Hyperosmolality tolerance of freshwater largemouth bass (Micropterus salmoides) to brackish at early ontogenetic stages: Eggs, embryos and yolk-sac larvae

Abstract

Salinity is recognized as a pivotal factor limiting the migration of freshwater fish to brackish environments. The largemouth bass (LMB, Micropterus salmoides), a globally translocated freshwater fish, exhibits estuarine distribution, yet its hyperosmoregulatory capacity during early ontogenetic stages remains inadequately understood. To investigate the impact of freshwater salinization, a series of experiments were conducted in the Pearl River Delta, China. The study aimed to elucidate the osmoregulatory abilities of LMB eggs and embryos, assess the salinity toxicity on hatching, and explore the acute effects of hyperosmolality on yolk-sac larvae. Our results revealed that freshwater-activated mature eggs and naturally fertilized oocytes maintained nearly identical osmotic homeostasis, with diameters of 1.38 {+/-} 0.068 mm and 1.37 {+/-} 0.054 mm, respectively. Furthermore, both exhibited peak water excretion at a salinity of 15.0 ppt. Remarkably, a reduction in water permeability was observed in hyperosmotic environments. Spontaneous hatching rates increased from 27.5 {+/-} 14.4% in the 1.0 ppt group to 75.1 {+/-} 12.0% in the 6.0 ppt group under fluctuating temperature conditions. Yolk-sac LMB larvae consistently reduced survival time from 12.5 d at 1.0 ppt to 50.7 {+/-} 2.1 min at 35.0 ppt. Similarly, more developed larvae also experienced a decrease in survival time. Logistic regression models fitting lethal time with salinity indicated a sharp decrease between 10.0 ppt and 20.0 ppt. These findings offer practical insights for predicting distribution patterns and enhancing aquaculture technology for LMB. Moreover, they may contribute theoretically to the broader understanding of the osmoregulatory mechanisms of freshwater fish.