Subterranean Fishes of the World

These are the only true synonyms. Since the cave fish has often been regarded as conspecific with Astyanax mexicanus (de Filippi, 1853), and with Astyanax fasciatus (Cuvier 1819), it has often been referred to under these names. Reddell (1981:238 243, 325) provides a list of names used and a full bibliography to that date.

Country

Types

Holotype of Anoptichthys jordani: UMMZ 113514, adult 51mm SL. Paratypes of Anoptichthys jordani: UMMZ 114486, 4 specimens; BMNH 1951.12.3:14‑17; plus others.

Distribution

Type locality: La Cueva Chica, San Luis Potosí, México. Sites from which this species has been recorded are distributed from 21^o50’ – 23^o10’N, 98^o50’ – 99^o14’W, see Mitchell, Russell and Elliott (1977) and Elliott (2018) for full details. Caves in which the fishes are found in seven areas:

The known distribution of Astyanax jordani. * type locailty, ⁺ caves not yet sampled for genetic studies and therefore of high priority to do so. Information from Elliott (2018)

Habitat

Most of the populations are known from lakes and streams within the vadose region of the caves. Two exceptions to this are the populations in Sótano de Soyate and Sótano del Venadito which are almost certainly living in the low level phreatic zone of the karst area. In particular the Soyate population is in a huge deep lake-like area which is probably the main groundwater flow in this area. During high flow the water levels rise considerably and fishes from the phreatic populations may become washed into vadose areas and become stranded there for a time.

Systematics

This fish is the most extensively studied of cave dwelling animals. The reason for its pre-eminence is that it is easy to breed in captivity and interfertile with the epigean Astyanax mexicanus. These properties allow very detailed studies, and in particular genetical ones, to be made easily. A large number of publications have resulted from these studies (see Elliott (2018) and this web site for bibliographies) but there has been great confusion as to the name and status of the animal. The following discussion outlines the problem and poses a solution.

The first population to be discovered, in La Cueva Chica, was described by Hubbs and Innes (1936) as a new genus and species, Anoptichthys jordani. In the following ten years two other populations were discovered in La Cueva de El Pachon and La Cueva de Los Sabinos. Alvarez (1946, 1947) included them in the genus Anoptichthys as Anoptichthys antrobius and Anoptichthys hubbsi respectively. It is now known that the cave fishes are widespread (Mitchell, Russell, and Elliott 1977), (Reddell 1981:238‑243, 325), (Elliott 2018)  and there are now 31 known sites.

In their original description of Anoptichthys jordani Hubbs and Innes (1936) recognised that: "Anoptichthys agrees with the genus and subgenus Astyanax in all apparent characters other than those associated with blindness and subterranean life". The epigean Astyanax mexicanus (for a discussion of the Astyanax mexicanus / Astyanax fasciatus “problem” see Miller and Smith 1986) and the hypogean Anoptichthys jordani are not only very similar in morphology, but they possess the same (or very similar) karyotype (Kirby, Thompson, and Hubbs 1977) and are interfertile. These, plus the sympatric distribution, provide strong evidence that Anoptichthys jordani is very closely related to, and in all probability, directly evolved from, Astyanax mexicanus. There is therefore no reason for recognising a distinct genus for the cave fishes. (See Greenwood (1976), Roberts and Stewart (1976), and Banister and Bunni (1980) for discussions of this philosophy). So, should the cave and surface fishes be regarded as members of the same species (with specifically adapted local races) or as distinct subspecies or species within the genus Astyanax ? Most authors have uncritically accepted that they are conspecific because of the interfertility. However there are a number of trenchant differences between the epigean and hypogean populations. They differ in fright reaction (Pfeiffer 1977), feeding behaviour (Schemmel 1980), the distribution and density of the taste buds (Schemmel 1980), metabolic rate (Huppop 1986), the neuromasts of the lateral line (Teyke 1990) and in competitive abilities within the cave environment (Wilkens and Hüppop 1986). Although the epigean fishes can survive and breed in darkness (contrary to the evidence of Rasquin and Rosenbloom 1954) they are out-competed by the better adapted cave fish (Wilkens and Hüppop 1986). It is certain that the hypogean fishes could neither compete nor survive on the surface. The genetic distance (as calculated by Nei's D parameter) is also relevant to this discussion. Values for D of 0.142 (Pachon population) and 0.105 (Sabinos population) (Chakraborty and Nei 1974) fall within the ranges for both subspecies (0.004 ‑ 0.351) and species (0.004 ‑ 3.000) and above that for local races (0.000 ‑ 0.049) as given by Nei (1987). A simple reading of the biological species concept (Mayr 1970:12) would confine both surface and cave fishes to the same species as a result of their interfertility. Rosen (1979:275‑278; see also Kottelat (1997:10-20)) has argued however that the biological species concept is worthless as a determiner of relationships since its primary definer, reproductive compatibility, is a primitive (plesiomorphic) attribute of members of a lineage. It therefore has no power to specify relationships within a genealogical framework. In the present case it is obvious that the cave form exhibits a number of autapomorphies (see above). The two forms should therefore be considered as good separate sister species which have not yet attained reproductive isolation.

The taxonomic consequences of this would be: one species of cave‑dwelling fish Astyanax jordani (Hubbs and Innes 1936) with one junior objective synonym, Anoptichthys jordani Hubbs and Innes 1936, and two junior subjective synonyms, Anoptichthys antrobius Alvarez 1946 and Anoptichthys hubbsi Alvarez 1947.

The above discussion is based principally on morphology. Recent work by Richard Borowsky (e.g. Borowsky 1994, 1996; Borowsky and Espinasa 1997; Borowsky and Wilkens 2002; Espinasa and Borowsky 2000, 2001), indicates that there may have been at least three independent invasions of surface fishes all of which now exhibit similar troglomorphic facies. If this is the case, and we accept that genetic characters are useful, then there should have three different names (see Kottelat (1997) for an excellent discussion of these points). Recently Dowling, Martasian and Jeffery (2002), Strecker, Bernatchez and Wilkens (2003) and Wilkens and Strecker (2003) have also shown that some cave populations are independently derived from surface fishes.

Oliveira et al. (2011) made an extensive ingroup study of the Family Characidae. Their results strongly support a sister group relationship between Astyanax jordani and a group containing Astyanax aeneus, Bramocharax caballeroi and Bramocharax baileyi (Figure below), and explicitally not a sister group relationship with Astyanax mexicanus, which is sister to the four species listed above.

The above discussion was largely written for the first edition of my book (Proudlove 2006) and needs revising in the light of much information discovered in the 14 years since it was writen. However, it is worth empasising that the conclusion of the discussion, that the cave fishes should be called Astyanax jordani, is strongly supported in the two most recent reviews of the these fishes, their habitats and their history (Wilkens and Strecker 2017:70-74 and Elliott 2018:39-44) and in an extensive phylogentic study of the Family Characidae (Oliveira  et al. 2011) - see phylogenetic tree below.

Biological Notes

Here is a different justification for the cave animals being a separate species to the surface fishes.

Abstract              

The most intensively studied cave animal is a characid fish from Mexico, originally described as Anoptichthys jordani. Two other species of Anoptichthys are synonyms of Anoptichthys jordani, and the genus Anoptichthys is an synonym of Astyanax. For many years the cave fishes have been considered to be conspecific with Astyanax fasciatus because the cave fishes are interfertile, under some conditions, with surface dwelling Astyanax fasciatus. In fact the cave and surface fishes are not interfertile under most, natural, conditions and are reproductively isolated by competitive exclusion. Under the tenets of the Biological Species Concept the cave fish are a separate species. Under the Phylogenetic Species Concept they are also separate species as they possess a significant number of unique character states. Since they are good species under these two species concepts they are also good species under the Evolutionary Species Concept. The cave fishes are a separate species Astyanax jordani.

Introduction     

The cave-dwelling characid from Mexico is well known. It has received more study than any other cave animal and is available in pet stores and aquaria the world over. The reasons for this prominence include the facts that it is easy to breed in captivity and interfertile with the sympatric Astyanax fasciatus. These properties allow very detailed studies, and in particular genetical ones, to be made easily. A large number of publications have resulted from these studies (see Reddell, 1981 and Wilkens, 1988 for bibliographies) but there has been great confusion as to the name and status of the animal. The following discussion outlines the problem and poses a definitive solution.

The first population to be discovered, in La Cueva Chica, was described by Hubbs and Innes (1936) as a new genus and species, Anoptichthys jordani. In the following ten years two other populations were discovered in La Cueva de El Pachon and La Cueva de Los Sabinos. Alvarez (1946, 1947) included them in the genus Anoptichthys as Anoptichthys antrobius and Anoptichthys hubbsi respectively.  It is now known that the cave fishes are widespread, Mitchell et al., (1977) document 29 populations (see also Reddell, 1981:238‑243, 325). The recognition of the original three populations as separate species is no longer supportable. The character states used to separate the three (details of the subdivision of the suborbital bones) is likely to be highly variable among the 29 known populations, principally because all have severe modification of the eye area as a result of eye loss during regressive evolution. In their original description of Anoptichthys jordani Hubbs and Innes (1936) recognised that: "Anoptichthys agrees with the genus and subgenus Astyanax in all apparent characters other than those associated with blindness and subterranean life". The epigean Astyanax fasciatus (previously thought to be Astyanax mexicanus, see Miller and Smith (1986) for details) and the hypogean Anoptichthys jordani are not only very similar in general morphology, but they possess the same (or very similar) karyotype (Kirby, Thompson, and Hubbs, 1977) and are interfertile under some conditions. These features, plus the sympatric distribution, provide strong evidence that Anoptichthys jordani is very closely related to, and in all probability, directly evolved from, Astyanax fasciatus. There is therefore no reason for recognising a distinct genus for the cave fishes. (See Greenwood (1976), Roberts and Stewart (1976), and Banister and Bunni (1980) for discussions of this philosophy). Since Alvarez’ species were informally synonymised with Anoptichthys jordani, and since workers dropped the genus Anoptichthys, informally considering it a junior synonym of Astyanax, the cave fishes have been most commonly referred to as the “cave form” of Astyanax mexicanus (earlier works) or A. fasciatus (more recent works). This was done on a tacit, but incorrect, acceptance of one of the tenets of the Biological Species Concept. In this paper I demonstrate that under this concept, and one other, the cave fishes are a different species from the surface fishes. The rationale for the present work is set out by Mayden and Wood (1995:110), “When entities subsumed under one binomial are actually behaving as distinct evolutionary entities, we perform no service to them nor to the biological community by treating them as a single species”.

Considering these animals as separate species is not unprecedented. Reddell (1981: 238-243), Miller and Smith (1986) and Espinosa Perez et al. (1993), in a checklist of Mexican fishes, all called the cave fish Astyanax jordani. Eschmeyer et al. (1998:816) consider that Anoptichthys jordani is valid as Astyanax jordani.

Cave populations and their relationships.

Troglomorphic Astyanax are now known from at least 29 locations in the Sierra de El Abra, Sierra de Guatemala and other areas in San Luis Potosi, Mexico (Fig. 1) (Mitchell et al., 1977). It has been suggested, on genetic grounds, that the populations vary in phylogenetic age, with some populations evolving in caves for longer than others (Wilkens, 1988). In particular, the isolated Micos population is thought to be considerably younger than most of the Sierra de El Abra populations.  Borowsky and Espinasa (1997) provide DNA evidence for three separate evolutionary lines: “Northern” (Sierra de Guatemala and Nicolas Perez), “Southern” (Sierra de El Abra), and “Micos” (Fig. 2). Espinasa and Borowsky (2001) demonstrate that the southern line, which contains the type localities of all three Anoptichthys species,  derives from  common ancestral stock, most likely due to a single colonisation event (Fig. 3). The southern population is therefore probably a single species, separate from the other two lineages. If further work confirms that the other lineages are distinct they will require descriptions and names. The present discussion relates only to the southern populations.

Species concepts

There has recently been much interest in species concepts (reviews and discussion in Kimbel and Martin 1993, Nielsen 1995, Claridge et al. 1997, Kottelat 1997, Mayden 1999 and Wheeler and Meier 2000). Although there is still much disagreement about the “best” concept (see e.g. the debate in Wheeler and Meier 2000) Mayden (1997, 1999) has provided a significant step forward with his division of species concepts into primary and secondary. The primary, non-operational, concept is the Evolutionary Species Concept (ESC; “An evolutionary species is an entity composed of organisms that maintains its identity from other such entities through time and over space and that has its own independent evolutionary fate and historical tendencies.” (Wiley and Mayden 2000a:73)). All other concepts (Mayden (1997) discusses 25) are considered as secondary, operational, concepts. Each of these attempts to discover biodiversity in a different way. Many overlap in their methods and results whereas many others are very different in methods and results. Until this breakthrough, positions on concepts were often very entrenched to one particular concept. Mayden has shown us that each of the secondary concepts is valuable in its own right. Each will discover a portion of the total biodiversity and, used together under the over-arching primary concept, will reveal the maximum information about global biodiversity. Here I use two of the secondary concepts, the Biological Species Concept (BSC) and one version of the Phylogenetic Species Concept (PSC) to show that the cave and surface fishes are separate species.

Biological Species Concept. - “I define biological species as groups of interbreeding natural populations that are reproductively isolated from other such groups.” (Mayr 2000:17).

It is undoubtedly true that the cave fishes and surface fishes can interbreed under laboratory conditions (reference) and that F₁, F₂ and backcross individuals can be obtained (Wilkens 1988). It is also true that under certain conditions in the wild they can interbreed. The laboratory crosses tell us nothing as they are artificial. Interbreeding in the wild takes place only under very unusual conditions. Of 29 known sites interbreeding has only been observed in nine and of these only in three is it of significant magnitude (Mitchell et al. 1977:72-76). Under the most usual cave conditions of food scarcity  the two forms cannot interbreed (Wilkens, 1988:344-347). Where surface fishes are found with cave fishes the former are so out-competed for food that they starve. In this condition they cannot produce eggs or sperm and so cannot breed. A further factor impedes successful breeding. There is more yolk in cave fish eggs (Huppop and Wilkens, 1991) and homozygous cave fish eggs are have a competitive advantage over heterozygous (cave x surface) eggs (Wilkens 1988:346). In the terms of the BCS this a pre-zygotic isolating mechanism driven by ecology and behaviour. Under unusual cave conditions where there is plenty of food the surface fishes can obtain enough and in La Cueva Chica, which has a large bat roost, the two fishes do interbreed and the Chica fish population is a hybrid one (Mitchell et al. 1977; Romero 1983). Some hybridisation has occurred in La Cueva de El Pachon (Langecker et al. 1991). In Sotano de Yerbaniz, which is food poor,  large numbers of surface fishes enter the cave yet there is a very small number of hybrids (Mitchell et al. 1977:74). It is notable that the probably unrelated Micos fish  “are already reproductively isolated from their epigean neighbours even though they occupy only an intermediate stage with respect to the constructive adaptations developed in phylogenetically old cave forms.” (Wilkens, 1988:346). A prime tenet of the BSC is that groups which are reproductively isolated from one another are separate species. (This is also the prime tenet of the Hennigian Species Concept, see Mayden (1997) and principally Meier and Willmann 2000)). Most cave fishes are isolated from surface fishes by competitive isolation (the competitive-exclusion principle of Mayr 1970:43-44) and therefore the cave and surface fishes are not members of the same species under the BSC. They are not forms a polymorphic species because such species, despite often great morphological variability, breed freely at contact zones (Mayr, 1970:17). They are not subspecies of A. fasciatus because subspecies are not reproductively isolated from other subspecies.

Metrical measures of similarity/difference can also be applied to the BSC where they measure “reproductive isolation and evolutionary independence” (Mayden 1997:399; the “Genetic Species Concept”). Two of these types of measure are available. Avise and Selander (1972) used electrophoresis to study allozyme variation in epigean and hypogean fishes. They use the data to calculate Rogers’ coefficient of genetic similarity (their Table 6). Similarity values for conspecific populations tend to lie in the high 0.80s and 0.90s with genetic similarity among congeneric species  usually much lower, although there is overlap. Avise and Selander (1972) provide values for various congeneric species (0.61, 0.76, 0.77, 0.32, 0.21, 0.50), and various conspecific populations (0.97, 0.98, 0.88, 0.95, 0.97, 0.75, 0.89). Their calculation for the similarity between cave and surface fishes is 0.82. While this is not so low as the congeneric species listed, it is intermediate between these values and the predominant 0.90s of the conspecifics. It is as we would expect of species in the process of separation. This intermediate value does not unequivocally support the stance, often stated (e.g. Wilkens 1988:273, Romero 1983), that cave and surface fish are conspecific. Chakraborty and Nei (1974) calculated Nei’s standard genetic distance (Nei's D parameter) between epigean and hypogean fishes. Values for D of 0.142 (Pachon population) and 0.105 (Sabinos population) fall within the ranges for both subspecies (0.004 ‑ 0.351) and species (0.004 ‑ 3.000) and significantly above that for local races (0.000 ‑ 0.049) as given by Nei (1987:241-242). Neither of these measures provide unequivocal support for the fishes being separate species since they are intermediate between fully separated species and populations within a species. However we are sure that the cave fishes evolved from the surface fishes and these values show us that this process is either still occurring or only recently stopped. They certainly are not supportive of a single, genetically unified species.

Interfertility under some, rather unusual and atypical,  conditions (i.e. absence of total reproductive isolation) has long been used to confine the two type of fishes into one species. The above discussion should finally lay to rest this misguided notion. Wiley and Mayden (2000b:157-158) put the case in context: “As ichthyologists, we know of no recently evolved and closely related species of North American freshwater fish that is 100% reproductively isolated from its sister species. What do we mean by closely related ? We mean species pairs that are young enough and whose biogeographic relationships are such that there is no reason to think that there is an extinct sister species left out of the analysis ... We can produce F₁ hybrids if we have a mind to do so.”

Phylogenetic Species Concept. - There are a number of different formulations of the PSC. Mayden (1997) identifies three and two are discussed in Wheeler and Meier (2000). Here I use the definition of Wheeler and Platnick (2000) which is also that of Cracraft (1997), both of which stem from the studies and thoughts of Eldredge and Cracraft (1980) and Nelson and Platnick (1981). This is the PSC₁ of Mayden (1997).

“We define species as the smallest aggregation of (sexual) populations or (asexual) lineages diagnosable by a unique combination of character states.” (Wheeler and Platnick 2000:58)

The cave fishes have a significant number of very distinctive and diagnosable character states (Table 1). They are quite clearly a different species from the surface fishes under this version of the PSC.

Evolutionary Species Concept. -

                The cave fishes clearly “maintain their identity” and have their own “independent evolutionary fate” within the caves of central Mexico. They are therefore good Evolutionary Species.

Discussion

There is debate among those studying speciation about the extinction of ancestors at a speciation event. One view (e.g. Meier and Willmannn, 2000) is that ancestors, also called stem species, must become extinct when species evolve from them. The present case provides strong, empirical, evidence that this cannot be the case. The presence of an evolved, or evolving, cave species (Astyanax jordani), over a small part of the range of the species it is evolved, or evolving, from (Astyanax fasciatus), can in no way affect the realness of the species Astyanax fasciatus. It exists as a perfectly good species, totally unaltered over the majority of its range, despite the evolution of a new species from it in a part of Mexico.

Why is the “cave form” not an evolutionarily significant unit (ESU) of Astyanax fasciatus ? There is no consensus on the definition of an ESU (see Cracraft 1997:332-335 for discussion) and Cracraft suggests that “its objective use is virtually precluded” because of this. The prime reason for not recognising the cave fishes an ESU of Astyanax fasciatus is that any group which is sufficiently distinct to be considered as ESU is, prima facie, a distinct species under at least the PSC. There is no reason to subdivide “species” into smaller units if it can be shown that such “species” consists of groups which have an “independent evolutionary fate”. The PSC and ESC show us how to determine species and there is no justification for defining sub- or infra- specific groupings.

Taxonomic consequences

The final taxonomic position is: one species of cave‑dwelling fish Astyanax jordani (Hubbs and Innes, 1936) with one junior objective synonym, Anoptichthys jordani Hubbs and Innes, 1936, and two junior subjective synonyms, Anoptichthys antrobius Alvarez, 1946 and Anoptichthys hubbsi Alvarez, 1947. The genus Anoptichthys Hubbs and Innes, 1936 is a synonym of Astyanax Baird and Girard, 1854.

Conservation implications

There is little doubt that subpopulations exist. Mitchell et al. (1977:76-79) document that animals from eight caves are discreet when subject to a morphometric analysis. The recognition of Astyanax  jordani as separate from Astyanax fasciatus does not preclude the existence of subpopulations (demes) within the jordani clade. Continued genetic studies, or  detailed morphological analysis, will reveal these. Once revealed we need to determine the relevant level of protection.

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Wilkens, H. 1988. Evolution and genetics of epigean and cave Astyanax fasciatus (Characidae, Pisces). Support for the neutral mutation theory. Evol. Biol. 23:271-367.

Table 1. Morphological, behavioural, physiological and genetic differences between the surface (epigean) and cave (hypogean) fishes

^* mean lipid to lean dry weight ratio

⁺ C = La Cueva Chica. CS = Commercial stock. Probably derived from the Chica population. Pa = La Cueva de El Pachon. Pi = El Sotano de Las Piedras. S = La Cueva de los Sabinos. Y = El Sotano de Yerbaniz. (All are from the “southern” population). Mi = La Cueva del Rio Subterraneo (unrelated Micos population)                                                                                                                                                                                                            

Museum Holdings

Anoptichthys jordani: BMNH 1973.9.10:20

Anoptichthys hubbsi: BMNH 1951.12.3:14‑17 (Paratypes)