Reproductive biology of the Amazonian amphibian fish the splash tetra Copella arnoldi with emphasis to histological characterization

Abstract

This study describes important components of the reproductive biology of the Amazonian amphibian fish Copella arnoldi. The species sex ratio biased to female 1.8 to 1 male and the length-weight relationship also shown differences between sexes (b value = 1.854 for females and 2.235 for males), although both presented allometric growth (b < 3), “being lighter for its length”. The results also show the importance of the histological analysis to describe the gametogenesis and, consequently, define the reproductive cycle of a fish species. It showed both, males and females, spawning capable during a whole year, whereas the gonadosomatic index pointed spawning capable females during the winter and summer, and spawning capable males only in the autumn. The species presents the rare type of semicystic spermatogenesis and the spermatids complete their differentiation into the tubular lumen, forming a Type I spermatozoa. Both the characteristics are described for the first time for the Lebiasinidae family. Such data can contribute to the discussions about the position of the Lebiasinidae family within the Characiform order, which still does not have a conclusion. Moreover, understand the reproductive biology of a fish species helps its preservation in its habitat and may be applied in restoration programs.

KEYWORDS: fish reproduction, Lebiasinidae, oogenesis, reproductive cycle, spermatogenesis

INTRODUCTION

The Amazon region stands out by its big natural diversity (Alho et al., 2015), including a unique and differentiated characterization in the terrestrial and aquatic habitats. The aquatic environment, for example, faces natural changes along the year, influenced by alternated rainy and dry periods, causing small streams emergence and/or disappearance (Junk et al., 2010). Those cyclical alternations may generate instability to the endemic species, which must, in response, develop new strategies that allow their reproduction and, consequently, survival.

The high adaptability of Amazonian fish species is unquestionable, which reflect in the elevated number of described species in the Amazonian basin (2.406 validated species), the highest freshwater biodiversity on earth, corresponding to 15% of all identified species (Jézéquel et al., 2020). However, the increasing anthropogenic threats in this environment, such as the constant logging, and the catastrophic predictions of the global climate changes for a near future (IPCC, 2021), make the knowledge about the reproductive biology of fish species an urgent matter, in special those whose basic biology are poorly studied.
Tagliacollo et al. (2021) show the number of threatened Neotropical freshwater fish species increased in 59%, emphasizing the need for such type of studies.

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