Fish Pancreas: Endocrine Layers, Exocrine Functions, and Beyond

The pancreas in fish plays a vital role in metabolic balance through endocrine and exocrine functions. While it shares similarities with the mammalian pancreas, its structure and function vary across species, influencing blood sugar regulation, digestion, and adaptation to aquatic environments.

Examining pancreatic tissue organization, endocrine cell roles, and exocrine secretions clarifies how this organ supports physiology. Species-specific variations highlight evolutionary adaptations affecting nutrient processing and digestion.

Tissue Organization

The fish pancreas differs structurally from its mammalian counterpart, often reflecting adaptations to aquatic environments. Unlike the compact mammalian pancreas, fish pancreatic tissue is frequently diffuse, interspersed among the liver, spleen, and intestines. Some species exhibit a hepatopancreatic configuration, integrating digestion and metabolic regulation with hepatic function. In contrast, elasmobranchs (sharks and rays) have a more consolidated pancreas resembling that of higher vertebrates.

The pancreas consists of endocrine and exocrine components. The endocrine portion forms clusters known as the islets of Langerhans, which vary in size, number, and distribution among species. Some fish have large, well-defined islets, while others have smaller, dispersed clusters. The exocrine tissue, responsible for digestive enzyme production, consists of acinar cells arranged in lobules and connected by a ductal network that facilitates enzyme secretion into the digestive tract. The degree of separation between these functional domains affects metabolic and digestive efficiency.

Vascularization plays a key role in pancreatic function. Blood supply to the islets ensures rapid hormone distribution for glucose homeostasis. In species with a diffuse pancreas, the proximity of endocrine and exocrine components to major blood vessels and digestive organs enhances nutrient sensing and enzyme secretion. Neural innervation, particularly from the autonomic nervous system, further modulates pancreatic activity, allowing dynamic responses to feeding and metabolic demands.

Endocrine Cells

The endocrine pancreas regulates metabolism, particularly glucose homeostasis, through hormone-secreting cells within the islets of Langerhans. These cells function similarly to their mammalian counterparts but vary in distribution and proportion among species. The three major endocrine cell types—beta, alpha, and delta cells—produce distinct hormones influencing energy metabolism.

Beta Cells

Beta cells produce insulin, which facilitates glucose uptake and storage. In fish, insulin secretion plays a central role in blood glucose regulation, though mechanisms differ from those in mammals. Fish generally exhibit lower baseline insulin levels and a prolonged response to glucose intake, an adaptation to variable dietary carbohydrate availability (Plisetskaya, 1998).

Beta cell distribution within the islets varies. In teleost fish, beta cells form dense clusters, while in elasmobranchs, they are more evenly distributed among other endocrine cells. Insulin secretion is influenced by amino acids, fatty acids, and neural inputs, reflecting dietary diversity. Some fish exhibit glucose intolerance, clearing glucose less efficiently than mammals due to differences in insulin receptor sensitivity and glucose transporter expression (Moon, 2001).

Alpha Cells

Alpha cells produce glucagon, which promotes glycogen breakdown and glucose release into the bloodstream. In fish, glucagon secretion helps maintain energy balance during fasting or food scarcity. Unlike in mammals, where blood glucose levels primarily regulate glucagon release, fish glucagon secretion is also influenced by amino acid availability and environmental conditions (Mommsen & Moon, 2001).

Alpha cell distribution varies. Some teleosts have alpha cells interspersed among beta cells, while others form distinct peripheral zones within the islets. This organization affects insulin-glucagon interactions and glucose metabolism regulation. Certain fish species exhibit a stronger glucagon response to fasting than mammals, an adaptation to prolonged food scarcity. Research suggests glucagon also plays roles in lipid metabolism and amino acid utilization (Plisetskaya et al., 1989).

Delta Cells

Delta cells secrete somatostatin, which inhibits insulin and glucagon release, helping maintain metabolic balance. In fish, somatostatin may also regulate digestion by modulating gastrointestinal motility and enzyme secretion (Conlon et al., 1997).

Delta cells are often scattered within the islets. Some species have delta cells near blood vessels, suggesting a role in fine-tuning hormone release in response to circulating nutrients. Multiple somatostatin isoforms exist in fish, with different physiological effects. For example, certain teleosts express somatostatin-14 and somatostatin-28, the latter having a longer half-life and stronger inhibitory effects on insulin and glucagon secretion (Sheridan & Plisetskaya, 2002). The presence of multiple somatostatin variants highlights the complexity of endocrine regulation in fish.

Exocrine Secretions

The exocrine pancreas produces digestive enzymes essential for nutrient breakdown. Acinar cells synthesize and store these enzymes before releasing them into the intestinal lumen via a ductal network. The composition of secretions varies by species, reflecting dietary needs.

Proteolytic enzymes such as trypsin and chymotrypsin play a key role in protein digestion. These enzymes are synthesized as inactive zymogens (trypsinogen and chymotrypsinogen) and activated in the intestine to prevent premature tissue degradation. Trypsin activity in teleost fish adapts to dietary protein intake, with carnivorous species exhibiting higher protease activity than herbivorous or omnivorous fish (Krogdahl et al., 2005). Variations in trypsin isoforms across species suggest evolutionary adaptations optimizing protein digestion under different environmental conditions.

Lipolytic enzymes, such as pancreatic lipase, aid in fat digestion. Fish often exhibit broader lipase activity than terrestrial vertebrates due to the high lipid content of aquatic diets. Marine species with piscivorous diets produce lipases with enhanced stability in cold-water conditions, ensuring efficient lipid hydrolysis despite low temperatures (Tocher, 2003). Bile salt-dependent lipase further enhances fat digestion by emulsifying lipid droplets, increasing enzymatic accessibility.

Amylase, which digests carbohydrates, varies significantly among fish species, largely influenced by dietary carbohydrate intake. Herbivorous and omnivorous fish produce higher amylase levels than carnivorous species, which rely less on carbohydrates for energy (Hidalgo et al., 1999). Fish generally exhibit lower starch digestion efficiency than mammals, possibly due to differences in enzyme kinetics and substrate availability. Environmental factors, such as water temperature and seasonal feeding patterns, may also influence amylase expression, demonstrating the dynamic regulation of pancreatic enzymes.

Role In Nutrient Processing

The fish pancreas coordinates digestive enzyme secretion with hormonal regulation of energy storage and utilization. As food is ingested, the exocrine pancreas releases enzymes to break down macronutrients, while the endocrine pancreas modulates glucose and lipid homeostasis to maintain energy supply.

Once absorbed, nutrients are distributed and stored under pancreatic regulation. Insulin promotes glucose uptake and glycogen synthesis, while glucagon mobilizes stored energy during fasting. Metabolic reliance varies by species—carnivorous fish depend more on lipid oxidation, while herbivorous and omnivorous fish metabolize carbohydrates more readily.

Species-Specific Differences

Pancreatic structure and function vary widely among fish species, reflecting adaptations to different ecological niches and diets. Some species have a compact pancreas, while others have a diffuse arrangement interspersed among other organs. These differences influence enzyme and hormone delivery, shaping nutrient processing efficiency.

Carnivorous fish, such as salmon and tuna, have pancreases optimized for high-protein diets, with elevated proteolytic enzyme levels. These species rely more on lipid metabolism, necessitating strong lipase activity. Herbivorous fish, such as tilapia, produce higher amylase concentrations to digest plant-based carbohydrates effectively. Some species, like catfish, adjust enzyme production based on diet, allowing them to thrive in diverse environments.

Temperature and habitat also influence pancreatic function. Cold-water species possess enzymes that remain active at lower temperatures, ensuring efficient digestion despite environmental constraints. These adaptations highlight the fish pancreas’s role in supporting diverse metabolic strategies across aquatic ecosystems.