Fish Embryo Development Stages

To better understand the different stages of fish embryo development, we should provide some background on basic embryology concepts. These include understanding different egg types and the phases which make up early embryonic development.

Different eggs are often differentiated by how much yolk is contained within and how it is distributed. The yolk is the part of the egg that provides nutrients to allow the embryo to survive. Some eggs do not contain yolk as they get their nutrition from other food sources such as a parasite feeding off a host body. The yolk is scientifically known as the vitellus, which might be helpful as we often think of yolks being those which belong to chicken or duck eggs. As we said above, the embryo forms from the union of an egg (or ovum) and the spermatozoa.

In fish, the transparency of their eggs permits researchers to directly observe cellular processes. This has made fish, particularly zebrafish, a model organism in developmental biology studies. Through these studies, scientists have gained insights into the mechanisms that govern cell division and differentiation, contributing to our understanding of human biology as well.

Types of eggs based on yolk distribution:

• Isolecithal eggs: the yolk is uniformly distributed throughout the interior of the egg. It is typical in cnidarians (such as jellyfish or coral), echinoderms (such as sea urchins), nemerteans (ribbon worms), and mammals (including humans).
• Telolecithal eggs: the yolk is contained within one pole of the egg in an uneven distribution. It is separate from the embryo. Most animals will develop their embryo from these types of egg, e.g., mollusks, amphibians, reptiles, and birds. The fish embryo will also develop from this type of egg.
• Centrolecithal eggs: as the name suggests, the yolk is held within the center of the egg. Cytoplasm surrounds the yolk which, in turn, surrounds the nucleus that will give rise to the embryo. It occurs in arthropods.

Types of eggs based on the amount of yolk:

• Alecithal eggs: these are small and contain little yolk.
• Mesolecithal eggs: of a medium size and with a moderate amount of yolk.
• Macrolecithal eggs: large eggs with a correspondingly large amount of yolk.

Typical phases of embryonic development:

• Segmentation: during this phase, a series of cell divisions take place that increase the size of the number of cells necessary for the second phase. This ends in a state called the blastula, which is a hollow sphere of cells carved out by segmentation.
• Gastrulation: a reorganization of the cells of the blastula takes place, giving rise to blastoderms (primitive germ layers). The primary layers are the ectoderm, endoderm, and, in some animals, the mesoderm.
• Differentiation and organogenesis: the tissues and organs will be formed from the germ layers, fixing the structure of the new individual being formed. The complexity of these processes underscores the intricacy of life itself, as each phase is crucial in ensuring the proper development of a viable organism.

Temperature is closely related to the incubation time of fish eggs and fish embryo development^[1]. The same occurs in other species. There is usually an optimal temperature range for incubation which can vary within a range of about 8 ºC (46.4 ºF).

In fish reproduction, eggs which hatch within this optimum incubation range will have a greater chance of developing well and reaching the hatching stage. Likewise, eggs incubated for a long period of time at extreme temperatures (those outside the optimum range for the species) will have a lower probability of a fish embryo forming. If they do, individuals born will have a greater chance of suffering from serious anomalies.

Moreover, environmental factors such as pH levels and oxygen concentration in the water can further influence the development process. Variations in these factors can lead to developmental delays or abnormalities, highlighting the delicate balance required for successful embryogenesis. Therefore, monitoring and maintaining suitable environmental conditions are crucial for the survival and health of developing fish embryos.

Now that you know some of the basic concepts of embryology, we can look at the specifics of fish embryo development stages. Fish are within the telolecithal egg category. This means their yolks are displaced to a different pole of the egg than that of the fish embryo. Here is how the phases break up:

Zygotic phase

The newly fertilized egg remains in the zygote stage until the first division. The approximate time in which this split takes place depends on the species and the corresponding optimal incubation temperature. In the zebrafish (Danio rerio), the first segmentation occurs about 40 minutes after fertilization. Although it seems that in this period there are no obvious changes (the zebrafish is one of the fish most often used in research), decisive processes are taking place inside the egg for further development. These processes include the replication of DNA and the initial establishment of embryonic axes, which are critical for subsequent development.

Segmentation phase

The egg enters the segmentation phase when the first division of the zygote occurs. In fish, the segmentation is meroblastic. This is where the division does not occur throughout the entire egg as it is prevented by the yolk. Instead, it is limited to the area where the fish embryo is located. The first divisions are vertical and horizontal to the embryo. These divisions occur very quickly and synchronously. They give rise to a mound of cells which settle on the yolk, constituting the blastodisc.

Gastrulation phase

During the gastrulation phase, a rearrangement of the cells of the embryonic disk takes place via morphogenetic movements. This is the information contained in the nuclei of the different cells which have already been formed. They are transcribed in a way that forces the cells to obtain a new configuration space. In the case of fish, this reorganization is termed involution^[2]. Similarly, the stage is characterized by a decrease in the rate of cell division with little or no cell growth.

During the involution, some cells of the blastodisc migrate towards the yolk to form a layer on top. This layer is known as the endoderm. The layer of cells which remains on the mound will form the ectoderm. At the end of the process, the gastrula will become defined and form the epiblast which results in the two primary germ layers^[3]. These germ layers are collectively known as blastoderms and consist of the ectoderm and endoderm. This stage is pivotal as it sets the foundation for organ and tissue formation.

Differentiation and organogenesis

During the differentiation phase in fish, the third embryonic layer appears. This is located between the endoderm and ectoderm and is known as the mesoderm.

The endoderm invaginates (folds into a pouch) forming a cavity known as the archenteron. The entrance to this cavity will be called the blastopore and it results in the anus of the fish. From this point, we can distinguish the cephalic vesicle (the part which forms the brain) and, on both sides, the optic vesicles (forming the eyes). After the cephalic vesicle, the neural tube will form and somites will be created on either side. These structures will end up forming the bones of the fish as well as muscles and other organs. This intricate orchestration of cellular movements and differentiation is essential for establishing the foundational body plan of the fish.

So, throughout the fish embryo development stage, each germ layer will begin the production of several organs or tissues:

Ectoderm:

• Epidermis and nervous system
• Beginning and end of the digestive tract

Mesoderm:

• Dermis
• Musculature, excretory organs, and reproducers
• Coelom, peritoneum, and circulatory system

Endoderm:

• Organs involved in digestion: internal epithelium of the digestive tract and related glands
• Organs in charge of gas exchange

Each of these layers plays a crucial role in the comprehensive development of the fish, illustrating the complex interplay of genetic and environmental factors in embryogenesis.

Another important factor of the fish embryo development stage is how they are fed by the mother. Humans are viviparous which means they develop inside the mother and are fed directly (through the umbilical cord). In the majority of fish, however, they are oviparous. This means they lay eggs and the fish embryo develops for the most part outside of the mother. This means they are more vulnerable to predators, but it also means the mother doesn't need to stay near the spawn.

Viviparous fish do exist, but are in the overwhelming minority. They include some species of shark such as lemon shark as well as fish species like the splitfin. These viviparous fish have evolved various adaptations to provide nourishment to their young, such as yolk-sac placentae or uterine milk. These adaptations ensure that the developing embryos receive adequate nutrition and protection until they are ready to hatch. Understanding these reproductive strategies provides valuable insights into the diverse life strategies employed by fish to ensure the survival of their species in varied environments.

If you want to read similar articles to Fish Embryo Development Stages, we recommend you visit our Facts about the animal kingdom category.