Polymorphism in cnidarians

What is Polymorphism in cnidarians?

• Polymorphism is the phenomena of the same type of organism occurring in several forms with distinct functions. Polymorphism is derived from the Greek words polys, which means numerous, and morphe, which means form.
• This polymorphism ensures a well-organized division of labour amongst multiple individuals. In coelenterates, diverse individuals merge to create a colony; hence, polymorphism is an essential characteristic of this phylum. The class hydrozoa finest exemplifies polymorphism.
• Polymorphism of coelenterates consists of two primary kinds, namely Polyps and meduase.
  • Polyps: Its morphology is tubular and the mouth is enclosed by tentacles at only one end, while the other end is typically linked to the substratum by a pedal disc.
  • Meduase: This variety of the Meduase is umbrella- or bowl-shaped, with tentacles along the margins and a mouth centred on the projection of the lowest concave surface.

The majority of polyps are sessile, whereas meduase types are motile. Nonetheless, there is a similarity between the two in their fundamental characteristics.

Importance of Polymorphism

Polymorphism is fundamentally a phenomena of labour division. Rather than assigning diverse functions to the components or organs of a single individual, distinct functions are allocated to distinct individuals. Consequently, polyps are responsible for eating, defence, and asexual reproduction, whereas medusa are concerned with sexual reproduction. Generally Polypoid forms consist of gastrozooids, dactylozooids, and gonozooids, while medusoid forms consist of pneumatophores, bracts, nectopore (or) nectocalyx (or) swimming bells, and gonophores.

A. Polypoid forms

1. Gastrozooids

• The healthful Gastrozooids refer to polyps.
• In the colony, only they consume food.
• Existence of mouth.
• Nematocyst batteries compose a single, long, and contractile tentacle.
• Tentilla are the lateral tentacle branches.

2. Dactylozooids

• They are known as palpons, tasters, or feelers.
• No mouth present.
• There is an unbranched, nematocyst-laden tentacle at the base of the organism.
• These zooids provide a defensive purpose.

3. Gonozooids

• These polyps are reproductive.
• Produces sexual medusa or gonophores.

B. Medusoid forms

1. Pneumatophore

• It is a gas-filled medusa with a bladder-like structure.
• Functions as a floater.

2. Nectophore (or) Nectocalyx or swimming zooid

• This muscular bell lacks manubrium and tentacles.
• Contributes to colony mobility by swimming.

3. Phyllozooid or Bract

• Also known as hydrophyllia
• Includes leaf-like structure.
• Defends other zooids throughout the colony.

4. Gonophores

• They are gonad-bearing.
• generating gametes

Patterns of polymorphism

Depending on the amount of zooids found in a colony, the degree of polymorphism varies substantially between various groups of hydrozoans. When a colony has two zooids, it is dimorphic; when it has three, it is trimorphic; and when it has numerous, it is polymorphic.

1. Dimorphic

• This is the simplest and most prevalent polymorphism pattern displayed by the majority of hydrozoan colonies. They have only two varieties of zooids, hence their dimorphic designation.
• Several hydrozoan colonies, such as Obelia and Tubularia, display dimorphic polymorphism. These are the two categories of zooids:
  • Gastrozooids: Gastrozooids are preoccupied with nutrition. Also known as hydranths.
  • Gonozooids: The focus of gonozooids is asexual reproduction. Also known as blastostyles.

2. Trimorphic

• In addition to gastrozooids and gonozooids, this polymorphic pattern also includes dactylozooids. Dactylozooids are nonfeeding, protective entities composed of nematocyct batteries.

3. Polymorphic

Polymorphism refers to the presence of zooid individuals with more than three distinct types. For instance, a colony of Hydractinia contains five distinct types of polyps, each with a specific function.

• Gastrozooids for consumption.
• Spiral dactylozooids for defence
• Long tentaculozoid structures for sensory function.
• Skeletozooids (Spiny projections of chitin) (Spiny projections of chitin).
• Gonozooids are utilised for reproduction.

Origin of Polymorphism

Polymorphism is one of the distinguishing features of cnidaria. There are numerous theories that explain the origin of polymorphism, as illustrated below.

Poly-organ hypothesis: Huxley (1859), Eschscholtz (1829), E.Metchnikoff (1874), and Muller proposed this theory (1871). A polymorphic colony is a single medusoid zooid, according to this theory. The numerous components of this medusoid zooid are considered modified organs. The various elements of the zooid, including as the manubrium, tentacles, and umbrella, proliferate independently of one another and have assumed distinct morphologies to accomplish distinct purposes.

Poly-Person theory: This hypothesis was initially proposed by Leuckart (1851), Vogt (1848), Gegenbaur (1854), Kolliker (1853), and Clans (1863), and subsequently substantially backed by E. Halckel (1888), Balfour (1885), and sedgewick (1888). This hypothesis argued that a colony is not a single individual, but rather a collection of individuals whose structure has altered owing to division of labour. These are all modifications of the primordial zooid Polyp.

Medusa theory : Haeckel (1888) suggested this theory as a compromise between the aforementioned hypotheses. The siphonophores that originated from the gastrula, according to this view, were medusoid individuals from which zooids or humans emerged by budding from the subumbrella.

Modifications of Polyp forms

• Gastrozooids are referred to as feeding polyps. They possess a mouth and a lengthy tentacle.
• Dactylozooid is often referred to as a protective polyp. They lack a mouth but have a lengthy tentacle at their base.
• Gonozooid is a synonym for reproductive polyp. It creates a sexual version of medusa.

Modifications of Medusa forms

• Nectophore is also known as a zooid that swims. They possess a strong bell devoid of tentacles.
• Pneumatophore is also known as a medusa resembling a float bladder. They are packed with gaseous secretions.
• Phyllozooids are also known as bracts. They are covered in nematocysts and aid in defence.
• Gonophore is also known as the gonad zooid. They could be masculine or female.

Defense Structure In Cnidaria

All coelenterates have specific protective features known as stinging cells or nematocysts in their body walls. Due to the existence of these cnidocytes, Coelenterata is also referred to as Cnidaria. Each cnidocyte contains a cnida, a fluid-filled membrane capsule. Cnidocytes contribute to not just defence but also movement, adhesion, and prey capture.

Coelenterates are characterised by the existence of these protective structures, which is one of their most prominent characteristics. They are cell organelles found in specialised cells known as cnidocytes or cnidoblasts and are not cells. Cnidoblast is derived from the Greek words “knide” (nettle) and “blast” (germ).

Cnidoblasts develop only from modified epidermal interstitial cells and are absent from the gastrodermis. When completely formed, cnidoblasts migrate to the tentacles via amoeboid migration through the mesoglea.

Structure of cnidoblast

Cnidoblasts are oval or spherical cells with a basal nucleus. Inside the cnidoblast is an oval or pyriform bladder known as the stinging capsule. This venomous capsule is also known as a nematocyst. The nematocyst consists of a small chitin bulb. This bulb is filled with deadly fluid or hypnotoxin, which is a protein and phenol mixture. This filament extends from the end of the bulb as a thin, long, hollow tube that is wound around the toxic sac. The name for this filament is thread tube. The thread tube’s base is enlarged to produce a shaft. Inside the shaft are three rows of barbs and three rows of barbules. The shaft is externally covered by a structure resembling a lid called the operculum.

The outside end of the cnidoblast extends freely beyond the epidermal surface as a cnidocil or trigger, a small, hair-like structure. Cnidocil’s centre core is encircled by groups of supporting rods. The centre core has fibres arranged in a 9+2 configuration, comparable to the cilium’s structure. Contractile muscle fibrils are present in the cytoplasm of cnidoblasts.

According to electron microscopic investigations, the cytoplasm of the cnidoblast contains endoplasmic reticulum, free ribosomes, Golgi bodies, mitochondria, and multi-vesicular structures.

Distribution of nematocyst

Nematocysts are dispersed singly or in small groups (very uncommon) throughout the epidermal region of cnidarian bodies. On the base disc, these specific defensive features are lacking. They are numerous in the mouth region and on the tentacles, where they form nematocyst batteries.

A nematocyst battery consists of two big nematocysts in the centre surrounded by 10-12 smaller nematocysts. Each of these large and small nematocysts is contained within a single giant epithelial-muscle cell. Cnidoblasts are not generated in the tentacles, but rather in the epidermis, before migrating to the tentacles. Following the movement of a significant number of cnidoblasts, some of them develop nematocyst batteries in the gastrovascular cavity.

Mechanism of Defense

The release or explosion of nematocysts occurs when cnidocil is activated by food, prey, or an adversary. The process of explosion is initiated by both the presence of food and physical contact, and not by either one alone. Hence, both mechanical stimulations, such as food contact, and chemical stimulations, such as an approaching foe, are involved in the action mechanism of nematocysts.

Despite the specific discharge mechanism and enzymes involved are unknown, it is abundantly clear that the response is entirely local and does not involve the nervous system. The nematocyst wall is impermeable to water except during the discharge process. Upon stimulation, the wall of the capsule abruptly becomes more permeable, resulting in a quick intake of water and an increase in osmotic pressure within the capsule. Now, as a result, the operculum is compelled to open, the coiled thread tube turns inside out, and the entire nematocyst bursts open. As the thread tube everts, the barbs and barbules inside the shaft unfold and extend to the exterior.

The discharged thread tube cannot be retracted; the nematocyst cannot be utilised again after it has exploded. Following the explosion, the cnidoblasts travel to the gastrovascular cavity, where they are digested. During 48 hours, the exploded nematocysts are replenished.

Types of nematocysts

Over 30 distinct forms of nematocysts are identified in the phylum Coelenterata. For particular species, their kind is constant. About Hydra, there are four fundamental types of nematocysts that fulfil various roles. This section has a description of each of them.

• Penetrant nematocyst: Penetrant nematocysts are also referred to as stenotele. In comparison to other types, these nematocysts are extremely massive and intricate. These nematocysts are pear-shaped and nearly fill the entire cnidoblast in which they reside. Its thread is also long and hollow, transversely coiled, and adorned with three rows of big barbs and three rows of little spines. When the thread tube is released, it bursts out with explosive force to puncture the victim’s body and inject the toxin, which paralyses or kills the victim. The hydra then captures the prey with its tentacles and pulls it into its mouth.
• Volvent nematocyst: Volvent nematocysts are also referred to as desmoneme. These nematocysts are tiny and pear-shaped. They contain a single loop of short, thick, and spineless Eleatic thread tube. When released, it coils firmly around the prey’s tiny projections, such as hair or bristles, preventing the animal from moving.
• Stereoline glutinant nematocyst: They are often referred to as little glutinous atrichous isorhizas. The form of these nematocysts is oval or elongated. They are without a shaft. They expel an unarmed straight thread tube with an open tip. This type of nematocyst is advantageous for adhesion and anchoring.
• Streptoline glutinant nematocyst: These nematocysts are also called as big glutinant isorhizas or holotrichous isorhizas. These nematocysts are oval or cylindrical in shape. Its thread tube is long with a thin shaft that wraps into three or four turns. It has a spiralling array of tiny spines. They are primarily effective for attaching small animals and impeding their movement.

Significance of Polymorphism

• Polymorphism and life-history are inextricably linked. The life cycle of monomorphic forms is straightforward (e.g., Hydra). With the introduction of polymorphism, reproductive capacities are divided. The polyp can only reproduce asexually, while sexual reproduction is restricted to the gonophores. Hence, the alternation between generation and metagenesis arises.
• Polymorphism is also concerned with labour distribution. Hence, polyps are primarily connected with feeding, testing, defence, and asexual reproduction, whereas medusas are concerned with sexual reproduction.

FAQ

References

• http://zoology.uok.edu.in/Files/cae2d08f-4f62-428e-b6ea-cf46cdccbf42/Custom/ZOO-102CR-1.4%20POLYMORPHISM.pdf
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