Reproductive Strategies and Development in Larvacea
Larvacea
Most are bisexual with gonads and a simple opening through a duct. External fertilization occurs, and there may be hatching within the atrial siphon. Indirect development occurs via a tadpole larva that undergoes metamorphosis.
1.1. Class Ascidiacea
a) Reproductive
Asexual Reproduction
All ascidians have great powers of regeneration, but only the colonial ascidians reproduce asexually. Asexual reproduction occurs by budding, but the mechanism is complex and highly variable, perhaps more than any other group of metazoans.
The yolk is called a tunicate blastozooide and originates from the oozooide (zooid developed from a fertilized egg). Depending on the species, there is considerable variation in the place where the buds that develop into tissues and germ are included. The most primitive type of budding appears in Perophora and similar species in which the buds arise from the stolon. In other families, the buds appear on the abdomen, post-abdomen, and even in early larval stages, as in the species Diplosoma (“two bodies”).
Sexual Reproduction
Female Reproductive System
– Ovary: consists of a vesicle whose wall is a germinal epithelium located in the visceral cavity, near the intestine.
An oviduct leaves the ovary and, parallel to the intestine, reaches the atrial cavity, opening near the anus.
Male Reproductive System
– Testicles are made of various tubes branching on the surface of the ovary and intestine, leading to an efferent duct, which is parallel to the oviduct.
With few exceptions, tunicates are hermaphrodites, but only a few species are self-fertilizing, and outcrossing is the property of the class. Normally, there is only one testicle and one ovary associated with each other.
The solitary ascidians generally produce small eggs with little yolk. The eggs go through the atrial siphon, and fertilization occurs in the seawater. The eggs of oviparous species are normally surrounded by special membranes that serve as floats. The eggs of colonial species are richer in yolk and are incubated in the oviduct or in the atrium, which sometimes presents special incubator bags. Hatching usually takes place during the larval stage, and the larvae leave the parent, but sometimes almost all development takes place within the atrial cavity. The development of brooding species with yolk-rich eggs is usually faster and condensed, forming incubators.
The segmentation is complete and results in a celoblastula. Gastrulation occurs by invagination and epiboly, and a wide archenteron occupies and obliterates all the blastocoel. The blastopore marks the posterior end of the embryo and is closed while the embryo elongates along the anteroposterior axis. Over the archenteron, a mediodorsal line forms a supporting rod (the notochord). The archenteron occurs laterally with mesodermal cells that form a cord of cells along each side. In this regard, the development does not follow the same pattern as amphioxus and other deuterostomes, as there is no formation of pockets of archenteron. There never appears to be a cavity or segmentation of the coelom. The ectoderm along the mediodorsal line differentiates to form a neural plate that sinks to the center and rolls up to form the neural tube.
a) Production of Embryos
In Ascidiacea, fertilization usually occurs externally (e.g., in solitary ascidians) or eggs hatch in the atrium if fertilization takes place within the animal (colonies). Although several species often develop in the atrial cavity.
The embryos can be released quickly into the marine environment or stored within the atrial cavity for a certain period of time. At the end of their planktonic life (which usually lasts about 4 days and during which they do not feed, but just look for a place to attach), the larva attaches to a substrate, initiating a process of metamorphosis in which the notochord and neural tube are reabsorbed, the pharynx becomes a large chamber or branchial pouch designed to collect food, and is covered on the outside with a tunic, consisting mainly of tunicine.
Colonial species can reproduce asexually through a process of budding. This form of reproduction allows for rapid proliferation when environmental conditions are favorable.
Class Thaliacea
They reproduce by eggs, shoots, or buds.
It is committed to producing the blastozooides.
The generation producing buds usually results in a free-living individual and fully developed called amnion.
The first blastozooides are produced by a late embryonic state and then fall suddenly into regression (Pyrosoma).
The early stages of embryonic development in the salp are completed with the care of the young, provided the eggs produced in isolation remain in the mother’s body: in the perisomidos in the gill chamber and in other salps in the ovary, specifically in the follicle. The follicle forms a lump within the cloacal chamber and develops a supportive epithelial ovarian structure through which the embryo is nourished, with a placenta covering the embryo. After birth, the mother may remain with the young animal and use this as an additional food source. The life cycles of all involving germination in Thaliacea are asexual. As a result, sexual reproductive stages occur as buds (blastozooides) of individuals formed from eggs (oozooides). It is often considered that the present Thaliacea exhibit metagenesis, but there seems to be no justification for considering these animals as metagenes, unlike the status ascribed to hydroid coelenterates.
The egg of Pyrosoma is large, yolk-rich, and discoidal, undergoing fragmentation. In the 8-cell stage, 4 cells located peripherally (merocitos) develop into a nutrient syncytial tissue called the periblasto. This connects the embryonic disc to the periphery and below the yolk. The training is an auxiliary periblasto widespread among meroblastic eggs of chordates and serves to supply the embryo. The embryonic disk soon presents several layers. The superficial cell layer is the epidermal ectoderm. In the deeper layers are different structures of the brain, pharynx, and the pericardium. Over the outline of the pharynx, ectoderm forms two invaginations, outlining the peribranchial chambers. These are joined at their base after forming the cloaca.
Reproduction in Cephalochordata involves separate sexes, with external fertilization. The larvae, which are swimmers, are born adult asymmetric and acquire bilateral symmetry.
a) Reproductive
Sexual reproduction involves separate sexes without dimorphism.
Gonads are numerous and arranged segmentally in the atrium; eggs and sperm are released into the atrial cavity and out through the atriopore. Fertilization is external.
The egg is 0.1 mm in diameter, with little yolk, and segmentation is total (holoblastic). The larvae mature rapidly and gradually assume the adult form.
The Cephalochordata are dioecious. Branchiostoma presents 26 pairs of gonads that often correspond with miometros 10 to 35, but asymmetrical gonads only appear on the right side of the body.
Order: Amphioxiformes
Reproductive System: separate sexes with external fertilization. Eggs and sperm exit through the atriopore.
– Fam Branchiostomidae: have a double row of gonads. Metapleural asymmetric folds. Ex: Branchiostoma.
– Fam Epigonichtidae: the gonads are only on the right side. Metapleural asymmetric folds may surround the anal area. Ex: Epigonichthys.