Claws conical, armed with three rows of minute spinules (=cilia of Paggi, 1976), ..... major changes in flora and fauna (Flores-Villela &. Gerez, 1994), including ...
Hydrobiologia 442: 41–54, 2001. M. Boersma & K.H. Wiltshire (eds), Cladocera. © 2001 Kluwer Academic Publishers. Printed in the Netherlands.
41
New and little known cladocerans (Crustacea: Anomopoda) from southeastern Mexico Manuel El´ıas-Guti´errez1 , Nikolai N. Smirnov2, Eduardo Su´arez-Morales1 & Noemi Dimas-Flores3 1 El
Colegio de la Frontera Sur. km 2 Carr. Chetumal-Bacalar Zona. Ind. # 2, Chetumal 77000, Quintana Roo, Mexico 2 Institute of Ecology and Evolution, Russian Academy of Sciences, Leninsky Prospekt 33, Moscow 117071, Russia 3 Instituto Mexicano del Petr´ oleo (IMP). Eje Central 152, Col S Bartolo Atepehuacan CP 07730, Deleg G.A. Madero, Mexico, D.F. Mexico
Abstract Mexico represents a transitional area in which two biogeographic regions meet. A survey of the cladocerans collected in true tropical waterbodies from the country revealed a diverse fauna. The environments examined included temporary pools in the forest, rivers, permanent lagoons and karstic sinkholes. New records or little known taxa include: Diaphanosoma brevireme, D. fluviatile, Pseudosida ramosa, Simocephalus mixtus, Onchobunops tuberculatus,Guernella raphaelis, Chydorus nitidilus, Notoalona cf. globulosa, Alonella brasiliensis and Leydigiopsis curvirostris. In contrast to high altitude systems, with clear nearctic affinity, the lowlands of southeastern Mexico showed a striking resemblance with the South and Central American cladoceran fauna. Apparently, orographic barriers control the distribution of the cladoceran taxa in this region. Introduction
Study area
Although the regional and local lists keep increasing (Elías-Gutiérrez et al., 1999), and several new species have been described recently, the cladoceran fauna of Mexico is still poorly known. From about 2 million km2 of territory, including 320 large drainage basins which comprise ca. 27% of the total surface of the country (Alcocer & Escobar, 1996), less than 1% of the countries’ major freshwater systems have been surveyed (Elías-Gutiérrez et al., 1999). Most studies deal with collections made in two of the smallest states of central Mexico (Elías-Gutiérrez et al., 1999), with a total area of 26 826 km2 . Large zones in the south and north do not have a single record of cladocerans (Suárez-Morales et al., 2000). A biological survey of several true tropical waterbodies of the southeast revealed 37 species, including 11 new faunistic records. The environments examined, included temporary forest pools, rivers, permanent lagoons and karstic sinkholes. The latter are characteristic of the Yucatan Peninsula. Many have a surface less than 1 ha, but are sometimes more than 40 m deep.
Surveys were conducted in 15 systems in the states of Tabasco, Campeche, and Quintana Roo (Fig. 1). All sites are located between 17◦ 530 to 19◦ 510 N and 87◦ 590 to 95◦ 010 W, at less than 200 m above sea level. Some environmental variables, dates, and the geographical location of the sites sampled are presented in Table 1.
Materials and methods Samples from each water body were collected at both littoral and limnetic zones. For littoral sampling, a conical net (40 cm diameter, 50 µm pore size, with a 1.5 m long handle) was hauled through macrophytes and over near-shore substrates. The limnetic environment was sampled by vertical and horizontal tows using a conical net of 0.50 m diameter, 1 m length and 50 µm mesh size. Samples were preserved with 4% sugar-formalin. Specimens were analysed and dissected under a stereomicroscope and identified with
42
Figure 1. Sampling localities in southeastern Mexico. 1. Puente Milagros; 2. Ucum; 3. Puente Sorpresas; 4. Calackmul; 5. Small puddle 1; 6. Kana lagoon; 7. Cenote Azul; 8. Chichancanab lagoon; 9. Amtun sinkhole; 10. La Esperanza lagoon; 11. Matillas lagoon; 12. El Pajonal; 13. Channel at river Hondo; 14. Small Puddle 2; 15. Catemaco lake.
the aid of standard literature (e.g. Goulden, 1968; Smirnov, 1971, 1992, 1996; Korovchinsky, 1992), and with original descriptions when necessary. Drawings were made using a camera lucida. The material was deposited in the Reference Collection (ECOCH-Z) held in the Laboratorio de Zooplancton of El Colegio de la Frontera Sur, Chetumal, Mexico.
A list of species recorded is presented in Table 2. The total number of taxa is 37. About 25% of these have been recorded in Central and South America, suggesting an affinity with these regions rather than with the geographically closer highlands of Mexico. About 5% are forms with nearctic affinity. Eleven records are new or relevant, and are discussed below with morphological remarks.
margin of valves lacking inflexion, armed with several setae and a row of 4–6 spinules between each two setae (Fig. 3). One dorsal spine near posterior margin of valve (Fig. 4). Postabdomen with wide dorsal proximal prominence and three spines on base of claw. Claws armed with a line of fine denticles. Diaphanosoma fluviatile has been found in South America, Nicaragua, and Haiti (Korovchinsky, 1992). The Veracruz record currently represents its northernmost geographical limit, and is a new record for Mexico. Catemaco Lake is a peculiar environment, with nine endemic fish species (Miller & Van Conner, 1997). Three of them, Bramocharax (Catemaco) caballeroi and two species of Cichlasoma, prey heavily on zooplankton, which could explain the low numbers of this species in our samples. Torres-Orozco & Zanatta (1998) reported D. brachyurum from Catemaco, with a low density (0.5 ind l−1 ) in July. Their material was probably D. fluviatile.
Diaphanosoma fluviatile Hansen, 1899 (Figs 2–4) Material examined: Several females from Catemaco Lake, Veracruz, Mexico. Length: 0.72–0.81 mm. Body elongated, head rectangular. Swimming antennae not reaching posterior margin (Fig. 2). Antennal setae 4-8/0-1-4. Ventral
Diaphanosoma brevireme Sars, 1901 (Figs 5–9) Material examined: Many females from three small ponds (see Table 2). Total length: 0.46–0.56 mm. Body elongated, with rounded head. Optic vesicle almost filling anterior part of head (Fig. 5). Swimming antennae strong, not
Results
43
Figures 2–11. Diaphanosoma fluviatile, parthenogenetic female, Catemaco lake. Figure 2. Habitus, lateral view; Figure 3. Posterior margin of the valve; Figure 4. Postabdomen. Diaphanosoma brevireme, parthenogenetic female, Puente Milagros. Figure 5. Habitus, lateral view; Figure 6. Ventral flap of valves; Figure 7. Posterior margin of the valve; Figure 8. Spines of postero-dorsal margin of valves; Figure 9. Postabdomen. Pseudosida ramosa, parthenogenetic female, Ucum river. Figure 10. Habitus, lateral view; Figure 11. Postabdomen.
44 Table 1. Location of the surveyed systems and environmental variables measured at the moment of collections. NA= Not available data; ∗ data from Torres-Orozco & Zanatta (1998); ∗∗ only littoral samples were taken Localities
Date of collection
Geographical coordinates
Maximum deep (m)
Secchi transparency (m)
Dissolved oxygen (mg/l)
Water temperature ◦C
Puente Milagros
March 19, 1997
18◦ 310 0400 N 88◦ 250 3100 W
0.84
0.84
3.7
26
Ucum
March 19, 1997
18◦ 300 2400 N 88◦ 300 5400 W
0.75
0.75
NA
23.8
Puente Sorpresas∗∗
March 19, 1997
18◦ 270 3900 N 89◦ 020 0500 W
0.45
0.45
9.03
23.7
Calackmul Small puddle 1
March 20, 1997 March 20, 1997
0.87 0.95
0.87 0.24
11.7 13.2
26.4 26.8
Kana Lagoon
May 2, 1997
19◦ 290 4700 N 88◦ 230 3600 W
4.0
2.4
Cenote Azul
August 20, 1997
18◦ 380 4800 N 88◦ 290 4200 W
66.7
7.67
Chichancanab lagoon
May 1, 1997
19◦ 510 –19◦ 570 N 88◦ 150 –88◦ 45 W
5.16
1.72
31.1
Amtun sinkhole
May 2, 1997
19◦ 380 330 N 88◦ 220 460 W
18 +
7.6
26.5
La Esperanza lagoon
May 28, 1997
19◦ 290 0500 N 87◦ 590 1700 W
10
5.52
7.7
31.7
Matillas lagoon
January 12, 1998
17◦ 530 –17◦ 520 N 92◦ 310 –92◦ 320 W
2.11
0.79
8.0
27.5
El Pajonal
January 12, 1998
18◦ 000 4000 N 92◦ 510 4800 W
1.66
0.58
8.3
30.3
Channel of Hondo river
January 27, 1998
18◦ 290 4900 N 88◦ 210 0100 W
0.84
0.84
5.7
25.7
Small puddle 2 Catemaco lake
October 13, 1998 October 14, 1998
18◦ 210 –18◦ 270 N∗ 95◦ 010 –95◦ 070 W∗
0.4 22∗
0.4 0.64
NA 10.2–6.0∗
34.7 29.3
18◦ 230 1800 N 89◦ 530 5100 W
reaching posterior margin of valves. Antennal armature 8 (sometimes 7)-4/0-1-4. Ventral margin of valves with wide ventral flap armed with 4–5 long, thin setae (Fig. 6). Posterior margin armed with 5–7 groups of small denticles separated by a larger one (Fig. 7).
31.2
6.4
31.8
Dorsal margin of valves armed with two spines (Fig. 8). Postabdomen relatively narrow, with heavy prominence near ventral base of claws. The latter armed with a row of fine spinules (Fig. 9). According to Korovchinsky (1992), this species is South American.
45 Table 2. Cladoceran species found in southeastern Mexico during the survey, where A = Puente Milagros, B = Ucum, C = Pte. Sorpresas, D = Calackmul, E = Small puddle 1, F = Kana Lagoon, G = Cenote Azul, H = Chichancanab lagoon, I = Amtun sinkhole, J = La Esperanza sinkhole, K = Matillas lagoon, L = El Pajonal, M = Channel River Hondo, N = Small puddle 2, O = Catemaco lake Species
A
Diaphanosoma brevireme Sars, 1901 D. fluviatile Hansen, 1899 Pseudosida ramosa Daday, 1904 Latonopsis australis group Ceriodaphnia cornuta group Ceriodaphnia dubia Richard, 1894 Simocephalus mixtus Sars, 1903 Moinodaphnia macleayi King, 1853 Scapholeberis sp. Bosmina sp. B. longirostris (O.F. Müller, 1785) Bosminopsis sp. Macrothrix n. sp. 1 Macrothrix sp. M. triserialis group Macrothrix spinosa King, 1853 Onchobunops tuberculatus Fryer Paggi, 1972 Guernella raphaelis Richard, 1892 Pleuroxus denticulatus Birge, 1879 Alonella brasiliensis Bergamin, 1935 A. excisa (Fischer, 1854) Chydorus nitidilus (Sars, 1901) Chydorus eurynotus Sars, 1901 Dunhevedia americana Rajapaksa & Fernando, 1987 Ephemerophorus barroisi (Richard, 1894) Alona karua King, 1853 Alona pectinata El´ias-Guti´errez & Su´arez-Morales, 1999 Alona affinis (Leydig, 1860) Disparalona hamata (Birge, 1879) Euryalona orientalis (Daday, 1898) Graptoleberis testudinaria (Fischer, 1848) Notoalona cf. globulosa Kurzia latissima (Kurz, 1874) Camptocercus dadayi Stingelin, 1913 Leydigiopsis brevirostris Brehm, 1938
•
B
C
D
•
E
F
G
H
I
J
K
L
M
N
O
• •
• • • • •
•
•
• • • • • • •
•
• •
• • •
•
• • • •
• •
•
•
• • •
•
• • •
•
•
•
•
• • •
• •
Its previous northernmost record was Guatemala, although it also has been recorded in Cuba. In our survey, it was observed in small ponds only. Its record in Mexico represents the new northern limit to its known distribution in the Americas.
• •
Pseudosida ramosa Daday, 1904 (Figs 10 and 11) Material examined: Five females from Ucum stream, an intermittent tributary of the Hondo river, which represents the border between Mexico and Belize.
46 Total length: 1.00–1.2 mm. Body elongated, with rounded head. Ventral eye surrounded by numerous small hyaline lenses. Antenna strong, with large basipodite. Antennal setae 6-10/0-1-3. Antennulae with lateral outgrowth bearing sensory papillae (Fig. 10). Postabdomen small, with only one row of lanceolate anal teeth arranged in clusters (Fig. 11), instead of two as described by Korovchinsky (1992). Each cluster with members decreasing in size proximally. Claws with small spinules on ventral side. Specimens were found in one location only, near Chetumal Bay, Yucatan Peninsula. They represent a link with the Central and South American fauna, where it has been found in low lands. It has also been observed in Cuba (Korovchinsky, 1992). Simocephalus mixtus Sars, 1903 (Fig. 12) Material examined: Numerous adult females, males, and ephippial females from several systems (Table 2). Female: Length 0.92–1.14 mm. This species is characterised mainly by the dorso-posterior valve prominence (Orlova-Bienkowskaja, 1998) with a diameter larger than the body length. The dorsal portion of valves protrudes backwards. Ocellus elongated. A clear deep depression on the ventral margin of the head is observed. Postabdominal claws biserially armed with a row of fine setules. Postabdomen with anal bay armed with small teeth. This species, apparently a cosmopolitan form in the Northern Hemisphere, has never been recorded in South America (Orlova-Bienkowskaja, 1998). In the Americas, the southernmost record of S. mixtus is in the insular Caribbean (Orlova-Bienkowskaja, 1998). However, our record marks a new southern limit of its known distributional range in the continental America. Onchobunops cf. tuberculatus (Fig. 13) Material examined: Four parthenogenetic females and a juvenile from El Pajonal and La Estancia Vieja, Tabasco (last locality not figured in Table 2). Total length 0.607–0.90 mm. Body suboblong with narrower posterior margin. Head with an anterior protrusion occupied by the compound eye and a protruding tubercle on the posterior part of the head. Ocellus smaller than eye. Labrum oval, with two triangular projections. Antennules rod-like, with several rows of spinules and two ventral setae near tip. Postabdomen bilobed, with five marginal spines surrounded by groups of small spinules. This taxon is also a South American form, with previous records in Argentina
and Nicaragua (Smirnov, 1988, 1992; Silva-Briano, 1998). In Mexican material, we found a body shape varying from typical semicircular to an elongated ellipsoid form (as in Fig. 13). This variation may relate to the presence of eggs in the embryonic chamber, but further material is needed to document the relationship between the South American and the Mexican taxa. The Tabasco records represent the northernmost record of the genus in the Americas. Although Silva-Briano (1998) synonymised Bunops (an apparent cosmopolitan taxon) and Onchobunops, here we retain the traditional classification, until more material of O. tuberculatus or related taxa has been examined. Guernella raphaelis Richard, 1892 (Figs 14 to 27) Material examined: 100+ specimens from a small puddle 2 (see Table 2) in Tabasco. Description: Total length 0.26–0.39 mm, with an average of 0.32 mm. Body rounded, head broad (Fig. 14). Head shield and valves with an ornamentation represented by a reticulum of poligonal shapes. Eye large, ocellus relatively smaller. Behind the head there is a big, rounded head pore (Fig. 15). Labrum rounded. Antennulae wide, ornamented with 6-7 transversal rows of minute spinules. Only visible in lateral view, a seta is inserted near the antennule base. Ventrally there are two setae, on the distal third (Figs 16–17). Five apical spinules surround the nine aesthetascs. Antennae with basipodite covered by rows of fine spinules and with two long soft setae, one near the basis, the other on the distal third (Fig. 18). Antennal formula 0-1-1-3/1-1-3, spines 0-1-0-0/0-0-1. Leg 1 with large ejector hooks, inner distal lobe with one seta, outer distal lobe with four setae, one of them shorter (Fig. 20). Endites 1–3 as in Figure 21. Exopodite in leg 4 with a small ciliated receptor (Figs 22–23). Exopodite in trunk limb IV with four setae (Fig. 24). Postabdomen with rows of minute spinules scattered on its surface (Fig. 24). Claws conical, armed with three rows of minute spinules (=cilia of Paggi, 1976), one internal, another external inserted along concave side, third one between both (Figs 25–26). Setae natatoria with long distal end covered by long setulae (Fig. 27). Ciliates (Peritricha) and numerous bacteria were found commonly associated to the cuticle of valves and antennulae (Figs 15 and 17). Guernella raphaelis has been found commonly in the tropics from both, the Old and New World (Smirnov, 1992). Frey (1988), based on the size-distribution of material from Sri Lanka, Congo, and Florida, suggested that the European and the American populations could
47
Figures 12–13. Figure 12 Simocephalus mixtus, parthenogenetic female, Ucum river, lateral view. Figure 13 Onchobunops tuberculatus, parthenogenetic female, lateral view. La Estancia Vieja (Tabasco State), October 9, 1997.
represent different taxa. This author stated that even SEM analysis (never published) was not conclusive at that moment. Comparison of our material with drawings of this taxon by Paggi (1976) from Argentinian specimens and by Silva-Briano (1998) from material from Papua New Guinea, showed differences mainly in the exopods of limbs III and IV, and in the antennular and antennal armature. We could not observe the triangle-shaped ciliated lamella near the postabdominal claws reported by Paggi (1976), his observations were made on light microscopy and both claws are extremely small. Maybe these ‘lamellae’ were part of the claw itself. We illustrate the features of the Mexican material with SEM photographs as an aid to develop future studies to clarify the status of Guernella as a single taxon or as a species-group. An overall revision of this genus should include the study of material from all around the world, including the type locality in Africa, as suggested by Frey (1988).
men elongated, rectangle-shaped, with 13 anal spines, and no lateral armature, only two or three rows of small denticles. Claws with two basal spines, different in size and a pecten of small denticles on the dorsal surface (Fig. 29). This species has been found in Brazil (Sars, 1901), Guatemala (Goulden, 1966), Nicaragua (Smirnov, 1988), possibly Haiti (Frey, 1982), and Argentina (Paggi, 1972). The latter author pointed out that this taxon could be related to the ‘barroisi’ group. Later, Frey (1982) re-allocated the ‘barroisi’ chydorids in a new genus, Ephemeroporus, due to the lack of a cephalic pore. We agree with Frey (1982) who, after the analysis of original samples by Sars (1901), concluded that C. nitidilus is a true Chydorus in spite of the lack of these pores. This is also the northernmost record for C. nitidilus in the Americas. Its possible conspecifity with a similar organism from New Zealand should be revised.
Chydorus nitidilus (Sars, 1901) (Figs 28 and 29) Material examined: Three females from El Pajonal, Tabasco. Total length: 0.30 mm. Species characterized by the formation of a denticle on the posteroventral corner of the valves. Body rotund, with a well defined posterior margin. Ocellus smaller than eye. Rostrum longer than antennulae. Labrum broad, slightly curved, with blunt apex (Fig. 28). Postabd-
Notoalona cf.globulosa (Figs 30–35) Material examined: Seven females from El Pajonal, Tabasco. Total length: 0.36 – 0.43 mm. Small animals, body rotund. Valve sculpture similar to that noticed by Sars (1901), with longitudinal and transverse striae, but not as marked as he figured them. Head wide, with one cephalic pore divided into two curved peanut-shaped halves. Ocellus small (Fig. 30). Labrum with a blunt
48
Figures 14–19. SEM micrographs of Guernella raphaelis, parthenogenetic female, Small puddle 2, Tabasco State. Figure 14. Habitus, lateral view; Figure 15. Head pore; Figure 16. Antennulae, frontal view. Arrow points to frontal pore; Figure 17. Antennulae, lateral view, showing epibionts; Figure 18. Right antenna, side view; Figure 19. Thoracic limbs and postabdomen, ventral view.
49
Figures 20–25. SEM of Guernella raphaelis (Cont.). Figure 20. Outer and inner distal lobes, leg I; Figure 21. Leg I, endites 1–3; Figure 22. Leg IV. Arrow points to a ciliated receptor; Figure 23. Detail of ciliated receptor in leg IV; Figure 24. Postabdomen and claws, side view; Figure 25. Postabdominal claws, side view.
50
Figures 26–27. SEM of Guernella raphaelis (Cont.).
Figure 26. Postabdominal claws, ventral view;
apex, and a variable arrangement of small denticles (usually two) on the anterior margin (Fig. 31a–c). Antennulae shorter than rostrum, with a lateral setae on the proximal third, and nine aesthetascs. Antennae with setae 0-1-3/0-0-3; spines 0-1-1/1-1-1 (Fig. 32). The spine of the second endopodal segment hyaline. Leg I (Fig. 33) with an outer distal lobe armed with a long hyaline setae, inner distal lobe with three pectinated setae, two of them shorter. Endite 3 armed with three bilaterally ciliated setae. Endite 2 with one cilated setae and two larger, pectinated setae. Endite 1 with two hyaline, unisetulated setae and one specialized finger-like branched setae, with small cilia along one side. Leg 2 (Fig. 34), exopodite reduced, endopodite with 8 scrapers, six with a similar structure, an enlarged base and a distal segment bearing a pecten of strong, sclerotized teeth. Scrapers 7 and 8 with two segments, distal one of both armed with short setules along one side. Postabdomen (Fig. 35) narrowing distally, with a deep concavity before the preanal corner, armed with 15–17 anal spines, variable in arrangement, some of them forming close pairs. Lateral setae forming 10–12 groups, each one decreasing in size proximally. Claws armed with a basal tooth, and a small pecten, with the distalmost member clearly distinguished by its large size. The genus Notoalona was established by Rajapaksa & Fernando (1987) for the Alona-like chydorids with the head pore represented by two bean-like thickenings, trunk limb I with a unique branched setae in posterior lobe, and submarginal setae in the carapace. The original taxon, N. globulosa, was described by Daday (1898) from Ceylon, and later redescribed
Figure 27. Setae natatoria.
by Rajapaksa & Fernando (1987), with an ample distribution in Asia and Africa. They also described N. freyi from Florida, but failed to clarify the status of other N. globulosa-like material from the Americas. Our material is more similar to N. globulosa mainly in the features of the postabdomen. Sars (1901) described Alonella sculpta from Ipiranga (Brazil), related to I. globulosa. The status of this taxon was not clarified by Rajapaksa & Fernando (1987), although they studied the type material and other localities. We concluded that our material is similar to sculpta sensu Sars (1901) and could be conspecific with the material described later by Daday (1905) from Paraguay, by Brehm (1956) from Venezuela, by Rajapaksa & Fernando (1987) from Costa Rica, Guatemala, El Salvador, and Florida (U.S.A.), and with the taxon reported by Smirnov (1988) from Lake Nicaragua. The relations of these populations should be clarified based on more abundant material. Alonella brasiliensis Bergamin, 1935 (Figs 36–39) Material examined: Several adult and preadult females from Rio Hondo and Puente Palmiras at San Luis Potosí State. Part of the material were some pre-reproductive instars (size 0.303 mm–0.323 mm), which presented two connected head pores (Fig. 36). In the adult stages, these pores are absent. Changes in the postabdomen, mainly at the pre-anal region, are figured (Figs 37 – 38). All remaining features of adult parthenogenetic females are as described by Smirnov (1984, 1988) (Figs 38–39).
51
Figures 28–35. Chydorus nitidilus, parthenogenetic female, El Pajonal. Figure 28. Habitus, lateral view; Figure 29. Postabdomen. Notoalona cf. globulosa, parthenogenetic female, El Pajonal. Figure 30. Habitus, lateral view; Figure 31a–c. Variation in labrum shape; Figure 32. Antenna, side view; Figure 33. Leg I; Figure 34. Leg II; Figure 35. Postabdomen.
52
Figures 36–40. Alonella brasiliensis, juvenile instar, Puente Palmiras (San Luis Potos´ı State), October 3, 1996. Figure 36. Habitus, lateral view; Figure 37. Postabdomen. Alonella brasiliensis, parthenogenetic female, Puente Palmiras (San Luis Potos´ı State), October 3, 1996. Figure 38a. Postabdomen; Figure 38b. Postabdominal claw, inner view; Figure 39. Antenna, side view. Figure 40. Leydigiopsis brevirostris, parthenogenetic female, lateral view. El Pajonal, Tabasco State.
These animals were found in the border between Mexico and Belize and in a location in the northern Mexico (at San Luis Potosí State) named Puente Palmiras (21◦ 400 3000 N, 98◦ 580 1600 W). They are also South American representatives with previous records in Brazil and Nicaragua. Hence, Puente Palmiras is now regarded as the northernmost record for A. brasiliensis in the Americas. Leydigiopsis brevirostris Brehm, 1938 (Fig. 40) Material examined: Several females from El Pajonal, Tabasco (Table 2). Total length: 0.73 mm. Body widely rounded. Valves with a row of long setae on the ventral mar-
gin and an inner row with thin setulae. Posterior margin with a dorso-ventral lining of inner spinules. Rostrum long and narrow. Antennulae short, with nine aesthetascs of different size, larger ones slightly longer than rostrum. Some of these aesthetascs have a heavily sclerotized basis, with a thicker appearance. Labrum plate triangular. Antennae with setae 0-0-3/01-3; spines 1-0-1/1-0-1. Postabdomen widely rounded, heavily armed with groups of anal spines and lateral setae. This genus has been found in South America, although there is a record of a single specimen from Thailand (Sanoamuang, 1998). Smirnov (1971) recognized four species whose diagnostic characters are
53 more or less obvious. The studied biological material of this taxon is still scarce at the species level, and little is known about its biology and morphological variability.
north. However, it seems that South American cladocerans extend much farther north than calanoid copepods, which show a weak South American influence (Suárez-Morales et al., 1999).
Discussion
Acknowledgements
The records presented here extend the Mexican lists (Elías-Gutiérrez et al., 1999; Suárez-Morales et al., 2000) to 121 species, and the new totals for the families Sididae, Chydoridae and Macrothricidae are 10, 47, and 11 species, respectively. However, this number is still relatively low when compared with that of North America (Villalobos-Hiriart et al., 1993). It is expected that, being a mixture of nearctic and neotropical fauna, the potential number of species in Mexico is much higher. In Mexico, there is a strong influence of the nearctic fauna, which is represented mainly by a diverse assemblage of daphnids and moinids on the central plateau (Elías-Gutiérrez et al., 1999). They were scarce in this study. The fauna detected in this work is mainly composed of sidids, macrothricids and chydorids. Many have clear neotropical affinities, and distributional patterns limited to the southeastern lowlands, including the Yucatan Peninsula and the Gulf of Mexico coast. Our results and the analysis of records already published (Elías-Gutiérrez et al., 1999) on the distribution of Cladocera in Mexico, suggest that the real geographic barrier for both fauna (nearctic, neotropical) is the Western Mountain Chain (Sierra Madre Occidental) that runs north–south along the west of the country, separating the coastal plains from the inland regions. Although at the same latitude, both sides of this chain have quite different climatic conditions, the coast being a true tropical environment, but the central zone more temperate, due to the effect of altitude. These topographic and climatic variations cause major changes in flora and fauna (Flores-Villela & Gerez, 1994), including Cladocera. In order to complete the picture of anomopod and sidid distributions along this transitional zone, surveys of the fauna of the East Coast are urgently needed. The East Coast is also isolated by a mountain chain, so it is possible that both coastal plains represent ‘corridors’ along which a southern fauna penetrates north, while the center is a southward passage for the northern fauna. Most species from both environments do not mix, constituting characteristic assemblages. To date, we do not know how far the neotropical cladoceran fauna penetrates
The Comisión Nacional para el Estudio y Conocimiento de la Biodiversidad en México (CONABIO) partially supported this study (Grant-S050). The Mexican Consejo Nacional de Ciencia y Tecnología (E130.2605) and El Colegio de la Frontera Sur granted the visit of N.N. Smirnov to Mexico during 1998. María Guadalupe Nieto López from El Colegio de la Frontera Sur at Tapachula kindly assisted us with the SEM work.
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