age of specimen. Peduncle long and slender. Peduncle article 4 about same length as article 5. Flagellum with up to 19 articles. Flagellum setae long.

Mandible with spine row with 8 spines. Palp tri­articulate, article 2 longer than either article 1 or 3, slen­ der and setose.

Maxilla 1 with inner lobe conical, ending apically with 1 long seta. Outer plate with 9 spines. Palp biarticulate, second article with 4 distal sharp cusps, 4 spines and 6 setae. Maxilla 2 normal.

Maxilliped with inner plate reaching to the end of palp article 1 and with several plumose setae; outer plate reaching to end of palp article 2, inner margin of outer plate lined with 8 chisel teeth, round apex with 5 setal spines, outermost two plumose. Palp with four articles, article 4 with nail as long as rest of article.

Coxae 1­4 longer than broad. Coxal plate 1 expanded and rounded distally. Coxae 1 and 2 lower posterior angle with slit and bearing plumose setae on lower margin with row of setae overlying them.

Gnathopod 1 simple and linear, shorter than gnathopod 2; basis long, ischium and merus short, carpus slightly longer than propodus; dactylus shorter than propodus, arched, and bearing several setules along inferior margin and one seta on outer margin; palmar portion weakly developed; armed with sets of comb spines and setal spines.

Gnathopod 2 very slender, longer than gnathopod 1, carpus more than 1.5 times as long as propodus, with heavy sets of setae on distal margin; propodus heavily covered with spinose setae; dactylus about two­thirds of propodus, slightly curved with simple setae on flexor margin.

Pereopod 3 and pereopod 4 almost similar. Pereopod 3 merus longer than carpus and propodus combined. Both merus and carpus bearing few long spinose setae at distal anterior margin; carpus with 1 seta on extensor surface; propodus with few simple setae on the extensor surface; dactylus very long and slender, almost straight and significantly longer than both propodus and carpus combined. Pereopod 4 coxa rectangular with rounded distal corner. Pereopod 4 similar to pereopod 3 but with much longer merus and with setae on both sides; distal anterior edge of the merus not produced.

Pereopod 5 with basis having anterior margin fringed with plumose setae, posterior margin with rounded lobe. Carpus slightly longer and broader than propodus and endowed with set of 4­5 comb spines and two short spines, posterior margin with 2 sets of short spines. Propodus with very long setae on the distal end; dactylus very short.

Pereopod 6 with basis roughly rectangular, anterior margin with 3 plumose setae and several spines, posterior margin rounded without any ornamentation. Carpus slightly longer than propodus, anterior margin with several spines, and 2 sets of spines on lateral surface, distal margin with long set of 4­5 comb spines and two short spines. Propodus less broad than carpus, with several spines on anterior margin. Dactylus very short.

Pereopod 7 with basis straight, anterior margin with few short spines and rounded posteriorly, inferior margin of posterior lobe of basis expanded distally, passing distal end of ischium, distal margin fringed with plumose setae. Ischium longer than merus. Merus with large posterior setose lobe produced along entire margin of carpus, fringed by long plumose setae, anterior lobe slightly produced along anterior margin of carpus with 1 spine at blunt apex. Carpus with anterior edge slightly produced and notched. Propodus attached to posterior proximal portion of carpus, with notched distal anterior margin. Carpus and dactylus shorter than propodus. Dactylus broad at base and tapering distally, not curving forward.

Epimeral plate 1 rounded with spine at distal margin and plumose setae at posterior margin. Epimeral plate 2 with inferior margin rounded. Epimeral plate 3 posteroventral corner with acute spine.

Pleon segment 3 with posterior margin slightly convex, lower posterior margin produced into large acute tooth. Dorsal surface of segment with 2 setules.

Pleon segment 4 dorsal surface with prominent wedge­shaped dorsal carina ending acutely above segment 5 and bearing 2 setae at top. Pleon segment 6 with shallow lateral crests. Pleon segments from 1 to 6 with pair of setules.

Uropod 1 reaching as far as mid­rami of uropod 2, rami as long as peduncle, outer and inner rami equal. Inner ramus with 3­4 spines while outer ramus with 1 spine.

Uropod 2 with inner ramus slightly longer than outer ramus. Inner margin of inner ramus and outer margin of outer ramus spinulate. Outer rami with very long subterminal robust toothed seta.

Uropod 3 with peduncle strong, with 3 spines; rami equal in length, long, slender, lanceolate; outer ramus more slender than inner, broadest at proximal end, apical outer margin of inner ramus and inner margin of outer ramus strongly setose.

Telson longer than broad, deeply cleft for more than 70% of length; distal margin acute, apices of lobes with 1­2 setae, strongly notched on outer ridge, and dorsal surface of each lobe with 1­2 fine setae.

Male TL 5.5 mm. Male resembles female in shape of head and length of antennae, differing from female in characters usually sexually dimorphic as follows: presence of setal tufts on peduncle of antennae 1 and 2, gnathopod 1 propodus with palm strongly developed, armed with spines and spinose setae on palmar side. Spinose setae on Gn1 and Gn2 generally are much heavier in male. Hump on pleon segment 4 more prominent than in female. Dorsal surfaces of pleon segment 3 and pleon segment 4 cristate. Lateral surfaces of pereopod 7 basis with short plumose setae, uropod 3 with apical outer margin of inner ramus and inner margin of outer ramus strongly setose.

Remarks. The new species resembles the northwestern Atlantic species A uncinata , A. gibba , A. pugetica and A. mexicana . These resemble the new species in the length of antenna 2 that is almost equal to or exceeds the length of the body, antenna 1 shorter than antenna 2, large posterior lobe of the merus of pereopod 7, shape of pleon segment 3, and shape of uropod 3. However they differ from the new species in the shape of the head, presence or absence of corneal lenses, the relative lengths of antennae 1 segments, the shape of the segments of pereopods 7 and the shape of uropod 2. The species are compared in Table 1 . Of these species, A. uncinata most closely resembles A. mississippiana in lacking of corneal lenses. It differs from the new species in the shape of the head ( Fig. 5 ), length of the carpus of pereopod 7 and absence of notched anterior edge from the same segment; shape of dactylus of pereopod 7, shape of the dorsal process of pleon segment 4, absence of subterminal robust long seta of uropod 2, and in having two setae instead of one on the apex of the inner lobe of maxilla 1.

Discussion. According to Barnard and Thomas (1989), the Ampeliscidae is characterized by the extreme constancy of small morphological characters over wide geographic areas. Some of the species that are found in the Gulf of Mexico are also known from the northeastern Pacific with few morphological differences ( Barnard, 1954a; Mills, 1965). Barnard (1954a) examined the ampeliscids from the Caribbean Sea and the Gulf of Mexico, and compared them with those from northeastern Pacific ( Barnard 1954b). He found that the differences in the ornamentation of pleon segment 4 for at least 12 species were not enough to separate them as new species. This close relationship between the morphological characters of the northeastern Pacific ampeliscids and those from the northwestern Atlantic was confirmed by Mills (1965). His work related this affinity in the morphological characters to common ancestors that had an amphi­American distribution before the rising of the Central American isthmus. One of those supposed amphi­American species, A. cristoides Barnard, 1954 , was reinvestigated by Goeke and Heard (1983). They found that specimen from the Gulf of Mexico had a shorter antenna 1 of the female, stronger carina of the urosome, extra tooth on the inner plate of the maxilliped as well as well­developed lateral carinae of pleon segment 3. They described it as a new species, Ampelisca bicarinata . Barnard and Thomas (1989) separated A. burkei , from Florida, from its eastern Pacific twin A. lobata Holmes, 1908 , based on differences in shape and setation of article 4 as well as a single spine on article 5 of pereopod 7. These works demonstrated the subtle differences between the species of the Ampeliscidae and the need for care in identifying them.

Ampeliscids have been placed in species groups based on morphologically similar specimens (Kaïm­ Malka, 2000). Like Ampelisca mississippiana , the ampeliscid species A. pugetica , A. mexicana , A. uncinata , and A. gibba , from the northwestern Atlantic share common morphological characteristics including a large posterior lobe on the merus of pereopod 5, pleon segment 3 with a convex posterior edge bearing an acute spine; and uropod 3 having a lanceolate shape. They can be distinguished by features such as the shape of the head, presence or absence of corneal lenses, shape of the dorsal carina of pleon segment 4, shape of the merus and carpus of pereopod 5, shape of uropod 2, and ratio of article 2 to article 1 in antenna 1.

Ecological notes. Ampelisca mississippiana n.sp. was collected from the head of the Mississippi Canyon at a depth of approximately 480 m. The new species was a numerical dominant at the head of the canyon with densities reaching 26,000 ind.m ­2. A. mississippiana represented more than 85% of the total macrofauna at this location. The sediment at the head of the canyon is muddy, about 73% mud and 25% silt.

Why might A. mississippiana occur at such high densities in this location? Ampeliscid amphipods are known to be tube dwellers collecting suspended and settling matter or raking sediment for feeding ( Mills, 1967a). Canyons are known to be a good conduit of sediment and organic matter from the continental shelf to the deep water. Gardner (1989) found that the canyon focuses internal tides, which lead to mixing near the bottom and resuspension of particulate matter producing a nepheloid layer. These sedimentary conditions might explain the existence of such high densities of this species at this location. It is known also that tube builders can withstand the shear of the current and stabilize the sloppy mud.

The Ampelisca mat at the head of the canyon is likely to be of great ecological importance in recycling of organic carbon that might be transported from the shelf to the slope and the deep Gulf. The tubes stabilize sediments through reducing resuspension and erosion, which is a common ecological role for all the known sedentary tube burrowers that form mats at high densities ( Gage and Tyler, 1991).

Figures

FIGURE 1. Ampelisca mississippiana sp. nov.: female, 5.6 mm, holotype. Mississippi Canyon (28 32 ' 30.7 " N, 89 49 ' 43.9 " W).

FIGURE 2. Ampelisca mississippiana sp. nov.: female, 5.6 mm, holotype. Mississippi Canyon (28 32 ' 30.7 " N, 89 49 ' 43.9 " W).

FIGURE 3. Ampelisca mississippiana sp. nov.: female, 5.6 mm, holotype. Mississippi Canyon (28 32 ' 30.7 " N, 89 49 ' 43.9 " W).

FIGURE 4. Ampelisca mississippiana sp. nov.: Male, 5.4 mm. Mississippi Canyon (28 32 ' 30.7 " N, 89 49 ' 43.9 " W).

FIGURE 5. Heads of the ampeliscid species (A) A. mississippiana; (B) A. uncinata; (C) A. mexicana; (D) A. pugetica.

Tables

TABLE 1. Comparison between the major morphological characteristics of the new species A. mississippiana and other similar ampeliscids.

A. mississippiana	A. uncinata	A. gibba	A. mexicana	A. pugetica
Sources Present study	Chevreux (1887, 1900), Mills (1976, 1971).	Barnard (1960), Lincoln, (1979), Ruffo, (1982).	Barnard (1954a& 1960)	Barnard (1954a& 1960), Dickinson, (1982), Watling, (1995), Serejo et al (2000).
Size 5–5.7 mm	6.5 mm	7.5 mm	5 mm	9­11 mm
Lenses Absent	Absent	Present	Present	Present
Antenna 1 (A1) A1 = art 4 of peduncle of A2, Peduncle. Article 2 = 1.5 x length of article 1. A2: art 4 = art 5.	A1 <Peduncle of A2; Peduncle arti­ cle 2 = 1 to 1.5 x length of article 1. A2: art 4> art 5.	A1 = art 4 of peduncle of A2; Peduncle article 2 = 1.5 to 2.5 x length of article 1. A2: art 4 = art 5	A1 = peduncle of A2; Peduncle arti­ cle 2 = 1.3 x length of article1. A2: art 4 ˜ 1.3 x art 5.	A1 <peduncle of A2, Peduncle. Article 2 = 2 x length of article 1. A2: art 4 = art 5.
Maxilla 1 Inner lobe apex with one long seta	Inner lobe apex with two setae	Inner lobe apex with one seta	Inner lobe apex with two setae	Inner lobe apex with two setae
Pleon segment 4 Wedge­shaped dorsal carina with two setae at top.	Slightly elevated dorsal keel.	Angular dorsal keel.	Slightly elevated dorsal keel.	Saddle­shaped dorsal carina.
Pereopod 7 Iscium longer than merus. Carpus long (about twice as long as broad) and notched ante­ riorly. Dactylus straight.	Iscium longer than merus. Carpus short (about as long as broad), without a notch. Dactylus curving into a fine point.	Ischium is shorter than merus. Carpus short (about as long as broad).	Ischium is shorter than merus. Car­ pus is very short, without a notch.	Ischium subequal in length to merus. Carpus long (about twice as long as broad) and notched anteriorly.
Uropod 2 rami Unequal rami; outer ramus with sub­terminal long comb spine.	Unequal rami; outer ramus with­ out terminal seta.	Equal to slightly subequal rami; outer ramus with long seta.	Equal rami; apices of both armed with a single long spine.	Equal rami, apex of outer ramus with a long spine.
Telson Deeply cleft. Surface of each lobe with 1­2 setae.	Moderately cleft. Surface of each lobe with 4 setae.	Deeply cleft. Surface of each lobe with 4 setae.	Deeply cleft. Surface of each lobe with 3 setae.	Moderately cleft. Surface of each lobe with 3­6 spines