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Aspects of asexual reproduction in holothurians are discussed. Holothurians are significant as fishery and aquaculture items and have high commercial value.
The last review on holothurian asexual reproduction was published 18 years ago and included only 8 species. An analysis of the available literature shows that asexual reproduction has now been confirmed in 16 holothurian species.
Five additional species are also most likely capable of fission. The recent discovery of new fissiparous holothurian species indicates that this reproduction mode is more widespread in Holothuroidea than previously believed. New data about the history of the discovery of asexual reproduction in holothurians, features of fission, and regeneration of anterior and posterior fragments are described here.
Asexual reproduction is obviously controlled by the integrated systems of the organism, primarily the nervous system. Examples of enzymes ending in asexual reproduction molecular mechanisms appear to determine the location where fission occurs along the anterior-posterior axis of the body.
Alteration of the connective tissue strength of the body wall may play an important role during fission of holothurians. The basic mechanism of fission is the interaction of matrix metalloproteinases, their inhibitors, and enzymes forming cross-link complexes between fibrils of collagen. The population dynamics of fissiparous holothurians are discussed. Asexual reproduction is the most ancient mode of reproduction and is observed in representatives of all phyla of modern Metazoa [ 1 — 3 ].
Because asexual reproduction is closely related to the structure of an animal, its types are as diverse as the animals themselves [ 4 ].
The variety of manifestations of this phenomenon is even greater because asexual reproduction in different species has different biological functions, such as population growth, Examples of enzymes ending in asexual reproduction of body size, colonization of new sites, and survival under adverse conditions.
The evolution of multicellular organisms has apparently passed through repeated losses and restorations of various forms of asexual reproduction [ 3 ]. Among modern groups of asexually reproducing invertebrates, holothurians deserve special consideration because of their commercial value. Holothurians, or sea cucumbers, are a class in the phylum Echinodermata. Holothurians have elongated often worm-shaped bodies that are covered with various outgrowths.
Most holothurians are benthic organisms [ 56 ], although there are swimming species and most likely completely pelagic ones [ 7 ]. Holothurians are significant for commercial fishery and aquaculture. Approximately 66 holothurian species are commonly exploited throughout the world [ 8 — 11 ].
People in these regions consider holothurians not only a traditional commodity but also a commercial resource [ 912 ].
Moreover, holothurians are a major source of biologically active substances in biotechnology and medicine [ 813 — 16 ]. They have a wide array of vitamins, minerals, saponins, chondroitin sulfates, polysaccharides, sterols, phenolics, lectins, peptides, glycoprotein, glycosphingolipids, and essential fatty acids [ 13 ].
Thus, various aspects of biochemistry, physiology, and developmental biology of these animals are being actively studied. Holothurians are also notable because they possess diverse regeneration abilities [ 17 — 19 ].
Some species can expel their internal organs, mainly the digestive system, in response to various stimuli and then can quickly restore them [ 20 — 24 ].
Furthermore, many holothurians can regenerate after a transverse cut [ 25 — 28 ]. Some holothurian species are capable of asexual reproduction.
Most fissiparous holothurians live in tropical and subtropical zones. The only exceptions are Ocnus planci and O. In the southern hemisphere, Staurothyone inconspicua also occurs beyond the subtropical zone. This species, with probably dividing juveniles, was collected in Opossum Bay in southern Tasmania [ 29 ].
Because of the high commercial value of holothurians, researchers attempt to use their regenerative property and fission ability to develop cultivation methods and increase natural populations [ 30 — 36 ].
The last review on holothurian asexual reproduction was published 18 years ago [ 37 ]. This review included only 8 fissiparous species. Since then, asexual reproduction has been observed in additional species see, e. The discovery of new species indicates that this type of reproduction in the class Holothuroidea is more widespread than previously believed.
Moreover, new data on the regeneration, population dynamics, and other biological aspects of fissiparous species have been obtained. This information requires systematization. The goal of this review is to analyze the available data on asexual reproduction in holothurians.
All currently known fissiparous species of holothurians belong to two orders: Asexual reproduction was first described in dendrochirotids. Dalyellcited by Monticelli [ 39 ] observed dividing Ocnus as Cucumaria lactea and Ocnus as Cucumaria planci. Moreover, according to Monticelli [ 39 ], Semper described a case of fission in Havelockia as Cucumaria versicolor. However, both Dalyell and Semper stated only that fission occurred and did not describe the process.
Chadwikcited by Crozier [ 40 ] provided the first brief description of fission in O. Subsequently, Monticelli [ 39 ] published a comprehensive study of asexual reproduction in this species. Benham [ 41 ] apparently was the first to provide evidence of asexual reproduction among holothurians from the order Aspidochirotida. He observed four specimens of Holothuria difficilis as Actinopyga parvula which had lighter colored posterior regions and less developed tube feet in a collection of fixed animals from the Kermadec Islands.
These animals had regenerated their posterior regions as Benham [ 41 ] had believed. Crozier [ 40 ] was the first to directly observe fission in aspidochirotids. He described the fission process in Holothuria surinamensis and Holothuria parvula as Captiva. Deichmann [ 42 ] subsequently confirmed asexual reproduction in H.
Another species of fissiparous holothurians was noted only approximately 40 years later. Bonham and Held [ 43 ] provided data and a photograph of dividing Holothuria atra off the Marshall Islands. Harriott [ 44 ] then observed individuals of Stichopus horrensStichopus chloronotusand Holothuria edulis in the field that had undergone fission and were in the process of fission.
The specimens of S. In an article describing the neotype for Ocnus brunneusMcKenzie [ 46 ] indicated that this dendrochirotid species was capable of fission. However, the status of this species was not subsequently confirmed,
Examples of enzymes ending in asexual reproduction the studied individuals most likely belonged to O.
They reported that a new holothurian species, Squamocnus aureoruberfrom the coastal waters off Australia was capable of asexual reproduction. Additional fissiparous holothurians were described in the 21st century. The initial four species belong to the order Aspidochirotida. Sonnenholzner [ 50 ] reported asexual reproduction in Holothuria theeliwhich occurs along the coast of Ecuador. Data on the ability of larvae of the sea cucumber Parastichopus californicus to bud were also published that year by Eaves and Palmer [ 51 ].
This is the only species of holothurian for which asexual reproduction by larvae has been definitely established.
There may be other similar species [ 52 ]. Japanese researchers then observed an individual of Stichopus horrens in the process of fission off Okinawa Island [ 53 ].
However, as Byrne et al.
The ability to reproduce asexually was then established in Holothuria hilla [ 3854 ]. Furthermore, a specimen of Holothuria portovallartensis with a growing anterior end was described by Uthicke et al.
Another three species of fissiparous holothurians belong to the order Dendrochirotida. These species are the Australian Cucuvitrum rowei [ 29 ] as well as Cladolabes schmeltzii and Colochirus robustus [ 2526 ], which occur along the Vietnam coast. Additionally, juvenile Staurothyone inconspicua may possess the ability to perform transverse fission [ 29 ]. Thus, asexual reproduction has been currently confirmed in 16 holothurian species see Examples of enzymes ending in asexual reproduction 1.
Five more species, Havelockia versicolorStaurothyone inconspicuaStichopus monotuberculatusS. Moreover, some holothurian species most likely reproduce asexually in the larval stage [ 52 ]. Currently, there are data for only one holothurian species, P. In this species, The process of asexual reproduction begins in the doliolaria stage. The larvae constrict around the penultimate ciliary band. Subsequently, a deepening of the constriction at the posterior end causes a bud to form.
This bud a daughter organism retains
Examples of enzymes ending in asexual reproduction ciliary band and remains attached to the mother even after settlement. The bud is separated in the pentactula stage, and a normal Auricularia develops. In the Florida current of the Gulf Stream System, Balser [ 52 ] observed auriculariae of an unknown holothurian species that possessed lateral lobes. Under artificial cultivation the lateral lobes of these auriculariae separated from the individual and formed blastula-like larvae.
The larvae began gastrulation, but further development then stopped, and the animals died [ 52 ]. Asexual reproduction in adult holothurians occurs as transverse fission architomy and fragmentation. The most detailed description of the fission process is that by Monticelli [ 39 ] who differentiated three fission mechanisms in O. An analysis of the available fission data on holothurians shows that various holothurian species use different combinations of the methods described by Monticelli see Table 1.
Most holothurians, in which the fission process was observed, began by forming a constriction. Division into fragments proceeds either by the development and deepening of the constriction [ 49 ] or as a result of stretching and twisting at the fission site.
Usually the posterior sections of the body are attached to a substrate by the tube feet whereas the anterior regions move forward or twist Figure 1 [ 2656 ]. The closure of the wound resulting from fission has not been described but probably results from contraction of circular muscles in the body wall. The duration of the process varies from a few minutes, as in S.
The duration of fission most likely depends on the intensity of the transformation of the extracellular matrix of the body wall. Asexual reproduction is the formation of offspring without the fusion of gametes. Pollen is produced in the anthers, which are attached at the distal end of filaments.