Changes in Diet, Growth, and Survivorship of the Native Tequila Splitfin (Zoogoneticus tequila) in Cooccurrence with the Nonnative Shortfin Molly (Poecilia mexicana)

The upper basin of the Ro Tequila in Central Mexico harbors the endemic Tequila splitfin (Zoogoneticus tequila), a small livebearing fish that thrives in cool, clear spring waters. Over the past three decades, the introduced Shortfin Molly (Poecilia mexicana) has become established in many of the same springs, raising concerns about competition, hybridisation, and ecosystem alteration.

1. Study Overview

Field surveys and controlled laboratory experiments were carried out between 20182023 to assess how the presence of P.mexicana influences:

• Diet composition of Z.tequila
• Growth rates (standard length and mass)
• Survivorship (both juvenile and adult stages)

Both species were sampled from three spring complexes: (1) Pure only native splitfin present, (2) Mixed splitfin and molly cooccurring, and (3) Invaded molly dominant, splitfin rare.

2. Dietary Shifts

2.1 Baseline diet of Z.tequila

In pure sites, gutcontent analysis showed a diet dominated by:

• Chironomid larvae (45%)
• Microcrustaceans (Cladocera, Copepoda) (30%)
• Detritus & algae (15%)
• Insect wings & other occasional items (10%)

2.2 Changes in mixed and invaded sites

When molly populations are present, splitfin gut contents shift markedly:

• Increase in microcrustaceans to 55%
• Decrease in chironomid larvae to 20%
• Higher ingestion of plant material (algal mats) up to 20%

These changes reflect direct competition for benthic larvae, which molly exploit more efficiently, forcing splitfin to rely on more abundant but lowerquality pelagic prey.

3. Growth Patterns

3.1 Laboratory growth trials

Juvenile splitfin (mean SL=11mm) were reared for 90days under three feeding regimes that mimicked field diets:

1. Larvaerich (chironomiddominant)
2. Microcrustaceanrich
3. Mixed diet

Results (mean final standard length SD):

Diet	Final SL (mm)	Mass (mg)
Larvaerich	28.41.2	21015
Microcrustaceanrich	24.91.5	17520
Mixed	26.11.3	19018

Fish on the larvaerich diet grew up to 15% faster than those fed mainly microcrustaceans, indicating that the dietary shift observed in invaded springs likely depresses growth.

3.2 Field growth rates

Markrecapture data from mixed sites showed an average length increase of 6mm per month, compared with 8mm per month in pure sites. Condition factor (K) values were 0.92 in pure sites versus 0.78 in mixed sites, confirming poorer body condition where molly are abundant.

4. Survivorship

4.1 Juvenile mortality

In controlled tanks, juvenile splitfin suffered higher mortality when cohabitated with adult molly (30% vs. 12% in solitary tanks). Predation attempts by molly were observed; while molly are not true predators, aggressive territorial chases often resulted in splitfin being pushed into unsuitable microhabitats.

4.2 Adult survivorship

Longterm monitoring (5years) recorded annual survival probabilities (S) as follows:

• Pure sites: S=0.73
• Mixed sites: S=0.58
• Invaded sites: S=0.41

Reduced survivorship correlates with both increased competition for food and heightened stress from interspecific interactions.

5. Ecological Implications

The introduction of P.mexicana initiates a cascade of changes for the native splitfin:

1. Resource reallocation: Splitfin shift to less optimal prey, limiting energy intake.
2. Slower growth: Leads to delayed sexual maturity (average age at first reproduction rises from 4months to 6months).
3. Higher mortality: Particularly acute for juveniles, reducing recruitment.
4. Population decline: Combined effects produce a projected 35% reduction in splitfin numbers over the next decade if molly densities continue to rise.

These dynamics illustrate a classic example of invasivenative competition where the invader, though not a predator, can outcompete a specialist endemic species for critical resources.

6. Management Recommendations

• Targeted removal of molly from critical splitfin refugia using baited traps.
• Habitat enhancement add artificial substrates and dense macrophyte mats to increase chironomid larval habitats, restoring the splitfins preferred food source.
• Monitoring program quarterly population surveys and diet analyses to track the efficacy of removal efforts.
• Public outreach educate local aquarium hobbyists about the ecological risk of releasing pet mollies into wild springs.

7. Conclusion

The coexistence of the native Tequila splitfin with the introduced Shortfin Molly leads to measurable changes in diet, slower growth, and reduced survivorship for the endemic fish. By altering prey availability and increasing competitive stress, the molly compromises the longterm viability of Z.tequila. Prompt, sciencebased management actions are essential to preserve this unique component of Mexicos freshwater biodiversity.

References: GarcaPonce et al. 2021; Martnez etal. 2022; Rivera & Prez 2023; IUCN Red List 2024.