Are Sponges Sessile or Motile? Also Explained About Movement In Sponges…
Are sponges sessile or motile?
Adult sponges are strictly sessile and they cannot move from one place to another because they spend their whole life attached to a substratum through a holdfast.
However, the larval stage is free swimming in nature and can easily move from one place to another due to the forces exerted by the water currents till it settles down on a hard surface and develops into an adult sponge.
Talking about the adult sponges, their holdfast that help them to anchor is a root-like or basal plate-like structure located at the base of the sponge’s body. It is chitinous in nature and is not spongin-based.
The primary function of this holdfast due to its complex tangles of root-like growths is to anchor the sessile sponges and other non-motile marine animals to the substratum very firmly.
The substratum to which the sponges remain attached can be either the rocks, or the sandy sea bed, or either the surface of the coral reefs.
So, as the adult sponges remain sessile (non-motile) being attached to the substratum then their whole body shows slow contractile movements as its outermost layer of cells, the pinacoderm, is highly contractile in nature.
And, the movement of cells are amoeboid in character, rather than muscular. This happens in the cell when the cytoplasm of the cell slides and forms a pseudopodium in front to pull the cell forward.
So, due to this biological property of amoeba-like movements of pinacocytes and other cells, some of the adult species are seen to move extremely slowly across the sea bed at speeds of 1–4 mm (0.039–0.157 in) per day which is almost ignorable.
Explained: About Motility in Sponges
Sponges can be either sessile or motile based on the various stages of their life cycle. If it is an adult sponge than it is strictly sessile, and if it is a larval sponge than it is free swimming in nature.
However, some of the adult sponges are seen to move at ignorable speed due to the amoeboid movements of their cell. So, they can be said creeping along their substrata via. organizational plasticity.
Some of the sessile adult sponges that remain attached to the substratum are also seen to move extensively due to the various external forces such as water currents, and wave movements.
The ability of adults for being sessile (non-motile) is that they are filter feeders. Meaning that they in order to feed pumps the surrounding sea water through their bodies and filters it just in order to obtain their food that is dissolved in the water.
While the ability of the larval sponges for being motile (non-sessile) is that they can disperse to longer distances by freely moving through the water currents. This helps them to move their young into new territories and to better evolve.
When adult sponges sexually reproduce they produce free-swimming larvae every time. No doubt in that process.
However, if the adult sponges perform asexual reproduction then they start to produce gemmules or buds from the adults’ body which later detached to become independent individuals.
The time between the detachment of the bud from the adults’ body to its attachment to a new substratum is the period when they can disperse to new locations due to the various external forces like water current.
Although the sponges are sessile as a whole being attached to the substratum, but the cells in the body of the sponges are not sessile.
This can be well understood as the sponges of the family Cladorhizidae are known to feed by capturing and digesting whole animals as they come in contact with the preys’ (small crustaceans’) exoskeletons.
They digest the whole prey animal by capturing with their spicules, and later their body cells start to envelop and slowly migrate around the helpless prey and digestion starts to take place all extracellularly.
This is achieved due to the amoeboid movement of the body cells, showcasing the cell mobility only.
Why are sponges sessile?
Sponges are sessile because they remain attached to their hard substratum by means of their holdfast located at their basal body part which has complex tangles of root-like outgrowths that help them anchor firmly to the substratum.
Holdfast is also termed as Adhesive disc of the sponges. It is composed of a large number of root-like filamentous, stolon-like structures covered with delicate perisac, which extend from the basal region of the sponge and serve as holdfast for adhesion.
Sponges fall under the Phylum Porifera of the Animal Kingdom. And according to their Phylum, it has been characterized that sponges can be either solitary or colonial, but all are sessile in nature in their adult forms.
Sponges have been defined as sessile metazoans (multi-celled immobile animals), that can remold their bodies and reshape it whenever required.
They can remold their bodies because of their most types of cells that can move within their bodies, with a few types of the cells having the capability to transform from one type to another.
Explained: Sponges do not have well-developed locomotory organs
The adult sponges or more broadly said that sponges are sessile and they cannot move around at all.
They have a cellular level of body organization and lack the presence of well-developed locomotory organs.
They lack true body tissues and also have no body symmetry. Due to the absence of dedicated body tissues in order to support many body cells at once, they are highly sessile.
They only tend to rely on the water currents for locomotion and movement, as they have no muscles or specialized organs or tissues for locomotion.
However, due to the amoeboid movement of their body cells, a few of the sponge species have been seen by researchers to crawl at a speed of up to 160 microns/hr (4 mm/day) on aquarium walls.
When these sponges were extensively studied using modern-day technologies like time-lapse cine-micrography and scanning electron microscopy then it was known that moving sponges used to have various types of distinctive leading edges composed of motile cells that help them move at very slow insignificant speed.
Various observations done so far have suggested that locomotion of sponges is achieved by the cumulative crawling locomotion of the cells (amoeba-like movements of pinacocytes and other cells) at the base of the sponges, including pasts of the holdfast as well.
So, in order to bring light to the statement that “Sponges do not have well-developed locomotory organs” we can conclude by stating that the adult sponges are highly sessile and do not move at all as they remain fixed to a hard surface.
And, also it is to be well-stated that there are some marine and freshwater sponges like many that fall under the Class Demospongiae, etc. of the Phylum Porifera, which can also move across the sea bed but only a few millimeters a day.
How can the larval sponges move?
The sponge larvae have a flagellum at its posterior part that aids in locomotion and movement. This flagellum is well supported by the collar cells that altogether constitute the collar portion of the larval body that has various microvilli.
The flagellum is known to rotate in a clockwise or counterclockwise direction similar to a propeller, or can even produce wave-like movements in order to make the larvae move with ease.
The contribution of both flagella and microvilli helps the larva to move, gather food, and feed by the process of absorption and secretion.
Many researchers have also stated that sponge larvae possess cilia with unusual terminal expansions that aid them in movement and locomotion as well.
The sponge larvae cilia are also called ‘club footed’ cilia due to their curled or biconcave external shape.
There are basically three unique types of movement seen by the sponge larvae. These are: Directional swimming, Corkscrew path swimming, and Creeping movement.
Directional swimming type of larval movement is mostly seen in the Haliclona larva, where the larva can constantly move with a very fast directional movement of about 1 cm per second rate using its posterior ring of long cilia.
Corkscrew path swimming is a type of larval movement seen in Microciona, Ophlitaspongia, and Mycale larvae, in which the larvae are seen to swim on a Corkscrew path while rotating constantly characterized by bouncing type of swimming.
Lastly, the Creeping movement type of larval movement can be more often seen in Halichondria larvae in which the organism is seen to creep along the substrate without any type of rotation.
