Size at maturity is spatially and temporally modulated by thermal phases in an intertidal mollusc of fisheries interest

This study examined spatial and temporal variation in size at maturity (L50) of the broadcast-spawning mollusc Chiton articulatus, a small-scale fishery species distributed in the Mexican Tropical Pacific. Adult specimens were collected monthly over multiple years between 2011 and 2023 at seven intertidal sites spanning a latitudinal gradient, encompassing both cold (La Niña) and warm (El Niño) events of the El Niño-Southern Oscillation (ENSO), hereafter referred to as ‘thermal phases’. Total body length (TL), body weight and gonad weight were recorded, and the gonadosomatic index (GSI) was calculated for each individual. L50 was estimated annually at the population level using pooled individuals by fitting a logistic model to TL frequency distributions of mature adults (GSI ≥ 4%). Across the species distribution, estimated L50 values ranged from c. 50 to 64 mm TL, with the highest L50 values observed in Sinaloa (northernmost site), whereas the lowest occurred in Oaxaca (southernmost site). Notably, Nayarit (20°45′N; located near the tropical–subtropical climatic transition zone) deviated from the general latitudinal pattern, exhibiting consistently smaller L50 values (50.8 mm) than expected. Central populations (Jalisco, Colima, Michoacán and Guerrero) showed intermediate and relatively uniform L50 values ranging from 55.5 to 56.5 mm. Unexpectedly, L50 exhibited opposite responses to thermal phases at the two geographic range margins. At the northern distribution limit, warmer phases were associated with larger L50 values, whereas L50 decreased during colder phases in years (2021–2023). In contrast, at the southern distributional limit, larger L50 values were associated with cold phases. Overall, the findings indicate that spatial patterns provide an important geographic context for variation in size at maturity in C. articulatus and reflect the influence of thermal variability interacting with local ecological conditions. These results highlight the importance of spatially explicit reproductive monitoring and adaptive fishery management strategies, particularly under ongoing climate variability.