The constant growth of the world population has brought a tremendous increase in the discharge of a wide diversity of pollutants to aquatic ecosystems, causing deleterious effects on organisms and fisheries [1-3]. Aquatic organisms, including fish, uptake and accumulate pollutants directly from water or sediments, and indirectly via the food chain . Fish are often used as sentinels because they play a number of roles in the trophic web, bioaccumulate chemical substances, and respond to low concentrations of toxicants [5, 6].
Mugilidae (mullets) is a fish family widely distributed in tropical and subtropical waters around the globe, particularly in coastal and estuarine areas where they play an important ecological role, and provide biomass to support fisheries [7-9]. Over the past years, Mullets have been proposed as pollution bioindicators for environmental degradation . Mugilids, in particular species of genus Mugil, such as Mugil cephalus and Mugil incilis, have been extensively employed on environmental monitoring programs, as well as in toxicological studies in coastal zones impacted by human activities [1, 10-16]. The use of these mugilids as sentinel species in these coastal systems arises from their wide geographical distribution; great abundance, salinity tolerance and bioaccumulation of land-based pollution, a feature largely enhanced by their consumption of benthic sediments along with their food [1, 17].
Mugil incilis, also known as mullet, lisa and lisa rayada, is one of the most abundant fish in the Caribbean. It is found in the western Atlantic Ocean, from Panama and Haiti to southeastern Brazil . It is mainly found in brackish estuaries, but also in marine and hyper-saline waters . Juvenile fish (<25 mm) are primarily planktonic or carnivorous feeders, whereas larger specimens switch their diet to detritus and benthic microalgae, ingesting large amounts of organic matter, sand or mud from the sediment . Mugil incilis supports commercial and recreational fisheries, owing to its high protein and vitamin content, being a stable food source in many countries . Its importance as food contrasts with its capacity to serve as intermediate host for several parasites . In Colombia, M. incilis is one of the most widely distributed fish in the Caribbean coast [13, 22].
Along the Caribbean coast, Cartagena Bay is considered one of the ecosystems with high economic and environmental interest, as it hosts a great diversity of biological resources, but in the other hand, it receives many anthropic pressures, especially from industrial activities, such as oil refinery, pesticide packaging, metallurgical industry, and boatyards, including also naval and commercial shipping harbors. The direct and indirect discharges of urban and industrial wastes and runoff have led to the chemical contamination of the Bay [13, 23, 24]. Some studies with native species of this ecosystem, such as M. incilis (Lisa), have shown anthropic contamination by chemical substances, both organic and inorganic, and biological stressors, such as parasites. Heavy metals, such as mercury, have been detected in this species [13, 25], as well as polycyclic aromatic hydrocarbons (HAPs) and perfluorinated octyl sulphonates (PFOS) in bile [14, 23], and organochlorine pesticides in muscle (β-HCH, Aldrin, 4, 4’-DDE and endosulfan . This species has also been found parasitized with nematodes of the Anisakidae family, in particular Contracaecum sp., as well as with Heterophydae trematodes, specifically Ascocotyle (Phagicola) longa in hepatic tissue [21, 26, 27].
As presented before, most studies on M. incilis from the Cartagena Bay environmental have included measurements of pollutant levels in different tissues. However, a better understanding of the impact of this pollution requires additional approaches to correlate the chemical or biological exposure with the molecular and physiological responses in these organisms, necessary to generate reliable information on their current health status and the effects of exposure to different pollutants [6, 28]. One of those approaches in the analysis of gene expression, a tool that provides multiple possibilities to evaluate molecular changes in exposed organisms [29, 30]. The understanding/comprehension of the biological response related to chronic exposition to environmental pollutants at the transcriptomic level is essential to safeguard the adaptive potential of populations under heavy anthropogenic pressure [31-33]. For example, de novo assembly of transcriptomes from important aquaculture species, such as Epinephelus coioides, Larimichthys crocea, Scophthalmus maximus, Lateolabrax japonicas and Labeo rohita (Hamilton) has revealed a huge number of molecular markers relevant in the immune response after exposition to pathogen microorganisms [34, 35]. Although experimentally controlled populations are ideal for evaluating the transcriptional profile during exposure to specific environmental stressors , the study of toxic effects and transcriptional response in natural populations is essential for understanding the synergistic effects of multiple environmental stressors under field conditions [29, 30].
In fish, liver has been the focus of toxicological numerous studies as it is sensitive to pollutant exposure [1, 12, 37]. In this perspective, the assessment of hepatic defense and damage responses in a resident fish species becomes highly relevant in biomonitoring studies, especially considering the liver performs complex biological functions that are essential for survival, including nutrient synthesis, metabolite transformation and storage, and detoxification processes [38, 39]. However, the analysis of fish liver transcriptomes has been limited to only a few species and to our knowledge; no studies have been reported on the liver transcriptome of M. incilis or in other mugilid species. The purpose of this study was to characterize the hepatic transcriptome of M. incilis in order to facilitate future studies on gene expression related to the effects of environmental pollution in this Caribbean wild species.