The Fish Heart

Among vertebrate species, fishes are the most abundant by far. Thus far, 25 000 species in 482 families and 57 orders have been formally discribed (Helfman et al., 1997).
While there are obvious fundamental differences between fish and humans, there is a high level of genetic conservation, and, although morphological differences might suggest distinct developmental processes, it is now clear that evolution has utilized the same basic building blocks to create even the most diverse structures and animals (6). This is evident through the conservation in the cardiac outflow tract and the arterial pole, where the beating myocardium forms a junction with the smooth muscle of the arterial system. Fish have gills and, with the exception of some airbreathing
fishes, do not acquire the secondary pulmonary circulation, which in land-based, obligate air-breathing animals sends blood to the lungs to exchange carbon dioxide and oxygen. Thus, the hearts of all fishes consist of a single atrium and a single ventricle. Even in the true lungfishes (Dipnoi) a single ventricle pumps blood from the heart, and it is primarily structural differences in the venous (inflow) and arterial (outflow) poles that allow artial separation of lung-oxygenated and system-deoxygenated blood. Such adaptations serve as a reminder that the evolution was likely a specialization in this region and that a partial or full separation of oxygenated and deoxygenated blood at the inflow could be a strong driving force for coincident or subsequent change at the outflow.

The Structure of the Fish Heart

Figure 1.  hearts of all fishes consist of a single atrium and a single ventricle. The heart of fish is the simplest  of vertebrate hearts. It contain a single atrial chamber connected directly to ventricle (thin walled atrium, and a more muscular ventricle) (14). The atrium pumps the blood into the ventricle, which in turn pumps the blood into the  an elastic compartment which does not pump, but has the ability to stretch and squeeze. The chambers maintain a unidirectional blood flow through the heart. The heart is composed of typical vertebrate cardiac muscle, although there may be minor differences in the distribution of spontaneously active cells, the rate and nature of spread of excitatory waves, and the characteristics of resting and action potentials between different fish and other vertebrates. Fish hearts lack sympathetic innervation.

In addition to the fish ventricle and atrium, attached at the cardiac inflow, and generally of similar volume to the atrium, is the sinus venosus, a structurethat receives venous blood from the systemic circulation through paired hepatic veins, anterior jugular veins and Cuvierian ducts. Depending on the species, attached at the cardiac outflow are described the conus arteriosus, the truncus arteriosus and, or the bulbus arteriosus. At the distal limit of these outflow structures, but lying outside the pericardial cavity, is the ventral aorta.
The hearts of all fishes (which is an enormously diverse group of animals) can be divided into four main groups of hearts based on  increasing complexity of ventricular structure.

Four Types of Fish Hearts

Figure 2. The four generalized heart types of fishes. First described by Tota et al. in 1983 and Tota in 1989 this generalization is based on ventricular morphology. The type I heart has a ventricular myocardium with no compact layer – and thus does not have coronary vasculature. The oxygen supply for this type of of myocardium comes entirely from lumenal blood. Hearts of types II, III and IV have trabeculae, a compacta and coronary circulation within the myocardium. Coronary capillaries are found only in the compacta in type II and have limited coronary circulation in the atrium. Hearts that have coronary vessels in the trabeculae and in which >30% of the ventricular tissue is compacta fall into the category type IV and have more extensive capillarization of the atrium. Image Credit: Grimes and Kirby

Generalized Heart Structures Found Within Various Groups of Fishes

Fig. 3 Generalized illustrations of heart structures found within the various groups of extant fishes. The terminology used in each case: dorsal is to the top and cranial to the right. Not drawn to scale. Myocardium-derived chambers have been shaded in red. Green shaded regions are non-myocardialand contain primarily smooth muscle and, or fibrous proteins. Endocardium derived valve structures are coloured yellow. av, atrioventricular; sa, sinoatrial; sv, sinus venosus. Note that the teleost bulbus arteriosus is never invested in myocardium and is probably akin to the arterial trunk of land-based vertebrates (Grimes et al., 2006). Moreover, as there is generally no valve at its distal end, it cannot properly be described as ‘an enclosed space or cavity’ Therefore, the bulbus arteriosus should not be considered a chamber of the heart.Image Credit: Grimes and Kirby

Hypothesized Phylogenetic Relationship Between All Extant Fishes

Figure 4. The Phylogentic relationship among fishses is based on morphology, palaeontology and molecular evidence. At least three branches of the phylogram are under continued dispute and are represented in grey. Hagfish is a eel-shaped slime-producing jawless, marine animal, which has a skull but not a vertebral column.  Living hagfish remain similar to hagfish 300 million years ago. Lmprey are an order of jawless, scaleless, fish-like vertebrates, characterized by toothed funnel-like mouth. Elasmobranchii is a subclass of Chondrichthyes or cartilaginous fish, that includes sharks and rays and skates. Teleost is a diverse group of jawed fishes. Lungfish (Dipnoi) are freshwater fish retaining characteristics primitive within the Osteichthyes (bony fish), including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed internal skeleton. Image Credit: Grimes and Kirby

There is a great deal of controversy surrounding the question of "what exactly defines a heart?" Is it the existence of dorsal vessel in Drosophila, or the circulatory pumps in organisms such as amphioxus? Perhaps it is the 'accessory' or secondary pumps in some vertebrates such as hagfish. All these issues tend to complicate the arguments regarding homology vs. homoplasty of the pumping organs. To learn more on this subject, see the Xavie-Neto et. al., 2007. I short, the authors of the aforementioned review define the heart as chambered pumping organ that is enclosed within a pericardial sac and possesses three tissue layers: endocardium, myocardium and epicardium, adapting an 'anatomical, homologous' concept of hearts, as opposed to the alternative 'analogical, homoplasic' concept. Thus, the above definition excludes the pumps of invertebrates, including Drosophila (which are composed of myoepithelium) and the accessory pumps powered by contractions of skeletal muscle.

At its simplest level, the vertebrate heart can be conceptualized as having two compartments: an inflow that receives blood from upstream and an outflow that delivers blood downstream. The atrioventricular valve(s) can be thought of as boundaries between these two compartments. Therefore, the sinus venosus and atria make up the inflow, while the ventricle(s) and everything extending from the ventricle(s) make up the outflow. Such a broad definition of the outflow tract has been proposed, based on both morphological criteria and patterns of gene expression (17). This ‘new point of view’ argues that the increasing complexity of hearts, from agnathans through to mammals, can be explained by the simple modular repetition of inflow and outflow units. However, it could equally be argued that both morphological considerations and gene expression patterns allow further, more detailed, structural analysis of the heart, and, while all vertebrates have inflow and outflow ‘modules’, some of the complexities of land-based, obligate air-breathing vertebrate hearts are difficult to explain by simple modular repetition.
For example, the outflow tract of the most ancient extant jawed vertebrates, the elasmobranchs  (sharks, skates and rays), has an elongated, myocardial conus arteriosus that is invested with several rows of valves, with the number of rows depending on the species. This structure is proposed to have been modified throughout evolution, with its equivalent being considerably shortened in the ancient teleosts, incorporated into the ventricle in the modern teleosts, septated in amphibians and reptiles, and compacted, remodelled and septated in birds and mammals.