Reproductive Strategy of the Annual Fish Leptopanchax Opalescens (Rivulidae) and Trade-Off Between Egg Size and Maximum Body Length in Temporary Wetlands

Abstract

Leptopanchax opalescens is a critically endangered small annual sh. Reproductive traits of this species were studied to improve our understanding of the strategies that facilitate the occupation of temporary wetlands. We compiled egg diameter and maximum total length (TLmax) data from 132 neotropical freshwater sh (83 genera, 43 families) to establish comparisons with this species. We used the egg diameter / TLmax ratio to test the hypothesis that annual sh in temporary wetlands have relatively larger eggs than non-annual species from perennial habitats (lakes, rivers). Fish were collected from Guandu River drainages (Brazil). DNA barcoding was employed to con rm the species identity. The phases of gonadal development and spawn type were described using histological techniques. Egg size and fecundity were determined with microscopic analysis. Females with batch spawning and males with continuous spawning were detected. The bath fecundity ranged from 22-32 vitellogenic oocytes (mean 27 ± 7 SD). Annual species presented greater relative egg size than perennial species (p < 0.001), which is an indication of greater reproductive investment at the expense of somatic growth in temporary wetlands. Larger eggs are advantageous for annual sh as their lifespan is limited, and they can allocate a greater amount of yolk reserves for long periods buried in the substrate under embryonic diapause. Large relative oocytes, spawning in batches, synchronous modal development of oocytes, continued production of sperm in males, and a complex process of embryonic diapause are reproductive traits that favor resilience of L. opalescens and other annual sh in temporary wetlands. IntroductionRivulidae (Cyprinodontiformes) is the eighth most speciose sh family in the World, with 471 valid species (Fricke et al. 2022), and the rst in number of threatened species in the Neotropical Region, where they occur. The threat is such that in a megadiverse country like Brazil, a single family of freshwater sh (i.e., Rivulidae) has a higher number of threatened species (130 spp.) than all classes of marine sh combined (Actinopterygii + Chondrichthyes + Myxini = 97 spp.) (ICMBio 2018). Habitat loss is the main threat to rivulids. The swamps and temporary wetlands where many of these animals inhabit have been drastically destroyed, due to deforestation, drainage and land lling resulting from agricultural and urban expansion (Loureiro et al. 2018;Castro and Polaz 2020). Loss of temporary wetlands from land alterations has signi cant ecological and social consequences (Calhoun et al. 2017), but freshwater research, policy, and conservation have historically neglected these habitats, focusing instead on larger water bodies and agship species (