Description of two new epigean species of the genus Hyalella S. I. Smith, 1874 (Crustacea: Amphipoda: Hyalellidae) from Jalisco, Mexico Author Marrón-Becerra, Aurora Universidad Nacional Autónoma de México, Posgrado en Ciencias del Mar y Limnología. Av. Ciudad Universitaria 3000, Coyoacán, Ciudad de México, México. E-mail: auro & ca- Author Hermoso-Salazar, Ana Margarita Universidad Nacional Autónoma de México, Facultad de Ciencias. Av. Ciudad Universitaria 3000, Coyoacán, Ciudad de México, México. & - 22 e & a 7749 cfa Author Ayón-Parente, Manuel Universidad de Guadalajara, Departamento de Ecología, Centro Universitario text Nauplius 2023 e 2023010 2023-05-22 31 1 22 http://dx.doi.org/10.1590/2358-2936e2023010 journal article 293306 10.1590/2358-2936e2023010 cb3febe4-b34d-4ea3-b687-2760e3212a66 2358-2936 10905014 601BE1CA-7CC6-428E-93D2-22E2A7749CFA Hyalella morronei sp. nov. ( Figs. 7–11 ) http://zoobank.org/ urn:lsid:zoobank.org:act: 9D8B4C87-ED22-4D47-A310-AD9CAA24F7B6 Etymology. This species is named in honor of Dr. Juan José Morrone, for his outstanding contributions to biogeography and especially for his interest in amphipods of the genus Hyalella . Material examined . Holotype , male, body size 5 mm , cephalothorax size 0.51 mm ( CNCR 35782 ), from Teuchitlán stream ( Fig. 1A, D ), La Vega reservoir, in Teuchitlán , state of Jalisco , Mexico ( 20°41’11.15”N 103°50’36.43”W ) 6 April 2018 , 1,288 m .a.s.l., temperature 24.2 °C, pH 6.9, DO saturation 112%, associated with the buried roots of littoral macrophytes, A. Marrón - Becerra, M . Ayón - Parente , and M. Hermoso-Salazar , collectors . Paratypes ( 30 males , 30 females ): males mean body size 4.6 ± 0.6 mm , mean cephalothorax length 0.46 ± 0.05 mm (n = 27), female mean body size 4.8 ± 0.5 mm , mean cephalothorax length 0.61 ± 0.13 mm (n = 30) (permanent slides CNCR 35783 , male paratypes CNCR 357784 , and female paratypes CNCR 35785 ), collected from the same locality as the holotype . Diagnosis . Pleonites 1−2 without dorsoposterior carina.Coxa 4 excavated posteriorly. Eyes pigmented, black. Antenna 1 shorter than antenna 2, without accessory flagellum. Antenna 2 less than half of body length, almost reaching the seventh pereonite.Maxilla 1 palp short,reaching less than half of length between base of palp and tip of setae of outer plate, with 1 short stout distal seta and without setules; inner plate slender with 2 strong pappose apical setae. Maxilla 2 inner plate with 2 strong pappose setae on inner margin and serrate setae on distal margin. Gnathopod 1 propodus hatchet-shaped, palm with transverse slope, inner face with 4 serrate setae, comb scales on distoposterior and distoanterior border, carpus inner face lobe with 2 serrate setae. Gnathopod 2, basis hind margin with 1 slender seta, merus without medial seta, palm with wide excavation. Uropod 1 male inner ramus without curved setae. Uropod 3, ramus longer than peduncle, styliform and with 5 cuspidate setae.Telson as long as wide, or longer, distal margin rounded or truncated with 2 long simple setae widely separated. Sternal gills on segments 3−7. Table 1. Comparison between the closest Mexican species of the genus Hyalella .

Hyalella marysolae sp. nov.	Hyalella morronei sp. nov.	Hyalella azteca ( de Saussure, 1858 )	Hyalella maya Marrón-Becerra, Hermoso-Salazar and Solís-Weiss, 2018	Hyalella cenotensis Marrón-Becerra, Hermoso-Salazar and Solís-Weiss, 2014 (female)	Hyalella tepehuana Marrón-Becerra, Hermoso-Salazar and Rivas, 2020
Locality	Tajo (river)	Teuchitán (river)	Veracruz	Aktun-Ha (cenote)	Aktun-Ha (cenote)	Durango (river)
Total body length (mm)	4.8	5	7.8	3.85−4.1	5.3	5.4−5.7
Dorsal projections or carinae on pleon 1−2	Present (short)	Absent	Present	Present (short)	Present (short)	Present
Head length vs second pereonite	Reaching half	Reaching two thirds	Reaching half	Reaching half	Equal in length	Reaching half
Eye form	Subquadrate/rounded	Subquadrate	Rounded	Subtriangular	Absent	Subtriangular
Antenna 1 length vs. relative body length	Reaching the middle of fifth pereonite	Reaching the fifth pereonite	Reaching the middle of third pereonite	Reaching one-third of third pereonite	Reaching the middle of third pereonite	Overreaching the middle of third pereonite
Antenna 1 peduncle vs. body length	Reaching one third of first pereonite	Reaching half of first pereonite	Reaching half of first pereonite	Reaching beyond half of first pereonite	Reaching half of first pereonite	Overreaching the middle of first pereonite
Antenna 1 proportion, articlesof peduncle	1.6:1.3:1	1.5:1.2:1	1.6:1.25:1	1.4:1.3:1	1.7:1.3:1	1.5:1.4:1
Antenna 1 flagellum, numberof articles	12	10	7−9	9	7	9−11
Antenna 1 aesthetasc in flagellum on articles	4−11 (1 pair)	4−5 (1), 6−7 (1 pair), 8−9 (1)	3−6 (1 pair), 7−8 (1)	−	−	4−1 (1 pair), 8−11 (1)
Antenna 2 length	Almost half of total body length	Almost half of total body length	Less than one third total body length	Almost half of total body length	Almost one third of total body length	Slightly longer than one third total body length
Antenna 2 length vs. total body length	Almost reaching the seventh pereonite	Almost reaching the seventh pereonite	Reaching the fourth pereonite	Almost reaching middle of sixth pereonite	Reaching one-third of fourth pereonite	Reaching middle of fifth pereonite
Antenna 2 flagellum number of articles	15	14	8−10	10	8	11−12
Antenna 2, flagellum relative length vs. peduncle	1.5×	1.8×	1.1×	1.2×	1.35×	1.25×
Palp of maxilla 1	With setules	Without setules	With setules	With setules	With setules	With setules

Table 1. Cont.

Hyalella marysolae sp. nov.	Hyalella morronei sp. nov.	Hyalella azteca ( de Saussure, 1858 )	Hyalella maya Marrón-Becerra, Hermoso-Salazar and Solís-Weiss, 2018	Hyalella cenotensis Marrón-Becerra, Hermoso-Salazar and Solís-Weiss, 2014 (female)	Hyalella tepehuana Marrón-Becerra, Hermoso-Salazar and Rivas, 2020
Maxilla 1 inner plate, number of setae	2−3	2	3	2−3	3	3−4
Maxilliped, palp, 4 th article inner margin setae	4	5	4	3	2	3
Relative length of nail vs. palp 4th article	2/3	1/2	More than 2/3	1/2	Almost as long as	2/3
Gnathopod 1, propodus form	Hammer	Hatchet	Hammer	Hammer	Hammer	Hammer
Gnathopod 1, propodus maximum length vs. width	1.5×	1.25×	1.6×	1.8×	1.6×	1.5×
Gnathopod 1, male propodus, inner face, number of serrate setae in a row	3−5	4	4	4	−	3+1
Gnathopod 2 basis length vs. width	Almost 4×	More than 3×	Almost 3×	3.5×	−	More than 3×
Pereopod 7, lobe length vs. distal margin of ischium	Overreaching	Shorter	Overreaching	Overreaching	Shorter	Overreaching
Pereopod 7 relative width of lobe vs. width of basis (measure at cleft)	2−2.2×	2×	2.5×	3×	1.7×	2×
Pereopod 7 number of serrations and setae	10−11 with setules	9−11 with setules	18 with setules but 3 to 5 stout setae	9 with setules	11 with setules but 3 stout setae	14 with setules but 1−2 with stout setae
Telson	W ≥ L	W ≥ L, or L>W	W>L	W>L	W>L	L>W
Telson distal simple setae	Two widely separated	Two widely separated	Two apposed	Two widely separated	Absent	Two widely separated

Description of males based on holotype ( CNCR 35782) and paratypes . Body, pleonites 1−2 without dorsoposterior carina ( Fig. 7A ). Epimeral plates 1−3 slightly acuminate. Coxae 1−3 ( Fig. 7A ) subequal in shape, subrectangular, longer than wide; coxa 1shorter than coxae 2−3.Coxa 4wider than coxae 1−3, with deep posterior excavation; coxae 1−4 slightly overlapping anterior coxa, distal margin rounded with small setae widely separated, acumination absent. Coxae 5−7 reduced, shorter than coxae 1−4, bilobate except coxa 7, lobe of basis overlapping posterior lobe of coxa (except coxa 7); coxa 5 with 2 subequal lobes, anterior lobe slightly shorter than posterior, overlapping coxae 4 and 6; coxa 6 with 2 unequal lobes, anterior lobe reduced; coxa 7 with 1 single lobe, anterior lobe absent. Head typically gammaridean ( Fig. 7A ), with smooth surface, total length less than the combined length of first two pereonites, reaching two-thirds of second pereonite, rostrum absent. Eyes pigmented, medium, subquadrate, and black, located between the insertions of antennae 1 and 2. Antenna 1 ( Fig. 7A, B ) less than half body length, about 65% the length of antenna 2 but longer than peduncle of antenna 2, almost reaching to fifth pereonite; peduncle longer than head, reaching half of first pereonite, article 1 and 2 subequal in length, article 1 wider than articles 2−3, article 2 longer and wider than article 3, article 3 shorter and thinner than articles 1−2 (proportions 1.5:1.25:1), article 1 with 1 medial cuspidate seta, and 1 cluster with 3 cuspidate setae at distal end; f lagellum with 10 articles reduced gradually towards distal portion, flagellum longer than peduncle; aesthetascs present on articles 4−8, 4−5 (1 aesthetasc), 6−7 (1 pair), 8−9 (1 aesthetasc); accessory flagellum absent. Antenna 2 ( Fig.7A, C ) almost 1.4 times longer than antenna 1, almost half of total body length, almost reaching seventh pereonite; peduncle reaching third pereonite, articles gradually increasing in length and decreasing in width, article 5 longer than article 4 (about 1.3 times); flagellum with 14 articles, longer than peduncle (about 1.8 times); flagellum without aesthetascs. Figure 7. Hyalella morronei sp. nov. , male holotype 5 mm (CNCR 35782), Teuchitlán stream, Teuchitlán, state of Jalisco, Mexico. ( A ) Lateral habitus; ( B ) antenna 1; ( C ) antenna 2. Scale bars: 100 µm. Mouthparts: upper lip ( Fig. 8A ) distal margin rounded with numerous setules, being longer and more distant towards lateral end. Lower lip ( Fig. 8B ) subquadrate, outer lobes without notches or excavations; mandibular projection of outer lobes widely rounded; without inner lobes. Left and right mandibles ( Fig. 8E, F ) without palp, asymmetric. Incisor with 6 teeth. Left lacinia mobilis similar to incisor process, with 5 teeth; setal row on left mandible with 3 main pappose setae plus accessory setae. Right lacinia mobilis more reduced than left, with 2 pairs of asymmetric L-shaped teeth, fused at base, proximal pair subequal to distal, inner margin denticulate; near lacinia mobilis base with setules on inner margin; setal row on right mandible with 2 main pappose setae plus accessory setae and with setulae near molar process. Molar process large, cylindrical, and triturative, accessory pappose setae in both molars. Maxilla 1 ( Fig. 8C ) palp short, vestigial, uniarticulated, longer than wide, distally pointed with 1 short stout distal seta, reaching less than half of palp length, setules absent; palp length less than half distance between base of palp and base of seta on outer plate; inner plate slender, shorter than outer plate, with 2 pappose apical setae; outer plate with 9 apical stout serrate setae and with setules. Maxilla 2 ( Fig. 8D ) plates subequal in length, width, and shape; inner plate shorter and slender, with 2 pappose setae on mid-distal margin, and with simple setae and 5 long serrulate setae on distal margin; outer and inner plates with large simple distal setae ( type A2) and few setules on both distal surfaces and margins. Maxilliped ( Fig. 8F ) inner plate lobe longer than outer plate lobe; distal margin slightly convex; inner plate distal margin with 3 cuspidate setae of equal size and with plumose setae, inner margin with several pappose setae; outer plate, inner and distal margins with numerous simple setae. Palp composed of 4 articles subequal in maximum length; first article with 2−4 simple setae at inner distal end and 2 setae at outer distal end; second article with numerous simple setae on inner margin and 2−3 at outer distal end; third article with several setae on distal margin, distal end on outer margin with 7 simple setae and comb scales, inner margin with at least 8 setae and 6 at distal end; fourth article unguiform, longer than nail, with comb scales, inner margin with 5 setae near distal half, outer margin with 1−2 setae near nail base; nail almost half length of fourth article, with serration on distal half. Gnathopod 1 ( Fig. 9A ) subchelate, propodus hatchet-shaped, shorter than gnathopod 2. Basis elongated, maximum length almost 3.3 times longer than maximum width; medial seta absent, posterior distal end with cluster of 2 setae. Ischium short, subquadrate, longer than wide, length longer than maximum width of basis, and as long as maximum length of merus; distal posterior end with 1 cluster of 2 setae. Merus longer than wide, at one-third of the posterior margin with comb scales; almost at distal margin with 3−6 setae. Carpus longer than wide, longer and slightly wider than propodus, with strong, short, and wide posterior lobe forming one scoop-like structure open to inside; mesial surface of lobe with 2 serrate setae, outer surface with comb scales and serrate setae (10−12), anteriodistal end with 5 simple setae. Propodus maximum length 1.25 times longer than wide; inner surface near distal margin with 4 serrate setae in 1 row; distal anterior end with 2 clusters of approximately 5 setae; distal half of anterior and posterior surfaces with comb scales; palm transverse, posterior distal end with 1 robust seta and cup for dactyl; posterior surface, near distal third with 1 long, simple seta. Dactyl claw-like; nail present; anterior surface near proximal third with 1 plumose seta, with comb scales on anterior surface. Gnathopod 2 ( Fig. 9B ) subchelate. Basis elongate, more than 3 times longer than wide; posterior margin with 1 slender seta.Ischium short, subquadrate, as long as merus. Merus short; distal end of posterior margin with 4 simple setae, medial setae absent; distal quarter of posterior inner and outer surfaces with comb scales. Carpus shorter than propodus, length similar to merus maximum length, slightly exceeding1.5 times width of merus; anterodistal end with 2 simple setae;posterior lobe scoop-like, elongate, almost 1.5 times width of merus, with several submarginal pappose setae and comb scales. Propodus robust, subrectangular, 1.5 times longer than maximum width; palm oblique, as long as posterior margin; slope slightly irregular, with several long simple setae, few short and medium setae; distal margin of palm with 1 truncated process near insertion of dactyl and with 1 wide posterior excavation at base; palm posterior distal end with 2 robust setae, comb scales, and cup for dactyl. Cup for dactyl as long as the space between it and the truncated process. Dactyl claw-like, congruent with palm, occupying less than half of cup of dactyl, without comb scales; proximal third of outer margin with 1 plumose seta; inner margin slightly crenulated with separated setules. Figure 8. Hyalella morronei sp. nov. , male holotype 5 mm (CNCR 35782), Teuchitlán stream, Teuchitlán, state of Jalisco, Mexico. Mouth parts: ( A ) upper lip; ( B ) lower lip; ( C ) left maxilla 1; ( D ) left maxilla 2; ( E ) left mandible; ( F ) right mandible; ( G ) left maxilliped. Scale bars: 100 µm. Figure 9. Hyalella morronei sp. nov. , male holotype 5 mm (CNCR 35782), Teuchitlán stream, Teuchitlán, state of Jalisco, Mexico. ( A ) Male gnathopod 1; ( B ) male gnathopod 2. Female paratype 4.8 mm (CNCR 35785). ( C ) Female gnathopod 1; ( D ) female gnathopod 2. Scale bars: 100 µm. Pereopods 3−7 ( Fig. 10A, E ) simple, gradually expanding posteriorly.Pereopod 5shorter than 4and 6. Pereopod 3 ( Fig. 10A ) basis elongate; posterior margin with 1 simple seta; anterodistal and posterodistal ends with simple setae. Ischium subquadrate; posterodistal end with 1 pair of setae. Merus longer than ischium (almost 2.5 times the maximum length); anterior margin without robust seta;posterior margin with 4 clusters of slender simple setae (3 clusters of 2 setae plus1 single separated seta); anterodistal and posterodistal ends with 1 cluster of 3−4 slender setae, anterodistal end with 1 robust seta. Carpus shorter and more slender than merus; posterior margin with 4 stout setae plus accessory setae; posterodistal end with at least 4 slender setae; anterodistal end with at least 2 setae. Propodus shorter than anterior margin of merus, more slender than carpus; posterior margin with 7 stout setae plus accessory seta; anterodistal end with 6 simple setae. Dactyl claw-like, length less than half of propodus; nail present; first proximal third of anterior margin with 1 plumose seta; anterior and posterior margins with 1 simple seta near base of nail. Pereopod 4 ( Fig.10B ) similar in shape to pereopod 3 but slightly longer; coxa 4 wider than coxa 3 with posterior excavation; basis posterior margin with 1 simple seta; propodus posterior margin with 7 setae. Pereopods 5−7 ( Fig. 10C, E ) similar in shape; basis posterior lobe rounded and denticulate. Pereopod 7 ( Fig. 10E ) basis lobe widely expanded posteriorly, shorter than ischium distal margin;wider than lobes of pereopods 5 and 6; basis lobe, width almost 2 times as wide as basis (measured at cleft between basis and basis lobe); posterior margin with 9−11 serrations, each with one setule, without stout seta on distal margin; anterior margin with 4 stout setae and 3 at distal end. Dactyl length less than half of propodus. Figure 10. Hyalella morronei sp. nov. , male holotype 5 mm (CNCR 35782), Teuchitlán stream, Teuchitlán, state of Jalisco, Mexico. ( A ) Pereopod 3; ( B ) pereopod 4; ( C ) pereopod 5; ( D ) pereopod 6; ( E ) pereopod 7. Scale bars: 100 µm. Pleopods 1−3 similarinsizeandshape, notmodified, biramous, elongated, ramus longer than peduncle, rami multi−annulated, with numerous plumose setae. Pleopod 1 ( Fig. 10F ), inner margin of peduncle distal end with two short retinacula (coupling hooks). Uropod 1 ( Fig. 11A ) longer than uropod 2 ( Fig. 11B ); peduncle longer than rami, proximal half of dorsal margin with 3 dorsal cuspidate setae with accessory seta, inner and outer distal ends with 1 cuspidate seta; rami subequal, inner ramus slightly longer, with 2 dorsal cuspidate setae with accessory seta and 5 distal cuspidate setae, outer ramus with 3 dorsal cuspidate setae with accessory seta and 4 distal cuspidate setae; male without curved setae on inner ramus. Figure 11. Hyalella morronei sp. nov. , male holotype 5 mm (CNCR 35782), Teuchitlán stream, Teuchitlán, state of Jalisco, Mexico. ( A ) Uropod 1; ( B ) uropod 2; ( C ) uropod 3; ( D ) telson. Scale bars: 100 µm. Uropod 2 ( Fig.11B )longer than peduncle of uropod 1; peduncle as long as rami or slightly shorter, with 2 dorsal cuspidate setae (with accessory seta) on distal half and 1 cuspidate seta at distal end; rami subequal, inner ramus slightly longer with2dorsal cuspidate setae with accessory seta and 5 cuspidate distal setae with accessory seta, outer ramus with 2dorsal cuspidate setae with accessory seta and up to 4 distal cuspidate setae, the 3rd cuspidate seta is shorter with accessory seta. Uropod 3 ( Fig. 11C ) longer than peduncle of uropod 2; peduncle subrectangular, wider than ramus with 5 robust distal cuspidate setae (with accessory seta) of variable length; inner ramus absent, outer ramus uniarticulate, slender, longer than peduncle, about 4 times the length of larger seta of peduncle, basal width about 2 times apex width of ramus, with 3 slender apical simple setae and 1 connate seta. Telson ( Fig. 11D ) entire, as long as wide, distal margin rounded with 2 long simple setae widely separated; dorsal surface bearing 2 clusters of 3 plumose setae near middle distal portion, near margin, symmetrically distributed. Coxal gills sac-like, present on segments 2−6. Sternal gills tubular, present on segments 3−7. Description of paratypes females. Females mean body size 4.8 ± 0.5 mm , mean cephalothorax length 0.61 ± 0.13 mm (n = 30). Gnathopod 1 ( Fig. 9C ), carpus with 4 setae on inner face lobe; propodus with row of 5 setae on inner face. Gnathopod 2 ( Fig. 9D ) smaller than male gnathopod 2, parachelate; palm slightly reverse oblique; basis posterior margin with2 setae. Propodus shorter than twice maximum width (1.6 times); outer face with 2 setae in 1 row and 3 large setae near palm, with 1 medial seta on mid-posterior surface; anterior and posterior distal half with comb scales. Pereonite 2 with 1 anterior excavation or notch for amplexus. Pereopod 7lobe with12 serrations and setules,without stout setae on distal margin.Oostegites subtriangular, with curled setae on margins, almost reaching half the length of merus. Telson similar in shape to male. Intraspecific variation . The peduncle of antenna 1 in some individuals of this species is as long as or even longer than the head (reaching half of the first pereonite). In maxilla 1, the palp nearly reaches or overreaches half of the distance between the base of the palp and the base of serrate seta on the outer plate. The telson can be as long as wide or longer. Remarks. One notable morphological feature of H. morronei sp. nov. is the hatchet-shaped propodus of gnathopod 1, which is 1.25× as long as wide (less than twice the maximum width), which distinguishes this species from the other species from Mexico ( Tab. 1 ). This morphology of gnathopods has been observed in hyalellids species from South America, including e.g. hyalellids from Chile ( Gonzalez, 2003 ); therefore, H. morronei sp. nov. constitutes the first North American species to exhibit this morphology. Hyalella morronei sp.nov. can be easily distinguished by the length of the flagellum of the antenna 2, it is almost twice the length of the peduncle, in other species from Mexico it is 1.1×−1.5× ( Tab. 1 ). The most morphologically similar species to H.marysolae sp. nov. is H. morronei sp. nov. and the differences are described above. H. morronei sp. nov. and H. marysolae sp. nov. have more articles in the f lagellum of antenna 2 (14 and 15 articles, respectively) and the antenna reaches the seventh pereonite; in contrast, in other species from Mexico , the flagellum is shorter and has 8–12 articles ( Tab. 1 ). Additionally, the form of the eyes in H. morronei sp. nov. and H. marysolae sp. nov. are subquadrate, whereas in H. azteca are rounded, in H. maya and H. tepehuana are subtriangular, and in H. cenotensis are absent ( Tab. 1 ). H. cheyennis Bueno, Oliveira and Wellborn, 2019 from Oklahoma , USA , as H. morronei sp. nov. , lacks projections or carinae on pleonites 1−2; in addition, both species have serrated setae on maxilla 2, and in the male gnathopod 2, the palm of propodus is as long as the posterior margin. However, these two species are separated from each other by the shape of the propodus of gnathopod 1: in H. morronei sp. nov. it is hatchet-shaped and in H.cheyennis is hammer-shaped. Moreover, H. morronei sp. nov. has one slender seta on the posterior margin of the basis of pereopods 3−4, while H. cheyennis has two setae; H. morronei sp. nov. lacks distal robust setae on the lobe of pereopod 7, and H. cheyennis has three robust setae; H. morronei sp. nov. has less robust setae on peduncles of uropods 1−2 (three and two robust setae, respectively), and H. cheyennis has six on uropod 1 and five cuspidate setae on uropod 2;and the ramus of uropod 3 is longer than the peduncle in H. morronei sp. nov. , which is shorter in H. cheyennis . Type locality. Teuchitlán stream ( Fig. 1A, D ), La Vega reservoir, Teuchitlán, state of Jalisco , Mexico ( 20°41’11.15”N 103°50’36.43”W ) . Habitat and conservation. This species was collected from the water column, associated with the roots of Lemna sp. , although the predominant vegetation was water lettuce Pistia stratiotes L., 1753. As in the case of H. marysolae sp. nov. , the habitat of this species is also threatened by anthropogenic activities. The Teuchitlán stream is impacted by caustic soda waste, molasses, cane bagasse, domestic plastics, and sewage discharge generated by the Teuchitlán people, in addition to nitrogen fertilizers from sugarcane crops ( R AMSAR , 2018). Documenting the diversity of species of highly impacted freshwater environments such as the Teuchitlán stream, is a crucial task as increased anthropogenic activity may threaten the biodiversity of these habitats. In summary, the new species in the state of Jalisco described in this paper, H. marysolae sp. nov. and H. morronei sp. nov. , can be distinguished from all other Hyalella species present in Mexico by several morphological characters, including the length of the antenna 2 being approximately half as long as the body length, composed of 14–15 articles, and the relative length of the ramus in the uropod 3, which is longer than the peduncle. The length of the antenna 2 distinguishes these species from those found in North America, except for Hyalella longicornis Bousfield, 1996 , which has less than 16 articles in the f lagellum of antennae 2, and more than 5 setae on the posterior margin of the basis of both gnathopods. In contrast, H. marysolae sp. nov. and H. morronei sp. nov. have only one simple seta. The findings concerning these two species contribute to highlighting the hidden diversity of the genus by showing that the biodiversity in the group is underestimated and increases to six the total number of species from Mexico .It is important to emphasize the need for further studies within this group of freshwater amphipods due to their importance in environmental and toxicological research, and secondly to record the unknown species before their habitat is lost. It is important to underline that we could preserve the unknown biodiversity of the group by enhancing the policies to protect freshwater bodies and raising awareness of water issues in the local communities.