Isaeopsis chiloensis,

Pérez-Schultheiss, Jorge & Pardo, Luis Miguel, 2020, A new crab-associated amphipod of the genus Isaeopsis Barnard, 1916 (Amphipoda Senticaudata: Ischyroceridae) from southern Chile, Zootaxa 4861 (1), pp. 107-119: 109-116

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Isaeopsis chiloensis

n. sp.

Isaeopsis chiloensis  n. sp.

( Figs 1–8View FIGURE 1View FIGURE 2View FIGURE 3View FIGURE 4View FIGURE 5View FIGURE 6View FIGURE 7View FIGURE 8)

Type material: 1 male holotype, 5.2 mm ( MNHNCL AMP-15807): Quellón, Chiloé , Región de Los Lagos, III- 2018, Leg. L.M. Pardo, on Metacarcinus edwardsii  . Paratypes  : 8 males ( MNHNCL AMP-15808): same data as holotype  . 2 females ( MNHNCL AMP-15809): same data as holotype  .

Additional specimens: 20 males ( MNHNCL AMP-15810): Muelle Carelmapu , Región de Los Lagos, 41°44’51.51’’S 73°42’18.28’’W, 19-XII-2004, Col. J. Pérez-Schultheiss, on Romaleon setosum  GoogleMaps  . 38 females ( MNHNCL AMP-15811), 1 dissected male ( MNHNCL AMP-15812), 1 dissected female ( MNHNCL AMP- 15813): same dataGoogleMaps  . 3 females ( MNHNCL AMP-15814): Canal de Chacao , a la cuadra de Carelmapu, Región de Los Lagos, 18-X-2004, Col. J. Pérez-Schultheiss, on Cancridae    9 females ( MNHNCL AMP-15815): Quellón, Chiloé , Región de Los Lagos, III-2018, Leg. L.M. Pardo, on Metacarcinus edwardsii    .

Type locality. Quellón , Chiloé, Los Lagos Region, southern Chile  .

Distribution. The species is known only from Carelmapu and Quellón, in Los Lagos Region. One record of immature specimens (not conserved) in Puerto Cisnes Bay, Aysén ( Figueroa-Muñoz et al. 2020), needs to be confirmed.

Etymology. The specific epithet alludes to the distribution of the new species, until now restricted to the inner sea around Chiloé Island, in Los Lagos region.

Diagnosis. Male gnathopod 2 basis not elongate, with anterior margin smooth; propodus posterior margin short, forming a distally truncated and granulated thumb; dactylus with posterior margin evenly concave. Pereopods 3–7 prehensile, with palm of propodus longer than posterior margin, lined with at least 5 strong setae, and dactyl not surpassing the palm. Uropod 3 outer ramus with a distal comb of fine and tight teeth.

Description. Male, body about 5.2 mm long ( Fig. 1AView FIGURE 1). Head 0.89 × length of pereonites 1–2 combined; rostrum small; anterior cephalic lobe small, with a well-defined distal obtuse tooth; eyes circular, moderate; inferior antennal sinus concave ( Fig. 2CView FIGURE 2). Antenna 1 setose on posteromedial margin; with peduncular articles 1–3 in length ratio of 1:1.5:1.3; peduncular article 1 stout, wider but shorter than articles 2–3; peduncular articles 2–3 similar in length, 3× longer than wide; accessory flagellum 2-articulate, second article minute, article 2 5x length of article 1, both 0.6 × length of peduncular article 1 ( Fig. 4View FIGURE 4 FAm); flagellum 5-articulate, slightly longer than peduncular articles 2–3 combined, each article with setae posterodistally ( Fig. 2AView FIGURE 2 1View FIGURE 1). Antenna 2 longer than antenna 1, also setose on

posteromedial margin; gland cone developed; with peduncular articles 3–5 in length ratio of 1:2:2.4; flagellum 6- articulate, articles with setae distally ( Figs. 2A2View FIGURE 2, 4AView FIGURE 4 2lView FIGURE 2, 4AView FIGURE 4 2mView FIGURE 2).

Lower lip inner and outer lobes distinct, apical margin with a patch of dense pubescence distally, mandibular process well developed ( Fig. 3View FIGURE 3 Ll). Left mandible with 4-dentate incisor and 4-dentate lacinia mobilis; accessory setal row with three broadened dentate setae; molar triturative with a long plumose seta; palp 3-articulate; article 2 somewhat expanded medially, distal margin oblique and medially with plumose setae; article 3 as long as article 2, clavate, apex blunt with numerous long setae ( Figs. 2View FIGURE 2 Mv, 2Md). Right mandible with 6-dentate incisor and 5-dentate lacinia mobilis. Maxilla 1 inner plate triangular with narrow distal half and completely naked, without setae; outer plate subrectangular, apex lined with eight robust setae, some bifid; palp bi-articulate, slightly curved, apex of article 2 blunt with six stout setae distally and four simple setae subdistally ( Fig. 3View FIGURE 3 MX 1). Maxilla 2 inner plate subrectangular, with several distal setae and on medial margin; outer plate slightly more slender than inner plate and distally slightly widened, apex lined with several distal and subdistal setae ( Fig. 3View FIGURE 3 MX 2). Maxilliped inner plate subrectangular, slightly widened distally and lined by a row of five distal slender setae, that continue in a row of at least ten plumose setae that cover most of the medial margin; distal margin forming a diffuse subquadrate plate in the distomedial half, carrying two short strong setae and a long and curve strong seta on distomedial angle; medial margin with a blunt and slender subdistal projection ( Fig. 3View FIGURE 3 MXP); outer plate elongate semiovoid, with a row of 4 long slender setae on apical margin, several slender setae, submarginal and marginal, along the medial margin and seven strong spaced setae along the medial margin, bigger toward the distomedial angle; palp slender, 4-articulate; article 1 oblique in distal margin; article 2 longest, lined with several slender setae on medial margin; article 3 slightly broadened in half, with a mediodistal row of at least 10 slender setae and 3 setae at laterodistal corner; article 4 short, oblong, with a distal-distomedial row of at least 7 slender setae ( Fig. 4View FIGURE 4 Mx).

Gnathopod 1 coxa subrectangular, with rounded angles ( Fig. 7View FIGURE 7 Cx1); basis with anterior margin expanded and straight, with some long setae and with anterodistal margin rounded; ischium short, with small anterior lobe; merus longer than ischium, with short anterior margin and a long posterior margin forming a distal rounded lobe with a group of apical setae; carpus trapezoidal, with the distal third of posterior margin convex and covered with setae; propodus longer than carpus, oval, with anterior margin slightly convex, posterior margin slightly shorter than half of anterior margin and shorter than palm, palm slightly convex, with several setae, defined by a stout seta, palmar angle not well defined, slightly convex; dactylus falcate, apex reaching the palmar angle, lined with small teeth on inner margin ( Fig. 5GView FIGURE 5 1View FIGURE 1). Gnathopod 2 enlarged ( Fig. 5GView FIGURE 5 2lView FIGURE 2, 5GView FIGURE 5 2mView FIGURE 2); coxa subquadrate, ventral margin straight, anterodistal angle and anterior margin convex ( Fig. 7View FIGURE 7 Cx2); basis slightly curved, gradually broadened to the apex, with slender setae in distal half of anterior margin, anterodistal angle produced laterally as a rounded lobe; ischium small, without setae; merus slightly longer than ischium, produced in a small rounded posterodistal lobe, anterior margin very small, hidden by carpus; carpus subtriangular, shorter than merus; propodus longer than distance from propodus to merus, subrectangular, anterior margin convex, with some sparse short setae along the distal half, posterior margin short, most of his length produced as a thumb with rugose apex; palm subparallel to anterior margin, almost three × the length of posterior margin, concave, covered with slender long dispersed setae in medial surface and with a distal obtuse tooth near the articulation with dactylus; dactylus falcate, not fitting palm, without small spines or minute setae on inner margin.

Pereopod 3 ( Fig. 6PView FIGURE 6 3View FIGURE 3) coxa subquadrate, with anterior and posterior margins subparallel, straight or posterior slightly concave, ventral margin convex ( Fig. 7View FIGURE 7 Cx3); basis anterior margin slightly concave, with a small rounded distal lobe provided with a slender seta, posterior margin convex, covered with several dispersed slender setae over its entire length and with a distal group of setae; ischium with setal group posterodistally; merus gradually expanded anterodistally with anterodistal corner produced with a distal group of slender setae, posterior margin with three or four groups of setae; carpus subrectangular, but slender proximally, slightly longer than half of merus, with a pair of setae at slightly produced anterodistal corner, and three or four group of setae on posterior margin; propodus longer than merus, narrow-ovoid, subchelate, anterior margin convex, with a long setae at anterodistal corner, posterior margin with a palm defined with a line of six strong setae; dactylus falcate, slightly shorter than the palm of propodus. Pereopod 4 ( Fig. 6PView FIGURE 6 4View FIGURE 4) similar to pereopod 3 in shape, coxa subquadrate, larger than coxa 3 ( Fig. 7View FIGURE 7 Cx4). Pereopod 5 ( Fig. 6PView FIGURE 6 5View FIGURE 5) coxa bilobate, anterior lobe subrectangular, much bigger than posterior lobe, expanded distally ( Fig. 7View FIGURE 7 Cx5); basis posterior margin expanded in a lobe, anterior margin straight with isolated marginal setae and a group of setae in anterodistal corner; ischium with a group of setae anterodistally; merus, anterior margin straight with two to four groups of setae, and a group of setae at anterodistal corner, posterior margin gradually expanded distally with four groups of setae, posterodistal corner slightly produced with a group of stout and simple setae; carpus subrectangular, smaller than merus, proximally narrow and widened gradually to distal end, posterodistal and anterodistal corners with a group of long setae; propodus oblong, subchelate, as long as merus, with two groups of paired setae on posterior margin and a group of setae on posterodistal corner, posterior margin with a palm defined with a line of six or seven strong setae; dactylus falcate, as long as 1/2 of palm, with a longitudinal pectinate row of sharp teeth at side. Pereopod 6 ( Fig. 7PView FIGURE 7 6View FIGURE 6) slightly longer than pereopod 5, coxa bilobate, both lobes subsimilar, anterior rounded, posterior with anteroventral margin nearly straight ( Fig. 7View FIGURE 7 Cx6). Pereopod 7 ( Fig. 8PView FIGURE 8 7View FIGURE 7) subsimilar to pereopod 6, coxa subrounded ( Fig. 7View FIGURE 7 Cx7).

Pleonal epimera posterior margin of each epimeron somewhat roundish expanded backward ( Fig. 7View FIGURE 7 Ep). Uropod 1 ( Fig. 8UView FIGURE 8 1View FIGURE 1) peduncle rectangular, longer than rami, with four lateral and three medial setae dorsally, inter-ramal process small, near 1/5 of the length of inner ramus; both rami linear and slender; outer ramus shorter than inner ramus with three dorsolateral and two dorsomedial setae, with a group of setae on apex; inner ramus with two medial setae, with a group of setae on apex. Uropod 2 ( Fig. 8UView FIGURE 8 2View FIGURE 2) peduncle rectangular, slightly longer than half peduncle of uropod 1, nearly as long as rami, with one seta dorsolaterally and one laterodistal seta; both rami linear and slender; outer ramus shorter than inner ramus with dorsolateral and dorsomedial setae, and a group of setae on apex; inner ramus with one or two medial setae and a group of setae on apex. Uropod 3 ( Fig. 8UView FIGURE 8 3View FIGURE 3) biramous, shorter than peduncle of uropod 1; peduncle rectangular with lateral and medial setae dorsolaterally and dorsodistally; rami styliform, outer ramus slightly longer than inner ramus, naked, apex slender and ctenate; inner ramus naked, apex without setae. Telson ( Fig. 8TView FIGURE 8) subquadrate, distally rounded, with a strong laterodistal seta in each side.

Female, body about 5.2 mm long ( Fig. 1BView FIGURE 1). Gnathopod 1 similar to those of male, dactylus with longitudinal lateral comb and a row of marginal spines in posterior margin; Gnathopod 2 not enlarged, similar to gnathopod 1, propodus with palm straight, as long as posterior margin, defined by a pair of strong setae, dactyl longer than palm; oostegites well developed, margin lined with long setae.

Ecology. The specimens were collected on the body surface and gills of the cancrid crabs Metacarcinus edwardsii ( Bell, 1835)  and Romaleon setosum ( Molina, 1782)  . Immature specimens from Puerto Cisnes Bay (Figueroa- Muñoz et al. 2020) were collected in samples containing Metacarcinus edwardsii  , but without evidence of direct association.

Remarks. A detailed comparison between both species of Isaeopsis  is difficult because the original description and illustrations of I. tenax  do not include details of several important characters. For example, it would be interesting to know the structure of the inner plate of maxilliped in this species, which in I. chiloensis  n. sp. has an unusual mediodistal large strong and curved seta. These observations would improve the generic diagnosis, but require the examination of the type material of I. tenax  .

However, the main characters of the genus are shared in both species and several differences could be identified. Isaeopsis chiloensis  n. sp. differs from I. tenax  principally in the structure of male gnathopod 2, the palm of pereopod 3–7 and outer ramus of uropod 3. In I. tenax  coxa 1 apparently is smaller than the following coxae, while in the new species, it is smaller, but follows the line of the coxae 2–4. The basis of gnathopod 2 of I. tenax  is elongated, with the anterior margin serrulate ( Barnard 1916: 268), instead in the new species it is not especially elongate, with the anterior margin smooth. In the new species, the posterior margin in the propodus of male gnathopod 2 is almost undeveloped, forming part of the thumb that define the palm, while in I. tenax  it is developed and slightly concave. Additionally, the dactyl of male gnathopod 2 is evenly concave in the posterior margin of I. chiloensis  n. sp., but bearing a convexity in the middle in I. tenax  .

The prehensile structure of pereopod 3–7 is more developed in I. chiloensis  , with the palm lined by five or more strong setae. In I. tenax  , at least in pereopod 3, there are only three strong setae in the palm (described and depicted as teeth by Barnard 1916), and in the pereopod 7, only one strong seta.

Other differences are in the outer ramus of uropod 3. In I. chiloensis  there is a comb of numerous slender and tight denticles, instead in I. tenax  , there are only two denticles and a distal hook.