Histological evidence and the existence of the cholesterol side-chain cleavage enzyme of the diminutive testis of the robust tonguefish Cynoglossus robustus (Pleuronectiformes: Cynoglossidae)

Abstract

Tonguefishes, commercially valuable marine flatfishes with a global distribution, are characterized by unusually small testes. Despite their economic importance, the detailed structure of these diminutive testes has not been adequately described. Mature testes of robust tonguefish (Cynoglossus robustus) were collected from the Seto Inland Sea, Japan. They were examined for gonadosomatic index (GSI) and histological characteristics. The localization of the cholesterol side-chain cleavage enzyme (P450scc) within the testis was also analyzed. The GSI of the sampled males was markedly lower than that of other bony fish. Histological analysis showed that the testes have a tubular structure, with spermatogenesis occurring in seminiferous tubules, which is less common among teleosts. The stage and structure of spermatogenesis varied depending on the location within the testis. In the cranial region of the testes, the spermatogonia and Sertoli cells were found only at the periphery. Meiotic spermatocytes, haploid spermatids and spermatozoa were primarily located in the inner part of the cranial region. Several seminiferous tubules containing only spermatids were observed in the caudal region. Strong positive signals for the P450scc antibody were detected in the interstitial Leydig cells surrounding the seminiferous tubules. This study provides a detailed description of the testicular structure in Cynoglossus robustus and contributes to understanding the relationship between testicular size, volume, and type in teleosts.