For veterinarians to better understand aquatic animal health, the most basic veterinary foundation begins with understanding piscine anatomy and physiology. General observations of fish behavior, anatomy, and food apprehension can provide more valuable diagnostic information than advanced laboratory testing alone. This article highlights anatomic differences of fish species occupying different environmental niches, using a case to show the use of modern veterinary diagnostics, and introduces additional topics in piscine gastroenterology related to toxins, nutrition, probiotics, and infectious diseases.

Keywords

Gastrointestinal tract

Gastroscopy

Buoyancy disorder

Taurine deficiency

Prebiotics

Probiotics

Thiaminase

Key points

Introduction

Gastroenterology in osteichthyes (bony fishes), which comprise most species of veterinary concern, such as salmon, koi, and catfish, and in chondrichthyes (cartilaginous fishes), which include sharks, rays, and skates, continues to be a growing area for research and new clinical treatments.1 Advanced diagnostic techniques can be practically modified and directly applied to fish patients for identifying gastrointestinal ailments. Prebiotics and probiotics may treat and prevent infectious diseases, could enhance oral medication and vaccine uptake, and can better maintain healthy and normal gastrointestinal flora in fish. More research investigating the normal intestinal flora of fish, oral pharmacokinetics for both drugs and vaccines, optimal nutritional requirements for various species, and the role of mucosal immunity on protection against common fish pathogens serves to improve the veterinary approach for a broader and more comprehensive understanding of fish gastroenterology.

Although no unique veterinary specialty is recognized for fish medicine or aquatic animal health in the United States, veterinarians have been expanding their experience, increasing their knowledge, and contributing to the veterinary literature for the last several decades to advance medicine and surgery for aquatic vertebrates and invertebrates. Several professional organizations have been created to support aquatic veterinarians, including the American Association of Fish Veterinarians, the World Aquatic Veterinary Medical Association, and the International Association of Aquatic Animal Medicine.

There are 3 extant classes of fish, which include agnatha (jawless fishes), chondrichthyes, and osteichthyes, with 60,229 described species and subspecies, and 32,590 species scientifically validated.2–4 Clinically, veterinarians may serve or consult with a wide variety of clients, including hobbyists or pet owners, laboratory researchers, pet and pharmaceutical industries, fisheries managers, public aquaria and zoologic gardens, public health, ornamental aquaria aquaculture, mariculture, and food production aquaculture. When presented with morbidity or mortality of an individual animal or population, often, a thorough medical history can provide invaluable information for making an expedient diagnosis; for population health, a quick diagnosis is critical for managing disease outbreaks, which can cause catastrophically high morbidity/mortality.

The piscine gastrointestinal tract

Fish comprise the 3 largest extant classes of vertebrates, and, given the great diversity across these classes, comparative similarities and differences in the piscine gastrointestinal tract are highlighted. For veterinarians to better understand aquatic animal health, the most basic veterinary foundation begins with understanding fish anatomy and physiology.

The piscine gastrointestinal tract begins with apprehension of food through the teeth, mouth, and pharynx, and then progresses down the esophagus, stomach, intestines, and pyloric ceca, with waste elimination out the cloacae, vent, or anus. The liver, pancreas, and gallbladder are vital to digestion, as in other vertebrates. Although the swim bladder has a role in buoyancy control for many teleost species, embryologically, it is derived from the esophagus.1

Overview of anatomy

Some of the earliest comprehensive works on understanding the gross anatomic and histologic differences of the gastrointestinal tract in fish were published in the 1930s. This series of research compared the gastrointestinal tracts of bottom-dwelling, predaceous, and planktivorous fishes. The following sections highlight anatomic differences among these groups.

General anatomic differences of bottom-feeder sea robin (Prionotus carolinus)

Sea robins opportunistically ingest whole prey items that drift past them on the bottom of the ocean, bay, or estuary.5 Because of this feeding strategy, this species has a wide mouth, leading to a short esophagus, consisting of a muscular layer, connective tissue layer, and mucosal layers, analogous to other fish and vertebrates.5 The stomach of Prionotus carolinus is tubular and has a pyloric cecal arrangement, conserved in other gurnard fish from this group.6 Grossly, the pyloric verses cardiac areas of the stomach are indistinguishable, but on histopathology, the large cardiac region consists of secreting gastric or peptic cells.5 Similar species in this grouping such as the monkfish (Lophius americanus), can ingest greater than 50% of their body weight at a single feeding, whereas other nonrelated fishes, like tetras, Characidae, have smaller stomach capacities, which hold roughly 10% of body weight. In a previous Veterinary Clinics issue, a radiograph of a sea raven (Hemitripterus americanus), after ingesting a lead weight shows the indiscriminate feeding behavior of this group.1 The intestine of these animals is roughly 3 to 5 times body length, with 3 distinct areas, the foremost being pyloric cecae.5 These fish have a delineated liver, pancreas, and gallbladder.

The liver in most fish is found in the anterior abdomen and can be a large discrete organ or can appear more disseminated, intimately associated in adipose tissue along loops of intestine or pyloric ceca. The pancreas grossly may appear as a discrete organ, or be diffusely found in the liver, only evident as discrete pancreatic tissue histologically. Although sometimes referred to by the misnomer hepatopancreas, this organ is not the same hepatopancreas as invertebrates, because in fish there is a true differentiation of pancreatic versus hepatic tissue when viewed microscopically. Unlike other vertebrates, histologic sectioning of the liver in fish does not show distinct chords and lobes, but rather consists of sinusoids that are distinguished by a separation of blood and bile on opposite sides. Lipid accumulation, especially for captive reared fish from a variety of sources, can abundantly occur in the hepatocytes; this hepatic lipidosis may or may not be a pathologic finding, depending on the nutrition, husbandry, species, and medical history for these animals. As a histologic example, shark hepatocytes normally have tremendous lipid accumulation caused by the production of squalene for buoyancy control, whereas this same appearance for a Prionotus carolinus, sea bass (Centropristes striatus), or minnow (Campostoma anomalum) liver...