Unraveling the Intricate Nervous System and Sense Organs of Palaemon Prawns: A Study in Marine Adaptation

The Palaemon prawn, a captivating marine creature, stands out for its intricate nervous system and extraordinary sense organs. These specialized features equip it to excel in diverse aquatic habitats.

Within its nervous system, a network of nerves and ganglia orchestrates complex physiological responses, ensuring swift reactions to environmental cues. Its centralized brain, situated atop its cephalothorax, coordinates essential functions and cognitive processes.

Its sense organs are a marvel of evolution, finely tuned to perceive the surrounding world. Compound eyes offer panoramic vision, detecting motion, light, and shadows with acute precision. Antennae, adorned with sensory hairs, act as chemical receptors, helping the prawn identify potential mates and food sources, as well as navigate through complex currents.

Remarkably adaptive, Palaemon prawns display impressive behaviors to secure their survival. Migratory patterns are dictated by environmental changes, allowing them to exploit favorable conditions. Moreover, they showcase social interactions in breeding colonies, reinforcing their species' continuity.

In sum, the Palaemon prawn's nervous system and sense organs exemplify nature's ingenuity, enabling it to thrive amidst the ever-changing dynamics of aquatic environments. Studying these remarkable adaptations sheds light on the intricacies of marine life and the fascinating interplay of evolution and survival strategies.

The nervous system of Palaemon resembles that of Annelidas, it shows

1. The central nervous system,
2. The peripheral nervous system, and
3. Autonomic nervous system.

Central Nervous System

It includes a pair of cerebral ganglia, a pair of circumesophageal connectives, and a double ventral nerve cord.

1. Cerebral ganglia:
   1. They lie in front of the junction of the esophagus with the cardiac stomach. It is a white bilobed mass.
   2. It is also called the brain.
2. Circum oesophageal connectives:
   1. These are nerve cords that originate from the postero-lateral parts of the brain and run downwards around the esophagus to unite with the ventral nerve cord.
   2. Each connective shows a small commissural ganglion which gives a mandibular nerve to the mandible.
3. Double Ventral nerve cord:
   1. It lies in the mid-ventral line of the body, it bears seventeen pairs of ganglia.
   2. Tare anterior eleven pairs of ganglia belong to the cephalothorax and are fused to form a single and large mass called the Cephalo thoracic ganglionic mass.
   3. The posterior six pairs of ganglia of the nerve cord belong to the abdomen and they remain separate.
   4. The sixth abdominal ganglion is larger than the others.

Peripheral Nervous System

It contains paired nerves that arise from the central nervous system to innervate body parts.

1. Nerves from the brain: They are 5 pairs.
   1. Optic nerves: The optic nerves extend outwards and forward through the eye stalks and innervate the eyes.
   2. Ophthalmic nerves: The ophthalmic nerves arise close behind the optic nerves. They supply the oscular muscles in the eye stalks.
   3. Antennulary nerves: The Antennulary nerves arise from the brain antero-ventrally. Each nerve enters the antennule of its side. It also gives the statocystic nerve which goes to the statocyst.
   4. Antennary nerves: The antennary nerves arise from the ventral side of the brain. Each nerve enters the antenna of its side.
   5. Tegumental nerves: The tegumental nerves arise behind the antennary nerves. They innervate the labrum.
2. Nerves from Cephalothoradc ganglionic mass: Eleven pairs of nerves originate from the cephalon thoracic ganglionic mass.
   1. A pair of Mandibular nerves to the mandible.
   2. A pair of Maxilhilary nerves to maxillulae.
   3. A pair of Maxillary nerves to maxillae.
   4. Three pairs of Maxillipede nerves to maxilliped.
   5. Five pairs of walking leg nerves to walking legs.
3. Nerves from Abdominal ganglia:
   1. Three pairs of nerves arise from each abdominal ganglion.
   2. The fifth abdominal ganglion gives off only two pairs of nerves.
   3. The sixth abdominal ganglion gives six pairs of nerves.

Autonomic Nervous System

• A slender nerve arises from the hind end of the brain and has two small visceral ganglia.
• The anterior visceral ganglion is connected with the commissural ganglia of the circum-oesophageal connectives by short transverse connectives.
• The posterior visceral ganglion gives two pairs of nerves to the muscles of the wall of the esophagus and stomach.

SENSE ORGANS OF PALAEMON

Palaemon shows different sense organs. They are

1. Statocysts:
   1. In Palaemon, a pair of statocysts can be seen.
   2. The precoxa of each antennule will contain a statocyst. A statocyst is a round sac.
   3. It is 1 to 1.5 mm wide. Its outer opening is covered by a small integument. Inside it, an oval ring of setae and a heap of sand grains will be present.
   4. A receptor seta has two parts 1) a swollen base and 2) a slender shaft. The base is articulated with the wall of the sac by a membrane.
   5. It is supplied with nerve from the statocysts nerve.
   6. The shaft is bent and fine bristles are present above the bent part.
   1. Working:
      • The statocysts are the organs that maintain equilibrium in the normal position. The sand grains lie on the floor of the statocyst.
      • The sand particles work like statoliths.
      • When the position of the animal in water is changed the sand grains press against some of the receptor setae.
      • Nerve fibers are stimulated. It is conveyed to the brain. The position is corrected.
2. Tactile Setae: The tactile setae are seen on the antennae and other parts of the body. A tactile setae has two parts.
   1. Stout Proximal segment: Articulating with the integument by a flexible arthrodial membrane.
   2. Distal segment: It is touch sensitive. It is also called the blade. It shows two rows of barbs.
3. Olfactory Setae:
   1. The olfactory setae are on the middle small feeler of each antennule.
   2. Each setae has a stalk and a blade.
   3. Each setae gets a small nerve from the antennary nerve.
4. Compound Eyes:
   1. There are a pair of compound eyes.
   2. They are stalked.
   3. The eye stalks are in orbital notches near the base of the rostrum.
   4. Each eye stalk has two segments.
      1. Proximal segment attached to the integument.
      2. The distal segment with an eye at its free end.
   1. Structure: The compound eye consists of a large number of Ommatidia. They are radially arranged.
      1. Dioptric part:
         • The eye is covered by a transparent cornea.
         • The corneal lens is biconvex.
         • It is secreted by a pair of congeal cells. Below these cells crystalline cells are under pressure.
         • The crystalline cone is secreted by four vitrellae cells.
         • All the above structures collectively form the dioptric part of the ommatidium.
      2. Retinal part:
         • Below the dioptric part a long, refractile rod, the rhabdome is present.
         • It is secreted and surrounded by seven long cells, the retinulae.
         • The end of each retinal cell is prolonged into a nerve fiber that joins the optic nerve.
         • The rhabdom and the retinulae from the retinal part of the ommatidium.
         • Each ommatidium is covered by pigmented cells which will separate it from the nearby ommatidium.
   2. Function:
      • During bright light, ommatidia are completely covered by the pigment cells.
      • The ommatidia are separate from one another.
      • Thus many images are developed. Such an image is called the opposition or mosaic image and the eye is said to have a mosaic vision.
      • During dim light pigment cells separate apart and expose the ommatidia. With the result, the rays of light entering several adjacent corneae will overlap The image is formed by overlapping points of light.
      • Such an image is known superposition image.