Morph-specific dorsal fin characteristics and sex steroids in muscles across life history stages in a sex dynamic marine fish, Lythrypnus dalli

Abstract While sex steroids orchestrate the organization and activation of external anatomical and behavioral phenotypes, tissue specific mechanisms that regulate these processes are largely unknown. To understand the regulation of both fixed and plastic morphological and endocrine characteristics across social contexts, we investigated the bluebanded goby, Lythrypnus dalli, a sexually plastic fish. The distribution and density of androgen receptors is sexually dimorphic in skeletal muscles of these fish and is associated with courtship displays in males. The supracarinalis muscle, which runs dorsal and parallel to the spinal cord is used for courtship displays by males and for aggressive displays by all L. dalli morphs. Here, we first investigated dorsal fin lengths across fish from different life history stages. We found that while the distribution of dorsal fin sizes exists on a continuum, 45.54% of males had dramatically longer dorsal fins compared to females, and dorsal fin lengths increased within 10 days of protogynous (female to male) sex change. Epaxial and hypaxial are trunk muscles that power all movements through association with all limb muscles in fish. We conducted two separate studies to examine sex steroid levels in skeletal muscles of fish housed either in larger open communities, or in smaller, isolated groups. In fish from community groups, supracarinalis muscles in males had significantly higher levels of 11-ketotestosterone and lower estradiol compared to females, while isolated groups had no sex differences. In fish from isolated groups, only the epaxial and hypaxial muscles had plastic endocrine responses such that within only 5 days of transition, protogynous fish had significantly higher 11-ketotestosterone compared to stable males and intermediate estradiol compared to stable males and females. Overall, endocrine changes preceded morphometric changes and exhibited plasticity across different environments and life history stages. These results support the presence of many traits, from external morphological features to the cryptic endocrine level, that contribute to the occurrence of multiple morphs across life history stages in L. dalli. The integration of these phenotypes is important to understand the mechanisms that regulate the functional performance of behavioral displays in specific social contexts.