Polymorphism Cnidaria
Polymorphism, derived from Greek poly (many) and morphe (form), refers to the occurrence of different types of
individuals or zooids within a single species.
Hydrozoan Polymorphism:
Coelenterates, including Hydrozoa, exhibit
polymorphism in two basic forms: the polyp and the medusa. These forms manifest
in different variations but can be classified based on their function.
Polyp:
Polyp zooids have a tubular body with a mouth
surrounded by tentacles. The other end is attached to a substrate via a pedal
disc.
Medusa:
Medusoid zooids have a bell or
umbrella-shaped body with marginal tentacles, and the mouth is centrally
located.
Patterns of Polymorphism
Polymorphism varies among hydrozoans. It can range
from simple to highly complex patterns.
- Dimorphism:
In colonies
like Obelia and Tubularia, two types of zooids are present:
Gastrozooids (Hydranths): Responsible
for feeding.
Gonozooids
(Blastostyles): Involved in asexual reproduction, producing medusae or
gonophores.
2. Trimorphism:
In some
species, a third type of zooid appears, called:
Dactylozooids: These individuals are specialized for defense
and protection.
3. Polymorphism:
In more
complex colonies like Hydractinia,
five types of zooids can be found:
Gastrozooids: For feeding.
Dactylozooids:
For protection.
Tentaculozooids: Sensory function.
Skeletozooids: Structural support through
spiny projections.
Gonozooids: Responsible for
reproduction.
Polypoid
Zooids
1. Gastrozooids:
These
zooids handle feeding for the colony. They have a mouth and tentacles armed
with nematocysts for capturing prey. The digested food is distributed through a
shared canal system called the coenosarc.
- Dactylozooids:
Also known
as palpons, these zooids serve a protective role. Although they resemble
gastrozooids, they lack a mouth and possess long, unbranched tentacles.
Medusoid
Forms
1.
Pneumatophores:
Pneumatophores act as floats. They are inverted medusan bells with a gas
gland that secretes gas into an air sac. This structure helps colonies like Physalia (Portuguese man of war) stay
afloat.
2. Nectocalyx (Nectophores):
These
bell-shaped zooids propel the colony by swimming. They do not have sensory
organs or tentacles but are muscular, aiding in locomotion.
Also called
hydrophyllia, these leaf-like structures protect other zooids in the colony.
These
zooids bear gonads responsible for producing sperm or ova.
Examples of Polymorphic Hydrozoans
Physalia (Portuguese Man of War):
This free-floating colony has a large pneumatophore
for buoyancy, with specialized zooids such as dactylozooids armed with stinging
tentacles for capturing prey.
Porpita:
Features a
medusoid disc and concentric gas chambers, with zooids like gastrozooids and
gonozooids for feeding and reproduction, respectively.
Theories on the Origin of Polymorphism
1. Poly-organ
Theory:
This theory
suggests that polymorphic colonies originated from a single medusoid
individual, with various parts of the medusa evolving into distinct zooids.
2. Poly-person
Theory:
According
to this view, colonies are made up of modified individuals that evolved from a
primitive polyp due to the division of labor.
3. Medusa
Theory:
Proposed as
a compromise, this theory suggests that siphonophores originated from a
medusoid individual, with various zooids budding off from it.
Significance
of Polymorphism
Polymorphism in Hydrozoa represents an evolutionary
adaptation for survival. It allows colonies to perform specialized functions
such as feeding, protection, and reproduction. This division of labor ensures
efficiency, giving polymorphic colonies a competitive advantage in their
environments.
