In this article we will discuss about Elphidium:- 1. Habit and Habitat of Elphidium 2. Structure of Elphidium 3. Locomotion 4. Nutrition 5. Reproduction and Life Cycle.
Habit and Habitat of Elphidium:
Elphidium is a marine form, found abundantly on the bottom of the ocean. It is found creeping about on sea-weeds to a depth of 300 fathoms (one fathom =1.8 meters). It also occurs in brackish waters.
Structure of Elphidium:
Shell:
Body of Elphidium is covered with a hard and translucent shell made up of calcium carbonate and small amounts of silica and magnesium sulphate. The shell is biconvex, polythalamus or multilocular (many chambered) and perforated.
The surface of the shell is chiselled. The chambers of the shell are V-shaped, laid down serially and arranged in a flat spiral in which each whorl of chambers overlaps the previous whorl, i.e., equitant.
The overlapping portions are known as alar processes. Due to the overlapping of the chambers only, the last chamber is visible from outside. The hinder margin of each chamber has a row of numerous minute backwardly directed, hollow, blind protoplasmic pockets called retral processes. The adjacent chambers remain separated from each other by perforated septa.
The chambers are interconnected or communicate with each other as well as with the exterior through minute pores present in the septa. The outer whorl opens to the outside by a row of large pores. The chambers of the shell originate from the initial chamber known as proloculum which may be small or large in size.
The small proloculum is known as microsphere and the shell having small proloculum shall be called microspheric, whereas the large proloculum is known as megalosphere and the shell having large proloculum is called megalospheric.
Cytoplasm:
The cytoplasm fills all the chambers of the shell, called inner cytoplasm. Besides, a thin layer of cytoplasm covers the shell from outside, called outer cytoplasm.
The cytoplasm is not differentiated into ectoplasm and endoplasm. However, the inner cytoplasm contains nucleus or nuclei, food particles, minute vacuoles, Golgi apparatuses, mitochondria, endoplasmic reticulum, ribosomes and brown granules or xanthosomes which are apparently waste matter. Contractile vacuoles are not found.
Nucleus:
The cytoplasm of megalospheric individuals contains single nucleus, while those of microsphoric individuals contains many nuclei. The nucleus is of vesicular nature and possesses many nucleoli in its nucleoplasm.
Rhizopodia:
The pseudopodia of Elphidium are in the form of numerous fine and often very long slender thread-like structures, which are often branched and anastomosing. This type of pseudopodia are characteristically called reticulopodia, rhizopodia or myxopodia. Each rhizopodium is made of an inner fibrillar axis and the outer fluid-like cortex.
The streaming circulation of cytoplasm has been observed in the rhizopodia. These are, in fact, temporary extensions of the outer cytoplasm and can be withdrawn within the shell. However, these are locomotory in function and often form feeding nets for catching diatoms on which animal feeds.
Elphidium is dimorphic and occurs in two forms, viz., megalospheric and microspheric (Fig. 16.2). The two forms are outwardly indistinguishable from each other but differ in internal organisation.
The megalospheric form has a large central chamber (proloculum), a large single nucleus and is relatively small in size; while the microspheric form possesses a small central chamber (proloculum), many small nuclei and is large. The megalospheric forms are said to be much more numerous than the microspheric forms.
Locomotion of Elphidium:
Elphidium creeps slowly with the help of its rhizopodia on sea-weeds at the bottom of the ocean. The rhizopodia are arranged in bundles around the shell. As referred to, rhizopodia contract and expand which bring about locomotion.
Nutrition of Elphidium:
Nutrition is holozoic. The food consists mostly of diatoms and algae; it also captures other Protozoa and micro crustaceans. The net-like rhizopodia are said to secrete an external mucus layer to entangle the food particles. The mucus layer also contains proteolytic secretions which help in paralyzing the prey and the process of digestion soon starts.
The entangled food in mucus is enclosed in a food vacuole and then the rhizopodia are withdrawn within the shell. The food is digested almost exclusively outside the shell and the digested products pass into the inner cytoplasm.
Reproduction and Life Cycle of Elphidium:
In Elphidium, Lister (1895) observed the development of the megalospheric form (sexual form) from the microspheric form (asexual form) by asexual reproduction. He noticed flagellated swarmer’s in megalospheric tests and considered them as gametes which through syngamy, gave rise to microspheric individuals.
Recent studies by Myers (1935-1940) confirmed the correctness of this view. In Elphidium crispa, there is no direct association of the megalospheric individuals during sexual reproduction. The flagellated gametes produced in each are set free in the water and the fusion of the gametes depends entirely upon the chance meeting.
Elphidium exhibits an alternation of generation in its life cycle. The megalospheric forms alternate with the microspheric forms. The microspheric forms always develop by the conjugation or syngamy and megalospheric forms develop without conjugation or syngamy. That means there is always an alternation of asexual (microspheric) and sexual (megalospheric) generations in Elphidium.
The microspheric form reproduces asexually by fission to produce a number of amoebulae. The inner cytoplasmic mass containing several nuclei creeps out of the shell and remains as a lump around it. A small amount of cytoplasm collects around each nucleus.
As a result, a large number of amoeboid cells (amoebulae) are formed. Each amoebula secretes the proloculum, forms rhizopodia, then it grows and forms other chambers of the shell to become a megalospheric form.
The megalospheric form reproduces sexually by syngamy or conjugation. During sexual reproduction in megalospheric forms, nucleus first breaks up into many small nuclei and the cytoplasm collects around each of these nuclei. The nuclei divide twice giving rise to a large number of tiny cells. The cells develop flagella and come out of the shell.
The biflagellate cells are haploid and known as isogametes. The isogametes of two different individuals fuse (conjugate) in pairs to form zygotes. (The fusion of similar gametes is known as isogamy). The zygotes, thus, formed develop into microspheric forms.
However, the life cycle of Elphidium may be summarised in the following way, that the microspheric forms produce amoebulae by asexual fission which develop into megalospheric forms. The megalospheric forms produce flagellated isogametes which after syngamy produce zygotes that develop into microspheric forms.
Thus, its life cycle clearly exhibits the phenomenon of alternation of asexual microspheric generation with sexual megalospheric generation.