Rev. Biol. Trop. 50(2): 659-667, 2002 www.ucr.ac.cr www.ots.ac.cr www.ots.duke.edu Redescription of the freshwater amphipod Hyalella faxoni from Costa Rica (Crustacea: Amphipoda: Hyalellidae) Exequiel R. González1 and Les Watling2 1 Facultad de Ciencias del Mar, Universidad Católica del Norte, Casilla 117, Coquimbo, Chile, Ph.: 56-51-209931, Fax: 56-51-209812, e-mail: [email protected] 2 Darling Marine Center, University of Maine, 193 Clark’s Cove Rd., Walpole, Maine 04573, USA, Ph.: 207-563-3146, Fax: 207-563-8407, e-mail: [email protected] Received 20-VII-2001. Corrected 21-XI-2001. Accepted 05-V-2002. Abstract.: Hyalella faxoni Stebbing, 1903 from Costa Rica is redescribed. The species was previously in the synonymy of Hyalella azteca (Saussure, 1858). The morphological differences between these two species are discussed. Key words: Amphipoda, freshwater, Hyalella, epigean, South America. Hyalella faxoni Stebbing, 1903 is part of onymized H. knickerbockeri (Bate 1862), H. what has been called the “azteca complex” dentata Smith, 1874, H. inermis Smith, 1875 named after Hyalella azteca (Saussure 1858), a and Lockingtonia fluvialis Harford, 1877, common freshwater organism generally under H. azteca, but did not mention H. faxoni. thought to be found all over North America, Weckel (1907) put H. faxoni in the synonymy Central America and probably also in northern of H. knickerbockeri, which she thought had South America. The original description by precedence over H. dentata. She did not see Saussure (1858), based on samples from a Stebbing (1906) who had already put H. “cistern” in Veracruz and Mexico City in knickerbockeri under H. azteca. She also does Mexico, was poorly documented and figured. not mention Saussure’s species, indicating that In North and Central America, most of the she considered it a valid separate species in the freshwater species of Hyalella recorded were sense of Smith (1874, 1875). assigned to H. azteca, however, seven other Several species in the “azteca complex” related species are known from the region have been the subject of genetic studies using (González and Watling 2002). allozyme (Duan et al. 1997, 2000, Hogg et al. The lack of evident morphological varia- 1998,) and allozymes and PCR (Thomas et al. tion and detailed study of H. azteca resulted in 1994, 1997, 1998, Witt and Hebert 2000). all the authors believing that the species was Most of the results show that populations of present all over America (Shoemaker 1933, what has been identified as H. azteca from Ruffo 1947) and many species have been North America have low levels of gene flow, placed in the synonymy of H. azteca. reduction in genetic variability, low heterozy- Stebbing (1903) described H. faxoni from gosity, unique alleles, and strong genetic differ- Costa Rica and reestablished the genus name entiation and divergence among the popula- after it was synonymized under Allorchestes tions. From these genetic studies it is clear that by Faxon (1876). Stebbing (1906) syn- H. azteca is a species complex. None of the 660 REVISTA DE BIOLOGÍA TROPICAL studies, however, mention any morphological Material Examined: Volcan Barva, Costa differences among the populations included in Rica. 2000 m.a.s.l (10° 08’ N, 84° 06’ W). the analysis. Material from Smithsonian Institution (Acc. # After the redescription of H. azteca based 96617). on material from the syntype series (González Diagnosis: Body surface smooth. Coxa 4 and Watling 2002), the morphology of the excavated posteriorly. Eyes pigmented. species described by Saussure (1858) was Antenna 1 shorter than antenna 2. Antenna 2 clearer. The new detailed morphology used for less than half body length. Mandible incisor the latter description, the lack of detail in toothed. Maxilla 1 palp minute, reaching less Stebbing’s description, and considering than half length the distance between base of Weckel (1907) nomenclatural changes, it is palp and tip of setae on outer plate; inner plate necessary to redescribe H. faxoni and establish slender, with four strong and pappose apical the differences with H. azteca. setae. Maxilla 2, inner margin of inner plate with two closely together strong pappose setae. Gnathopod 1 propodus length less than two MATERIALS AND METHODS times maximum width (quadrangular), hammer shape, inner face with five pappose setae, setose The 32 specimens examined are deposited scales on disto-posterior and disto-anterior bor- in the National Museum of Natural History, der. Gnathopod 2 propodus ovate, palm shorter USA, under the Accession Number 96617. The than posterior margin, slope slightly oblique, morphological characters were coded and anterior edge with a wide truncated or rounded descriptions generated, using the taxonomic process. Peraeopods 3 and 4 merus and carpus database DELTA (Dallwitz et al. 1999). The posterior margin with three hind marginal clus- terminology for setae follows Watling (1989) ters of long setae; propodus posterior margin and Oshel and Steele (1988). Measurements of with five to eight groups of setae. Uropod 3 the specimens were made from the tip of the peduncle with five strong distal setae; outer head to the base of the telson. This convention ramus same length as peduncle, basal width was chosen because of the variable position of more than two times tip of ramus. Telson wider the tip of the telson in different specimens. The than long, apically rounded, with two widely computer program Image-Pro Plus (Media apart, long simple setae, symmetrically distrib- Cybernetics 1997) was used to measure the uted. Sternal gills on segments 3 to 7. Female. specimens. Latitude and longitude were deter- Gnathopod 1 similar in size, and different in mined from a gazetteer. Localities, whose shape to gnathopod 2. Gnathopod 2 different record indicates a wide region, were assigned from male gnathopod 2 in shape and smaller, to a middle point of that region. propodus length two to three times maximum width, normally subchelate, palm slightly reverse oblique. RESULTS Description of male: Size, 8.7 mm. Body surface smooth. Epimeral plate 1, 2 and 3 Hyalella faxoni Stebbing, 1903 acuminate (Fig. 1 E). Coxae 1 to 4 subequal in Figs. 1-4 size and shape, slightly overlapping. Acumination in coxae absent. Coxa 1 same as Hyalella faxoni Stebbing, 1903: 928–930, 2 and 3. Coxa 3 narrower than 4. Coxa 4 deep- Fig. 61. er than wide, excavated posteriorly. Coxa 5 Type material: United States National posterior lobe deeper than anterior. Coxa 6 Museum. anterior lobe small. Type locality: Volcan Reventado, Costa Head smaller than first two thoracic seg- Rica (9°50’N, 83°56’W). ments, typically gammaridean, rostrum absent. INTERNATIONAL JOURNAL OF TROPICAL BIOLOGY AND CONSERVATION 661 Fig. 1. Hyalella faxoni Stebbing, 1903, male 8.7 mm. Symbols for figures are as follow. A, antenna; E, epimeron; G, gnatho- pod; H, head; L, lower lip; M, mandible; P, peraeopod; S, maxilliped; R, uropod; T, telson; U, upper Lip; X, maxilla. Lower case letters on the left side of capital letters refer to specimens cited in captions. Lower case letters on the right are as fol- low: l, left; r, right. The scale is indicated as a small bar on each appendices, “a” is equivalent to 192 microns; “b” is equiv- alent to 94 microns; “c” is equivalent to 47 microns; “d” is equivalent to 481 microns. 662 REVISTA DE BIOLOGÍA TROPICAL Eyes pigmented, medium, ovoid, located dactylus unguiform, shorter than third article, between insertion of antenna 1 and 2 (Fig. 1 H). distal setae simple and shorter than nail, inner Antenna 1 less than half of body length, border with setae, distal nail present (Fig. 2 S). shorter than antenna 2, longer than peduncle of Gnathopod 1 subchelate; carpus longer antenna 2; peduncle longer than head, article 1 than wide, longer and wider than propodus, longer than 2, article 3 the shortest; flagellum with strong and wide posterior lobe, produced of 12 articles, longer than peduncle, basal arti- and forming a scoop like structure, open to the cle not elongated; asthetascs on flagellum, inside, inner face with one to three pappose from articles 4 distally (Fig. 1 A1). setae, border pectinate and with several pap- Antenna 2 less than half body length; pose setae; propodus length less than two peduncle slender, longer than head, article 4 times maximum width (quadrangular), ham- shorter than article 5, setal groups on article 4 mer shape, with no setae on anterior border, and 5 scarce; flagellum of 15 to 17 articles, inner face with five pappose setae, setose longer than article 5, basal article not elongat- scales on disto-posterior and disto-anterior ed (Fig. 1 A2). border, palm slope transverse, margin straight, Basic amphipodan mandible (in the sense posterior distal corner with robust setae; dacty- of Watling 1993); incisor toothed; left lacinia lus claw-like, with one to three endal setae, mobilis with five teeth; setae row on left congruent with palm (Fig. 1 G1). mandible with three main setae without acces- Gnathopod 2 subchelate; basis hind mar- sory setae, right mandible with two main setae gin with four to six group of setae; merus with without accessory setae; molar large, cylindri- seven or more setae on posterior margin, pos- cal, and triturative, accessory seta present. tero-distal margin straight, distal corner round- Labrum ventral margin round. Lower lip outer ed; carpus posterior lobe elongated, produced lobes rounded without notches or excavations, between merus and propodus, border pectinate mandibular projection of outer lobes round with several pappose setae; propodus ovate, (Fig. 2 Ml; Fig. 1 L, U). scales on disto-posterior border, palm shorter Maxilla 1 palp uniarticulate, minute, than posterior margin, slope slightly oblique, reaching less than half length the distance margin irregular, few long setae, several medi- between base of palp and tip of setae on outer um size setae, anterior edge with a wide trun- plate, distal setae feeble; inner plate slender, cated or rounded process, posterior distal cor- smaller than outer plate, with four strong and ner with strong setae, and with cup for dactyl; pappose apical setae; outer plate with nine dactylus claw-like, congruent with palm, with stout and serrate setae (Fig.