Difference Between Sessile and Motile

The main difference between sessile and motile is that the sessile is a term used to describe non-motile life forms whereas the motile is a term used to describe freely-moving life forms. Furthermore, sessile life forms live attached to a substrate while motile life forms have cilia, flagella or limbs, which facilitate movement.

Sessile and mobile are two characteristics of both animals and plants, which describes the ability to move.

Key Areas Covered

1. What Does Sessile Mean
     – Definition, Features, Examples
2. What Does Motile Mean
     – Definition, Features, Examples
3. What are the Similarities Between Sessile and Motile
     – Outline of Common Features
4. What is the Difference Between Sessile and Motile
     – Comparison of Key Differences

Key Terms

Bilateral Symmetry, Cnidarians, Feeding, Motile, Petiole, Radial Symmetry, Sessile,

Difference Between Sessile and Motile – Side by Side Comparison

What Does Sessile Mean

A sessile organism does not have the ability of self-locomotion and is predominantly immobile. Sessile animals are attached to a substrate. Most sessile animals live in the sea. Sessile animals consume low amounts of food; hence, they maintain lower metabolic rates. The main benefit of being sessile is the lower energy requirement for mobility.

Most sessile animals show radial symmetry, making it easier for them to capture food coming from each angle. Some sessile animals move with the use of alternative methods such as water or wind currents. Other types of sessile animals can be motile during their adult stages, as in jellyfish. Some animals such as barnacles exhibit motile larval but, sessile adult life stages. Sessile animals such as corals lay down their own surface to grow. Another benefit of being sessile is the ease the reproduction due to the close proximity to their partners. However, sessile organisms have to develop mechanisms to be protected from predators; for example, the cnidocytes.

Figure 1: Orange Cup Corals (Tubastraea coccinea)

Sessile plants are identified by the lack of petiole, which is the supporting stalk to the leaves to attach to the stem. Hence, both leaves and flowers arise directly from the stem or peduncle.

What Does Motile Mean

Mobile refers to an organism’s ability to move. Motility is an active and spontaneous process of moving by consuming energy obtained from food. Higher organisms are motile. Motile organisms show the most efficient body form of moving through the environment, which is the bilateral symmetry with anterior, posterior, dorsal, and ventral parts of the body. The main benefits of being motile include the ability to actively seek out food, mating partners, and escape from predators. Due to the possession of bilateral symmetry, these organisms move into a defined direction. Therefore, they have developed sensory organs concentrated in the front part of the body.

Figure 2: A Pair of White-Tailed Deer (Odocoileus virginianus) Grazing from a Tree

Plant-like organisms such as algae are motile due to the presence of flagella. Male gametes of both animals and plants become motile with the aid of flagella. Some bacteria also possess flagella.

Similarities Between Sessile and Motile

• Sessile and motile are two characteristics of life forms, which describe their ability to move.
• Both animals and plants can be either sessile or motile.

Difference Between Sessile and Motile

Definition

Sessile refers to the ability of a life form to be attached to a particular surface throughout its life, being predominantly immobile while motile refers to the ability of a life form to move independently, using metabolic energy.

Attach to a Substrate

That is, the sessile animals always attach to a substrate while the motile organisms are freely-moving.

Type of Symmetry

Furthermore, the sessile organisms show radial symmetry while the motile organisms show bilateral symmetry.

Mode of Feeding

Moreover, the sessile animals are passive feeders, which are filter feeders or suspension feeders while the motile animals are active feeders who hunt for food.  

Nutritional Requirements and Metabolic Rates

Also, the sessile animals require low amounts of nutrients and slow metabolic rates while motile organisms require high amounts of nutrients as they show high metabolic activity.

Sensory Organs

In addition, the sensory organs of sessile organisms are equally distributed throughout the body while the sensory organs of motile organisms are concentrated to the front.

Advantages

Further on, the sessile organisms have lower energy requirements and easier reproduction while the motile organisms can gather more food by hunting or searching.

Disadvantages

But, looking at the disadvantages of both, predators can attack the sessile organisms easily while the motile organisms have to search for their mating partners.

Organisms

Some sessile animals are cnidarians such as corals, jellyfish, Hydra, and sea anemones and some echinoderms such as sea lilies and starfish while some motile organisms are mammals, reptiles, birds, algae, and some bacteria.

Conclusion

Sessile is the characteristic of being immobile by attaching itself to a substrate. But, motile is the characteristic of being motile actively and spontaneously. The main difference between sessile and motile is the ability to move.

Reference:

1. “Sessile – Definition and Quiz.” Biology Dictionary, Biology Dictionary, 29 Apr. 2017, Available Here
2. “Mobile vs. Motile – What’s the Difference?” Ask Difference, Available Here

Image courtesy:

1. “Coral Sol 01” By Maraguary – Own work (Public Domain) via Commons Wikimedia    
2. “White-tailed deer (Odocoileus virginianus) grazing – 20050809” By Raul654. – Own work (CC BY-SA 3.0) via Commons Wikimedia