Introduction

The Aeglidae are the most abundant and widely distributed freshwater decapod “crabs” in southern South America. Unlike true brachyuran crabs, however, in aeglids the fifth pair of pereiopods is reduced in size, lacking walking capacity (Lopretto, 1978; Martin & Abele, 1988); they also possess tiny chelae with which they groom branchiae and eggs attached to the pleopods of females and the underside of the abdomen (Martin & Felgenahuer, 1986) (Fig. 1, inset). All aeglids are primarily aquatic and occur in lakes, streams, and caves, at depths of down to 320 m in Chilean lakes (Jara, 1977), and at altitudes of up to ∼3,500 m in northeastern Argentinean cordilleras (Bond-Buckup & Buckup, 1994). Aeglids are the only anomuran family restricted to the Neotropical region of South America. Taxonomically, aeglids are included within the anomuran superfamily Galatheoidea, but there is some morphological evidence (e.g., gill structure and caparace sutures) and molecular data that suggest the Aeglidae should be in their own superfamily (Martin & Abele, 1986; Pérez-Losada et al., 2002b; Tudge & Scheltinga, 2002). From a conservation perspective, several of the known species are very restricted in distribution, and they and their habitats are considered threatened (Pérez-Losada et al., 2002a). From an ecological perspective, aeglids are unique because they are the only anomuran family entirely restricted to freshwater habitats.

Fig. 1
figure 1

Map of southern South America indicating the major river systems and distribution of the major clades (cf. Fig. 2) of species diversity in the aeglid crabs. A drawing of Aegla spinosa from Bond-Buckup and Buckup (1994) is shown as an inset

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The adult size of aeglids does not surpass 60 mm carapace length (CL), and color varies according to the substrate, from greenish brown to almost black; in some lacustrine populations blue, yellow, orange, and red specimens are also sporadically found (Jara, 1989). Aegla are dioecious; males lack pleopods, and genital pores open on the coxa of fifth pair of pereiopods at the tip of membranous tubes; female genital pores open on the coxa of third pereopods (Martin & Abele, 1988). Spawning occurs chiefly during the autumn, but some species spawn continuously throughout the year (Bahamonde & López, 1961; Rodrigues & Hebling, 1978; Bueno & Bond-Buckup, 2000). The incubation period lasts between four and eight months, so that an adult female produces no more than one clutch of eggs per annum. Fecundity depends on the size of females, ranging between 120 eggs at 12.5 mm CL to 400 at 22.5 mm CL female in A. laevis (Bahamonde & López, 1961); between 115 eggs at 14.99 mm CL to 368 at 19.18 mm CL female in A. leptodactyla (Buckup personal observation); and between 699 eggs at 29 mm to 1043 at 33 mm CL female in A. rostrata (Jara, 1977). Egg size ranges between 1.00 and 1.37 mm diameter (Jara, 1977). Recruitment normally occurs once a year, in springtime (August to October) (Bahamonde & López, 1961; López 1965; Bueno & Bond-Buckup, 2000), but for A. longirostri and A. castro, it seems that two recruitment periods exist (Swiech-Ayoub & Masunari, 2001; Delevati et al., 2005). Aegla species lack larval stages; offspring hatch as juvenile “crabs” of 1.13 - 1.58 mm CL (Bond-Buckup et al., 1999) that remain with the mother for 3–4 days before living independently (López et al., 2004). Population density varies widely, reaching up to 250 individuals/m2 (Bahamonde & López, 1961) in highly productive conditions.

The ecological role of Aegla species has not been assessed but their omnivorous diet includes periphyton, decaying allochtonous vegetable matter, aquatic invertebrates (Bahamonde & López, 1961; Burns, 1972; Lara & Moreno, 1995, Castro-Souza & Bond-Buckup, 2004), and fine particulate organic matter (Isler, 1988). Additionally, they constitute a relevant dietary item for the non-native rainbow trout (Oncorhynchus mykiss) in Chile and South Brazil and brown trout (Salmo trutta fario) in Chile (Burns, 1972; Arenas, 1978; Buckup, personal observation), and for the Chilean and Brazilian species of river otter (Lontra provocax) (Medina, 1998).

Species diversity

The present Aeglidae belong to a single genus, Aegla Leach, 1820, consisting of 63 described species (Bond-Buckup & Buckup, 1994; Bond-Buckup, 2003) (Table 1); including, newly described species based on recent molecular phylogenetic analyses (Jara et al., 2003). By our count, there are at least six additional species waiting to be described. Of these species, 57 are found mainly in rivers, only two in lakes, and four in cave habitats.

Table 1 Species from the freshwater anomuran crab family Aeglidae including the countries within which they are distributed
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Phylogeny and historical processes

Ortmann (1902) proposed that aeglid species from Chile represented the more primitive forms of the genus. However, Schmitt (1942) hypothesized that Aegla from the Atlantic side of South America were more primitive, and species ranging in the Chilean streams were more derived. Recent estimates of phylogenetic relationships based on a variety of molecular data support the Pacific origin hypothesis (Pérez-Losada et al., 2004) (Fig. 2) and suggest that the group, as a whole originated around 75 mya. The western Aegla species radiated, approximately, 40–45 mya (clades A and B—Fig. 2), but the speciation of the central and eastern taxa took place later, around 23–35 mya (clades C-E in Fig. 2) (see Pérez-Losada et al., 2004 for more detailed discussions on the phylogenetic relationships among all the aeglid species).

Fig. 2
figure 2

Bayesian and maximum likelihood estimate of phylogenetic relationships among the aeglid species using the GTR+Γ+I model of evolution. Clade support is graphically indicated as follows: ‒ bp ≥ 70% and pP ≥ 0.95,

figure a
50% ≤ bp < 70% and/or 0.75 ≤ pP < 0.95, and — bp < 50% and/or pP < 0.75. Branch lengths are shown proportional to the amount of change along the branches in the maximum likelihood tree with estimated divergence dates shown for the root and major clades (see Pérez-Losada et al., 2004, for details)

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Present distribution and areas of endemicity

The aeglids are distributed in southern South America (Fig. 1) including Chile, Brazil, Argentina, Uruguay, Bolivia, and Paraguay. Thus, all 63 species are contained within the Neotropical region. There are 16 species endemic to Chile, seven species endemic to Argentina and 36 species endemic to southern Brazil (Bond-Buckup et al. 2003). They occur in all the main rivers of southern South America, except the most southern Patagonian drainages.

Human related issues

Most Aegla species have very narrow distributional areas and are therefore of significant conservation concern. Of the 63 species currently recognized in the genus, using the IUCN Red List criteria (IUCN, 2001), we recognize 23 species or 36.5 % as under threat or endangered, mainly due to their narrow distributions and the rapid degradation of the freshwater habitats they occupy.

In Chile, aeglids are recognized as an important food item for exotic salmonid species (Salmo trutta fario and Oncorhynchus mykiss) which support a fast-growing fly-fishing sport industry, especially in the Llanquihue Lake region and South Brazilian Highlands. On the negative side, the fast-growing fruit and wine producing industries in Central Chile constitute a threat for conservation of aeglids, and benthic river fauna in general, due to the widespread use of biocides, which likely accumulate and have an impact in rivers and streams. Threats for conservation of aeglids also derive from silvicultural practices related to the establishment of extensive plantations of pine and Eucalyptus, mainly along the Coastal Cordillera and south Brazil. Additionally, it must be pointed out that the exaggerated use of pesticides in the widespread apple tree and potatoes cultivation and mainly, the hog raising activities along the majority of the South Brazilian Rivers, are an important menace to the native populations of aeglids, particularly in the states of Santa Catarina and Rio Grande do Sul, in Brazil.