Taxonomy & naming
Allotoca diazi was described by Seth Eugene Meek in 1902 in the Field Columbian Museum's Zoological Series, originally placed in the genus Zoogoneticus as Zoogoneticus diazi. The holotype is an adult female of about 2.5 in standard length (FMNH-3618), collected by Meek and F. E. Lutz at Lake Patzcuaro, Michoacan, between 18 and 22 May 1901; the type locality is the lake itself. Meek's own Zoogoneticus miniatus, described the same year, is now treated as a synonym, and the species passed for decades through the combination Neoophorus diazi (Hubbs & Turner, 1939) before being moved to Allotoca.
The transfer from Neoophorus to Allotoca rests on a convergence of morphological and molecular work in the early 2000s (Meyer et al. 2001; Doadrio & Dominguez-Dominguez 2004; Webb et al. 2004), which folded Neoophorus into Allotoca. Eschmeyer's Catalog of Fishes carries the species as Allotoca diazi (Meek, 1902), the authority kept in parentheses to mark the change of genus from the original Zoogoneticus. The fish sits in the subfamily Goodeinae of the family Goodeidae — the splitfins or live-bearing toothcarps endemic to central Mexico.
A point of historical confusion is worth noting: some of Meek's 1901 material from nearby Laguna de Zirahuen, long lumped with diazi, is now referred to the closely related congener Allotoca meeki, the Zirahuen allotoca. This restricts true A. diazi to the Patzcuaro basin. The specific epithet honours Jose de la Cruz Porfirio Diaz Mori (1830-1915), the long-serving President of Mexico at the time of the description.
Morphology
Allotoca diazi is a robust, moderately deep-bodied splitfin. Females reach the larger size — to about 4.5 in total length, with Miller and colleagues reporting standard lengths around 4.5 in — while males stay smaller, to roughly 4 in total length. The head is fairly blunt, the mouth large and protractile, and the jaws bear large conical teeth that suit a carnivorous habit; the gut is correspondingly short, about the length of the body.
Colouration is cryptic and unshowy. The ground colour is a light olivaceous, mottled with darker, irregular cross-bars, and there is an indistinct lateral band; the fins are largely plain. Larger live specimens can take on a reddish tinge over the body. This muted, broken pattern matches the clear-to-muddy spring and lake-margin waters the fish inhabits, where it offers camouflage rather than display.
As in all Goodeinae, the sexes are distinguished by the male's anal fin. Rather than the rod-like gonopodium of the New World poeciliids, male goodeids carry an andropodium — the first several anal-fin rays (about seven) are shortened and separated from the rest by a notch, forming a short, partially independent lobe used to deliver sperm. Males here also have a somewhat larger dorsal fin, while females run slightly larger overall, with a blunter head and a yellowish anal-fin base.
Habitat
The species is endemic to the endorheic (closed-basin) Lago de Patzcuaro drainage in the highlands of Michoacan, part of the upper Lerma system. Historically it lived in Lake Patzcuaro itself, but it is now effectively extirpated from the open lake, persisting there only as a remnant. Its stronghold today is the spring-fed water at the Molino de Chapultepec (Molino Viejo) near the town of Patzcuaro, including the Canal la Cienega — a small set of springs, channels and ponds that constitute a single evolutionarily significant unit.
The fish occupies lakes, spring-fed creeks and ponds with clear to muddy water and little to moderate current, over mixed substrates of mud, sand and rock. It favours shallow water, generally less than a metre deep, among aquatic vegetation — Potamogeton, Myriophyllum, Chara, Egeria, Lemna and the floating Eichhornia, along with green algae. The Chapultepec springs are noted for a characteristic milky-blue water; a field reading there in March 2017 recorded about 19.5 C, pH 7.22 and a conductivity around 1180 microsiemens.
Allotoca diazi shares its waters with a small community of Mesa Central fishes, including its congener Allotoca dugesii — notably one of the few same-genus goodeid pairs that do not hybridise — along with Alloophorus robustus, Goodea atripinnis, Skiffia lermae and the silverside-like Algansea lacustris. FishBase characterises the broader environment as tropical, roughly 22-27 C, with pH near 6.8-7.4 and hardness around 5-10 dH, and notes a tolerance of slightly brackish conditions.
Feeding
Allotoca diazi is primarily carnivorous. Dietary study (Kingston, 1979) and its anatomy point the same way: a large, protractile mouth, large conical teeth, and a short gut roughly equal to the body length are the equipment of a predator rather than a grazer. In the wild it hunts insects at the water surface and takes worms and other small invertebrates from among the vegetation and substrate. Its low trophic level estimate (around 2.8 on FishBase) reflects a diet of small animal prey rather than larger fish.
In the aquarium the species feeds readily and is not fussy. Keepers offer bloodworms, insect larvae, water fleas (Daphnia), mysids and newly hatched brine shrimp, and the fish also accepts prepared flake, granules and sinking tablets. A varied, protein-rich diet keeps condition and breeding readiness high. Because it is an active hunter from a well-oxygenated, flowing environment, frequent modest feedings suit it, paired with the large water changes its keepers use to maintain water quality.
Mating
Like all goodeids, Allotoca diazi practises internal fertilisation and bears live young. Fertilisation is achieved not with the rod-like gonopodium of poeciliids but with the goodeine andropodium — the notched, partly separated front rays of the male's anal fin, which channel sperm to the female. Males pursue and court females within the group; the species is described as not shy, and in the confined spring channels and aquaria it lives at fairly close quarters with conspecifics.
Reproduction is essentially year-round under suitable conditions. Mendoza's classic ovarian study (1962) found young present in females more or less continuously from April through January, with reproduction pausing only over the coldest part of the year. In cultivation this seasonal rhythm is reproduced deliberately: the fish needs a cool winter rest below about 20 C — even 15-16 C — for two to three months to halt fry production, after which it resumes spawning at around 20-21 C and stops again if the water climbs much above 25 C.
There is no parental care and no superfetation reported for this species; the female carries a single developing brood at a time rather than overlapping clutches. Sexual maturity comes early — within the first year of life — which, with the continuous breeding season, allows the small remnant populations to turn over quickly.
Breeding
Allotoca diazi is viviparous in the full goodeid sense: the embryos are nourished within the female through trophotaeniae, ribbon-like extensions of the gut that act as a placental analogue and draw nutrients directly from the ovarian fluid. This is matrotrophic live-bearing, distinct from the largely yolk-fed (lecithotrophic) gestation of guppies and most poeciliids, and the trophotaeniae of the fry are precisely the feature that gives the genus its name (Allotoca, "other birth").
Gestation is comparatively long. FishBase cites roughly 55 days; the Goodeid Working Group, following Mendoza, gives 60-75 days. Broods are substantial: Mendoza found an average of about 40 young per ovary, with embryos of near-equal size and only a tiny fraction (about 0.5 percent) of runts. Meek's own specimens bracket the range — a 3 in female held 28 young of about 0.5 in each, and an ~3 in female held 47 young of about 0.5 in each — so brood size scales with the size of the female. FishBase summarises the species as producing 10 to 50 young per brood.
The fry are born large and fully formed, able to swim and feed at once; newly born goodeids often still trail the remnants of their trophotaeniae for a short time. For aquarists this makes the species relatively straightforward to breed, the main requirements being the cool winter rest to drive the seasonal cycle and enough cover or separation to bring fry through. Early maturity then closes the loop within a single year.
In the aquarium
Allotoca diazi is a hardy and rewarding subject for the dedicated keeper, and is maintained in conservation aquaculture as much as in private tanks. It wants space and water movement that reflect its spring habitat: a tank of at least about 25 US gal, ideally low and wide rather than tall (a height of around 10 in is acceptable), with moderate-to-swift current and high oxygen — keepers aim for 8 mg/L or more. Structure helps: rocks, roots and clumps of vegetation in the corners give the fish cover without filling the open swimming space it uses.
Water management is the crux. The species is accustomed to clean, flowing, well-mineralised water, so large weekly changes of 60-80 percent are standard practice, and it does well outdoors from spring to autumn wherever daytime water stays above about 15 C. Temperature is used as a management lever rather than held flat: a cool winter period below 20 C pauses breeding and gives the fish a natural rest, after which warming to 20-21 C restarts it. It is not a shy fish and feeds eagerly on a varied diet of live, frozen and prepared foods.
Conservation
Allotoca diazi is assessed as Critically Endangered on the IUCN Red List (species 169372; assessment dated 17 April 2018) under criteria B1ab(i,ii,iii,iv)+2ab(i,ii,iii,iv), with a decreasing population trend. The listing reflects a tiny, fragmented range and severe decline: the fish has been lost from Lake Patzcuaro proper and now depends on a single small system of spring-fed channels and ponds at the Molino de Chapultepec near Patzcuaro, leaving it acutely vulnerable to habitat loss, water abstraction, pollution and introduced species. Lyons and colleagues (2019) likewise rank it as critically endangered and declining, and under Mexico's national standard NOM-059-SEMARNAT-2010 it is classified as P (en peligro de extincion, in danger of extinction).
The threats are those of the wider Mesa Central goodeid crisis: shrinking and degraded lakes and springs, falling water quality, and competition or predation from non-native fishes across the upper Lerma basin. With the wild stronghold reduced to a remnant, the margin for any further loss is slim, and the survival of the species in nature now hinges on protecting the few remaining spring habitats around Patzcuaro.
For this reason ex-situ conservation has become central. The species is maintained in coordinated aquarium-hobby breeding programmes — including an OVVOE (Austrian Association of Aquarists) studbook for the Allotoca of the Mesa Central — through the Goodeid Working Group network, which keeps genetically managed lines as insurance against extinction in the wild. The same hardiness that makes A. diazi tractable in the aquarium is, in effect, part of its survival strategy.