The elasmobranch digestive system - current status and future directions.

Area of Science:

• Evolutionary biology
• Comparative physiology
• Ichthyology

Background:

• Elasmobranchs exhibit unique gastrointestinal (GI) and metabolic adaptations, including an acid-pepsin stomach and spiral intestine, distinguishing them from other vertebrates.
• Their ureosmotic strategy results in nitrogen limitation, necessitating high-protein diets and efficient nutrient assimilation.
• The interplay between GI physiology, energy balance, and osmoregulation in elasmobranchs is an emerging area of research.

Purpose of the Study:

• To review nutrient assimilation processes in elasmobranchs, from digestion to absorption.
• To elucidate the physiological roles of accessory organs like the gallbladder and pancreas.
• To highlight the potential significance of the intestinal microbiome in elasmobranch nitrogen cycling.

Main Methods:

• Literature review of existing studies on elasmobranch digestive and metabolic physiology.
• Analysis of evolutionary firsts in vertebrate GI systems observed in elasmobranchs.
• Synthesis of current understanding regarding nutrient uptake, energy metabolism, and osmoregulation.

Main Results:

• Elasmobranchs were the first vertebrates to evolve an acid-pepsin stomach and a glandular pancreas.
• They possess a spiral intestine, an advancement over the typhlosole found in lampreys.
• Elasmobranchs rely heavily on ketones and amino acids, not fatty acid oxidation, for energy and are nitrogen-limited due to their ureosmotic strategy.

Conclusions:

• The elasmobranch gastrointestinal tract is central to nutrient assimilation, energy production, and osmotic balance.
• Accessory organs and the intestinal microbiome are crucial, yet understudied, components of elasmobranch physiology.
• Further research into the GI tract's role in elasmobranch energy and nitrogen balance is warranted.