**2. Erythrocytes and hemoglobin in teleost in the norm and under toxic effects**

#### **2.1 Erythrocytes and hemoglobin in teleost in the norm**

The erythrocytes of teleost fishes have a similar morphology to the erythrocytes of other non-mammalian vertebrates. They are nucleated cells with an elliptical to oval shape. Their cytoplasm is eosinophilic, and the nucleus is centrally located, deeply basophilic, with an oval to elliptical shape. Erythrocyte sizes range from 102 to 800 fl [3, 4]. Moderate anisocytosis and polychromasia are normal in teleost species.

Their life span is 13–500 days. The number of erythrocytes in the peripheral blood depends on many factors—fish activity, water temperature, and dissolved oxygen concentration, as well as other environmental factors and shows significant seasonal variability. Furthermore, it depends on age, sex, nutritional and reproductive status, and may differ between populations of the same species. It typically ranges from 0.5–1.5 × 106 /mm3 in less active species to 3.0–4.2 × 106 /mm3 in more active species. Antarctic icefishes, adapted to cold and well-oxygenated water, do not have erythrocytes.

The main erythropoietic organ in teleost is the head kidney. Erythropoiesis is similar to that of other vertebrates and involves the same precursors. Characteristically, in teleost fishes, the barrier between the hematopoietic tissue and the circulatory system is weak, and therefore in the circulating blood, many immature red blood cells are found, often constituting more than 10% of all erythrocytes. Immature erythrocytes are round rather than oval in shape, their cytoplasm is blue-stained, and their nucleus is larger and heterochromatic.

As in other vertebrates, fish erythrocytes contain tetrameric hemoglobins, but with different oxygen affinities. It is lower in species living in well-oxygenated water than in those exposed to hypoxia. Several hemoglobin isoforms with different oxygen affinities are often present in the blood of fish, which is considered an adaptation to variable oxygen concentration in the water. Fish erythrocytes are sensitive to environmental pollution, and their morphological assessment can be used as a bioindicator of toxicity [3].

#### **2.2 Hematological disorders in teleost as a biomarker for water pollution**

Hematologic studies in fish have been performed for more than 70 years now. Recently, they have gained great importance because they are an effective and sensitive indicator for evaluating physiological and pathological changes caused by natural or anthropogenic pollution of water sources. Hematological indicators are therefore considered an important tool to identify the functional state of the body in response to various stressors [5]. However, for the correct interpretation of the obtained results, it is necessary to consider a set of variables, such as reproductive cycle, age, sex, feeding behavior, stress, nutritional status, and water quality, as well as the habitat of the species, since poikilothermic animals are under the impact of environmental changes [6].

Hematological disorders in boteleost include anemia, polycythemia, abnormal morphology, and cytoplasmic and nuclear inclusions [4], and according to Witeska [3], they also include changes in the size and color of erythrocytes.

#### *Erythrocytes and Hemoglobin of Fish: Potential Indicators of Ecological Biomonitoring DOI: http://dx.doi.org/10.5772/intechopen.107053*

Anemia is well described in teleost. It is defined as a decrease in hemoglobin concentration below the reference level of an established threshold for a population of healthy organisms [7]. In teleost it is extremely difficult to establish reference values for hematological parameters, due to their strong variation caused by their poikilothermism and high sensitivity to the action of various factors (age, sex, water quality, photoperiod, season, diet, etc.). For example, hematocrit values strongly depend on the level of biological activity of the fish. Thus, actively swimming species such as tuna and other pelagic species generally have much higher hematocrit values than bottom-dwelling fish such as flatfish. Therefore, hematological parameters are very relative, and there is no clear definition of normal and abnormal values [8]. Therefore, anemia in fish is usually recognized by a significant reduction in red blood parameters compared with the values obtained for a reference group of animals not exposed to the specific damaging factor. According to Clauss et al. [4], hematocrit values (PCV) below 20% are indicative of anemia. The hematological parameters that are affected when anemia occurs in fish are mainly: red blood cell count (RBC), hemoglobin concentration (Hb), hematocrit, mean corpuscular volume (MCV), mean corpuscular hemoglobin content (MCH), and mean corpuscular hemoglobin concentration (MCHC). Examination of these indicators (along with WBC values) is recommended for routine monitoring of fish health status in fish farms [9].

Anemia in teleost fishes occurs under both natural and aquaculture conditions and can be caused by various effects, including the toxic effect of a number of pollutants in water bodies (nitrates, pesticides, heavy metal ions, cyanobacterial toxins, etc.) [3].

Depending on the causes of anemia, it can be hemorrhagic, hemolytic, or hypoplastic. Depending on the manifestations, the different types of anemia in fish are divided into microcytic, normocytic, and macrocytic (depending on the size of the erythrocytes), as well as hypochromic or normochromic (depending on the hemoglobin concentration) [4].
