San Marcos Redtail Splitfin: Caring for Xenotoca doadrioi
- aquaterraobsession
- Sep 7
- 10 min read
Updated: Sep 11
Comprehensive Biotope and Aquarium Care Sheet
:
This care sheet delivers a biotope-authentic protocol for Xenotoca doadrioi. It synthesizes the latest academic, conservation, and aquarist-community sources and offers detailed guidance for water chemistry, tank environmental setup, nutrition, social structure, captive breeding, and cohabitation, anchored by what is known of the fish’s natural habitats near San Marcos and Etzatlán.
Table of Contents
Introduction
The San Marcos Redtail Splitfin (Xenotoca doadrioi) is a striking and critically endangered livebearing fish endemic to the highland freshwater systems of Jalisco, Mexico. Only formally described as a distinct species in 2016, X. doadrioi is easily recognized by the males’ vibrant blue flanks and bright red caudal peduncle, contrasted with more subdued brownish females. This charismatic goodeid possesses significant scientific, ecological, and aquaristic importance: it plays a unique role in its native biotope, is emblematic of conservation concerns in Mexican inland waters, and has become a prized gem among responsible aquarium keepers globally.
Recent studies highlight not only the species’ adaptability and interactive behavior in well-designed aquaria, but also the severe threats facing wild populations—habitat degradation, irrigation, organic pollution, and invasive species introductions have eradicated over half of its historic populations. Responsible stewardship in the aquarium trade thus serves both species preservation and public education.
Summary Table: Key Care Parameters
Parameter | Recommended Range or Value |
Minimum Tank Size | 150 liters (39 gal); larger = better |
Water Temperature | 17–25°C (63–77°F); avoid >25°C long-term |
pH Range | 6.5–8.0 (neutral–slightly alkaline) |
General Hardness (GH) | Moderate–hard: 10–20 dGH |
Oxygen Level | ≥8 mg/L |
Substrate | Mud and gravel; some rocky areas |
Vegetation | Sparse to dense aquatic plants (varies by biotope) |
Water Movement | Moderate current |
Filtration | Strong; high turnover; aeration essential |
Water Change | 60–80% weekly |
Diet | Omnivorous (algae, aufwuchs, crustaceans, insects, vegetables) |
Social Structure | Colony (mixed ages/sizes); 8–10+ individuals ideal |
Compatibility | With fast, short-finned, robust native cohabitants |
Behavior | Active, semi-aggressive (esp. males) |
Breeding Temp. | Starts at 20–21°C |
Conservation Status | IUCN: Critically Endangered (CR) |
Taxonomy and Conservation Status
Xenotoca doadrioi was described in 2016, having been previously grouped under the redtail variant of X. eiseni. Intensive molecular and morphological work—especially that of Domínguez-Domínguez et al.—demonstrated X. doadrioi’s genetic distinctness. The species is classified as:
Order: Cyprinodontiformes
Family: Goodeidae
Genus: Xenotoca
Species: doadrioi
The IUCN Red List recognizes X. doadrioi as Critically Endangered (B1ab(i,ii,iii,iv)). Its populations have plummeted due to intensive irrigation, organic pollution, and catastrophic introductions of non-native competitors and predators (notably Oreochromis, Xiphophorus variatus, common carp, and bluegill sunfish). Of its native sites in the Laguna de Magdalena basin and upper Ameca drainage, more than half have been extirpated; many localities not revisited for a decade may no longer support any goodeid fish populations at all.
Moreover, X. doadrioi is not recognized in any formal risk category under Mexican conservation law NOM-059-SEMARNAT-2010 but is listed as endangered/declining by conservation and scientific authorities. Its rarity in the wild makes responsible captive maintenance and propagation a conservation priority.
Natural Habitat Characteristics
Geographic and Biotope Context
Xenotoca doadrioi is endemic to the endorheic (closed drainage) Laguna de Magdalena basin and upper Río Ameca system in Jalisco, central-western Mexico, particularly around Etzatlán and San Marcos. It naturally occurs in:
Spring-fed ponds (e.g., Hacienda San Sebastián)
Small reservoirs (e.g., Oconahua Dam)
Seasonally fluctuating streams (e.g., San Marcos and Sahuaripa streams)
Occasionally, shallow marshlands or irrigation ditches
Habitats range from relatively clear springs with submerged vegetation to turbid, organically polluted streams and open ponds with little or no aquatic plant life. Fish are often found in shallow waters (<1.5–3 m depth), over mud and gravel, sometimes with muddy or rocky patches; aquatic plant cover can be sparse or, in places, seasonally extensive due to floating plants or emergent vegetation.
Native Biotope Water Parameters
Natural and field-recorded values (where available):
Temperature: Typically fluctuates seasonally. Spring-fed sources: ~17–22°C; peaks up to 24–25°C in summer.
Spawning commences at 20–21°C in spring.
Prolonged exposure to >25°C leads to poor health and halt in reproduction.
pH: Not always specified but inferred as neutral to slightly alkaline (6.8–8.0), reflecting local limestone geology and resilience to moderate pollution.
Hardness (GH): Moderate to hard; wild habitats are generally not soft-water environments, with GH 10–20 dGH optimal.
Oxygen: High; 8 mg/L or above, especially in moving waters and springs.
Turbidity: Ranges from clear (El Moloya spring) to milky/cloudy (San Sebastián and Oconahua Dam), and at times highly turbid/polluted in seasonal streams.
Stability of water parameters and high oxygen levels are key, especially in closed tanks (see aquarium section).
Substrate and Vegetation in Native Habitat
Substrate:
Mud and gravel dominate the bottoms of most biotopes, occasionally intermixed with sand or organic detritus.
In seasonal streams and irrigation ditches, substrate can be unstable, sometimes silt-laden with a high organic load.
Vegetation:
Sparse or absent. Many core habitats (San Sebastián type locality, Oconahua Dam) are nearly barren of rooted aquatic plants.
Dense in others: El Moloya spring supports aquatic macrophytes. Seasonal streams (for example, San Marcos) support dense mats of
Water hyacinth (Eichhornia crassipes)—covers the entire surface during wet periods
Cattails (Typha spp.) and sedges (Cyperus spp.); along banks or in shallow reaches
Algae (aufwuchs), emergent vegetation, floating plants, and the presence of decaying macrophytes are all features in certain locations.
Native Cohabitant Species
Overview
The X. doadrioi biotope harbors a distinctive regional fish fauna—mainly endemic goodeids, poeciliids, and, due to invasive species introductions, various exotics. Many historical sympatric (cohabitant) populations have been severely reduced or extirpated due to human activities. The most important cohabitants (by frequency and ecological significance) are shown in the tables below.
Native Fish Species Table
Species | Status | Notes |
Xenotoca cf. melanosoma | Native, frequent | Sympatric in nearly all core habitats |
Goodea atripinnis | Native, frequent | Ubiquitous in springs, streams, and dams |
Poeciliopsis infans | Native, frequent | Small livebearer, coexists in most habitats |
Zoogoneticus purhepechus | Native, regionally limited | Present in El Moloya spring and vicinity |
Ameca splendens | Native, regionally limited | Found in El Moloya spring |
Allotoca maculata | Native, now likely extirpated | Historic records, not seen since ~2006 |
Algansea amecae | Native, now likely extirpated | Not seen since 1970s |
Moxostoma austrinum | Native, historic | Not seen since 1970s |
Xiphophorus variatus | Non-native, invasive | Competes and hybridizes with natives |
Oreochromis sp. | Non-native, invasive (tilapia) | Outcompetes and preys on juveniles |
Lepomis macrochirus | Non-native (bluegill) | Occasional in Oconahua and modified habitats |
Cyprinus carpio | Non-native (common carp) | Occasional in reservoirs |
Pseudoxiphophorus bimaculatus | Non-native, invasive | Has displaced natives in some sites |
Native Plant Species Table
Plant Species | Functional Role | Occurrence |
Eichhornia crassipes | Floating, covers stream surface | San Marcos stream (seasonal) |
Typha sp. (cattails) | Emergent, riparian, filter organic matter | San Marcos and similar streams |
Cyperus sp. (sedges) | Emergent, riparian, substrate stabilization | San Marcos and similar streams |
Hydrocotyle ranunculoides | Marginal, supports invertebrates | Associated with regional springs |
Lemna minor (duckweed) | Floating, nutrient uptake | Common in region’s shallows |
Cabomba sp., Potamogetonsp. | Submerged, oxygenation, fry shelter | Present in clear springs (less common in turbid habitats) |
Native Invertebrate Species
Wild-caught X. doadrioi contain the following as dietary and ecological associates:
Crustaceans: Small amphipods, copepods, ostracods (detected in gut analyses)
Insect larvae: Chironomids, mayflies (Ephemeroptera), caddisflies, mosquito larvae
Mollusks: Snails (Planorbidae, Physidae) in some locations
Others: Benthic algae (aufwuchs), detritus, microfauna
Many of these organisms contribute to nutrient cycling and serve as direct or indirect food items for the splitfins and their fry.
Aquarium Water Parameter Guidelines
Properly replicating the natural environmental envelope of X. doadrioi is crucial for their health, color, and reproductive fitness. Below are precise recommendations synthesized from field and aquarist sources:
Temperature:
Maintain between 17–25°C; optimal spawning starts at 20–21°C
Allow winter "dormancy" at ≤20°C for at least 2–3 months to mimic seasonal cycles and enhance subsequent spawning fitness.
Avoid sustained temperatures >25°C, which can cause metabolic stress, poor fry development, and bacterial/fungal outbreaks.
pH:
Neutral to slightly alkaline, 6.5–8.0; aim for slow and stable values.
Goodeids are resilient to minor fluctuations but will stress and suffer in very acid or very basic water.
General Hardness (GH):
Moderately hard to hard preferred, 10–20 dGH (179–357 ppm TDS as supported by field values).
Oxygen Levels:
Dissolved O2 ≥8 mg/L (essential due to the fish’s activity and tolerance for cool, moving waters).
Maximize with aeration, current, and regular water changes.
Water Changes:
Large and frequent (60–80% of tank volume per week); high waste production and sensitivity to organic waste accumulation require regular, significant water renewal.
Turbidity and Clarity:
These fish tolerate both clear and slightly turbid water, but stable, clean water is critical in aquaria.
Avoid organic overloads and poor hygiene, which cause stress and outbreaks of disease.
Filtration:
Powerful biological and mechanical filtration; sponge or matten filters recommended for safety of fry.
In summary: stability and cleanliness are more important than targeting any extreme “number.” Hard, well-aerated, neutral-to-alkaline, and unpolluted water is best.
Tank Setup and Aquascape Recommendations
Translating native biotope features to an aquarium requires careful attention to both environmental structure and social territory formation.
Minimum Tank Size: 150 liters (39 US gal); tanks with greater base area (footprint) are preferable over depth, as the species is benthopelagic and active on lower and middle levels.
Tank Height: Shallow (20–30 cm) is sufficient and beneficial; larger surface area supports better oxygenation and social diffusion.
Substrate: Replicate mud/gravel mixes. Washed gravel as the uppermost layer on top of a fine layer of sand or clay provides both a natural aesthetic and a surface for establishing beneficial biofilms. Avoid sharp or large rocks to prevent injury and substrate-related stress.
Hardscape/Decor: Use river rocks, weathered driftwood, and well-anchored roots for natural appearance and territory boundaries. Structure the tank with open swimming areas interspersed with complex structures to break line of sight and allow subordinate fish to escape aggression.
Vegetation:
Sparse rooted plants such as Sagittaria subulata, Bacopa caroliniana, Cryptocoryne spp.
Dense patches or floating plants (Eichhornia crassipes, Hydrocotyle sp., Lemna minor) in corners and along margins for shelter.
High lighting is encouraged for algae growth, on which the fish and their fry will graze.
Current and Aeration: Install a moderate water current (internal power filter, air stones, or sponge filters with good oxygenation capacity); mimics native conditions and prevents localized stagnation.
Filtration: Emphasize “overfiltration.” Waste production is high; use large sponge or matten filters, frequent siphoning, and backup aeration in warm months.
Water Change System: Automating large water changes is beneficial. Use dechlorinated tap water, prewarmed and matched to tank chemistry.
Safety and Cover: Hiding places—made of plants, roots, rocks, and inert ceramic items—are essential for fry survival and interrupting male-male chases.
Outdoor Housing Option: In mild climates or seasons, X. doadrioi can be kept in outdoor tubs or ponds from late spring to early fall, allowing for natural temperature and photoperiod cycles.
Diet and Feeding Regimen
In the wild, the San Marcos Redtail Splitfin consumes a rich spectrum of benthic and plant-based foods. Reflecting this in the aquarium is critical for their metabolic health, coloration, and breeding vitality.
Wild Diet:
Algae (aufwuchs), microalgal mats
Aquatic detritus
Small crustaceans (amphipods, copepods)
Insect larvae (chironomids, mayflies, caddisflies)
Occasionally, soft plant material
Aquarium Diet:
Staple: High-quality flake or granulate foods rich in vegetable matter (e.g., Spirulina flake, Omega Kelp Flakes).
Supplementary: Vegetable items such as blanched spinach, kale, zucchini slices, spirulina tabs.
Protein Source: Frozen or live brine shrimp (Artemia nauplii), Daphnia, occasional bloodworm or blackworm (sparingly, as excessive animal protein can lead to bloating and health issues).
Fry: As they are born large with fully formed jaws and trophotaenia (nutrient ribbon), fry accept the same foods as adults but may be supplemented with crushed or powdered fry food and microworms.
Feeding Frequency and Tips:
Twice daily feeding of main diet; supplement with fresh vegetables at least once weekly.
Grazing on algae in the tank is encouraged; avoid over-scraping hardscape and glass.
Avoid over-feeding; uneaten food quickly degrades water quality.
Occasional fasting (one day per week) is beneficial for gut health.
Behavior and Social Interactions
The social structure, aggression, and group dynamics of X. doadrioi are among the most important considerations for both welfare and breeding success.
Activity: Not shy and highly active, engaging in constant foraging, chasing, and displaying.
Intraspecific Aggression:
Especially among males of similar size—alpha males often establish clear dominance hierarchy.
Chasing and “fin-nipping” are common, particularly in small tanks or at high densities.
In larger tanks with complex environments and broken line-of-sight, aggression is diffused, especially in colonies of 8–10 or more fish.
Aggression much reduced when multiple age classes and both sexes are present.
Colony Structure:
Best kept in groups (minimum of six, ideally 8–10 or more).
Odd male-to-female ratios—approximately two females per male—reduce stress on females and curb relentless male harassment.
Introduction of small juveniles (~2–2.5 cm) at regular intervals helps maintain a peaceful flock-breeding colony.
Shoaling: While not a tight “schooling” fish, X. doadrioi regularly forms loose groups, exploring the tank cooperatively.
Fry Predation: Unlike many livebearers, adult X. doadrioi rarely prey on their own fry, provided sufficient hiding spaces and food; the presence of multiple age classes further ensures high fry survival.
Compatibility with Other Species
Native (biotope-accurate) cohabitants are recommended for authentic setups and species conservation. However, careful consideration must be given to behavioral compatibility, especially given the species’ semi-aggressive tendencies.
Best Tankmates:
Other goodeids of similar size and social structure (if source and population genetics are documented, to avoid hybridization), e.g.:
Goodea atripinnis
Poeciliopsis infans
Ameca splendens
Zoogoneticus purhepechus
Xenotoca cf. melanosoma
Regionally native poeciliids; only robust, short-finned species with similar water preferences.
Acceptable but with caution:
Bristlenose plecos (Ancistrus spp.)—documented as compatible due to non-competitive behavior and similar habitat use.
Fast, agile, short-finned minnows or tetras that tolerate moderate hard water and cooler temperatures.
Avoid:
All long-finned or slow-moving species (e.g., guppies, fancy-tailed livebearers), as X. doadrioi will nip fins.
Large, predatory, or overly aggressive species (e.g., cichlids, large sunfish).
All highly soft-water or strictly warm-water species.
Hybrid-prone or misclassified goodeids (to avoid genetic pollution).
Non-native invasive species that have caused harm in their home biotope.
Key compatibility principles: Avoid crowding, maximize cover and hiding spaces, select only robust, fast, and similar-sized cohabitants, and never mix with fish from untrusted sources (to prevent disease and hybridization).
Breeding Biology and Fry Care
Xenotoca doadrioi is a member of the Goodeidae with a unique reproductive mode called matrotrophic viviparity—meaning embryos are nourished by specialized extensions (trophotaeniae) attaching them directly to maternal ovarian tissue, instead of by solely a yolk sac.
Breeding Trigger: Spring temperature rise to 20–21°C; breeding slows or stops at temperatures above 25°C or below 17°C; may restart in autumn if high temperatures abate.
Gestation: Generally 7–8 weeks, varies with temperature and maternal condition.
Brood Size: Typically 5–50 fry; fry are large (8–11 mm at birth) and highly developed.
Fertility Details:
Female goodeids do not retain sperm—copulation is required prior to each brood.
Gravid spot sometimes visible; females become noticeably plump when close to giving birth.
Fry Care:
Fry are born with a visible trophotaenia (umbilical-like extension) that is absorbed within a few days.
Fry settle near the surface or among plants for several days, then join the active shoal, assuming basic adult diet.
Adult predation on fry is low, especially with solid hiding places or “fry net” refugia.
For maximum fry survival and to promote colony structure, introduce juveniles to tank as new cohort matures to 2–2.5 cm.
Colony Method: Natural approach; adults, subadults, and fry coexist, mimicking wild population structure and facilitating social acclimatization.
Selective Breeding and Conservation Responsibility: Given the species’ conservation status, breed and trade only pure, locality-documented lines—avoid hybridization, always preserve records of your fish’s origin, and share surplus fish with conservation-committed breeders, aquarists, or scientific societies.
Conclusions
Raising and propagating Xenotoca doadrioi in the aquarium—ideally in a naturalistic, biotope-faithful environment—represents a rare opportunity to preserve a unique evolutionary lineage on the brink of global extinction. By respecting its native requirements, hydrology, food web, social dynamics, and regional associations, aquarists can support both the vitality and conservation significance of this wonderful species. Every careful breeder contributes to safeguarding one of Jalisco’s most irreplaceable aquatic treasures.
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