B.Sc. Fish & Fisheries
1st Year / Semester-I
DSPMU Ranchi Syllabus
TAXONOMY OF CRUSTACEA (SHELLFISH SYSTEMATICS)
Biology, Classification, 19 Pairs of Appendages, Moulting Physiology & Commercial Indian Prawns
1. Introduction to Subphylum Crustacea
Crustaceans (कवचीय प्राणी) are predominantly aquatic arthropods widely celebrated as the "Insects of the Sea" due to their immense ecological diversity and abundance in marine, brackish, and freshwater biomes.
Standard Academic Definition:
"Crustacea is a major subphylum of bilaterally symmetrical, coelomate arthropods distinguished by having typically biramous (two-branched) appendages, two pairs of sensory antennae (antennules and antennae), a heavily calcified chitinous exoskeleton, and respiration taking place primarily through branchial gills."
"Crustacea is a major subphylum of bilaterally symmetrical, coelomate arthropods distinguished by having typically biramous (two-branched) appendages, two pairs of sensory antennae (antennules and antennae), a heavily calcified chitinous exoskeleton, and respiration taking place primarily through branchial gills."
Why Crustacean Taxonomy is Critical in Fisheries:
- Foreign Exchange Leader: Penaeid prawns and shrimps account for over 65–70% of India's total marine product export earnings (worth over ₹40,000 Crore annually).
- Hatchery Live Feeds: Lower crustaceans like Brine shrimp (Artemia salina) and water fleas (Daphnia, Moina) are the irreplaceable first live food for finfish and shrimp fry.
- Parasitology & Fish Health: Parasites such as fish lice (Argulus) and anchor worms (Lernaea) cause major economic losses in Indian carp aquaculture.
2. General Morphology of a Typical Decapod (Prawn / Shrimp)
The body of a decapod (e.g., Penaeus or Macrobrachium) is divided into two distinct tagmata:
- Cephalothorax (शिरोवक्ष): Formed by the fusion of 5 head segments + 8 thoracic segments = 13 segments. It is covered by a hard, calcified shield called the Carapace, which terminates anteriorly into a serrated, spear-like Rostrum.
- Pleon / Abdomen (उदर): Composed of 6 movable segments, ending in a terminal pointed spike termed the Telson.
Figure 1: Diagnostic Morphology & Landmarks of a Commercial Prawn
3. The 19 Pairs of Appendages in Decapods
Every typical malacostracan possesses a fixed plan of 19 pairs of appendages, all derived from a basic biramous (द्विशाखित) groundplan:
Figure 2: Architecture of a Generalized Biramous Appendage
| Region | Appendage Pair | Name of Appendage | Primary Function & Diagnostic Modification |
|---|---|---|---|
| Cephalic (Head) 5 Pairs |
Pair 1 | Antennules | Tactile, olfactory senses; houses the statocyst (organ of equilibrium) in base. |
| Pair 2 | Antennae | Sensory feelers; exopodite modified into an antennal scale (squama) for steering; coxa has green gland pore. | |
| Pair 3 | Mandibles | Heavily calcified jaws fitted with molar and incisor processes for masticating food. | |
| Pair 4 | Maxillulae (1st Maxillae) | Thin, foliaceous leaf-like food manipulators pushing food to mouth. | |
| Pair 5 | Maxillae (2nd Maxillae) | Bears the vibrating scaphognathite (gill bailer) to maintain respiratory water current. | |
| Thoracic (Pereion) 8 Pairs |
Pairs 6, 7, 8 | 1st, 2nd & 3rd Maxillipeds | Assists mouthparts in seizing, tasting, and tearing prey. |
| Pairs 9, 10, 11 | 1st, 2nd & 3rd Pereiopods | Chelipeds: End in pincers (chelae). Chelate in pairs 1–3 in Penaeids; 2nd pair massively enlarged in Carideans (*Macrobrachium*). | |
| Pairs 12, 13 | 4th & 5th Pereiopods | Non-chelate ambulatory walking legs used for crawling along benthic substrate. | |
| Abdominal (Pleon) 6 Pairs |
Pairs 14 to 18 | Pleopods 1 to 5 (Swimmerets) | Biramous swimming paddles. 1st pair in males modified into Petasma; holds spawned egg mass in caridean females. |
| Pair 19 | Uropods | Flat, broad fan-like paddles combining with median telson to form the tail fan for backward escape. |
4. Penaeid vs. Caridean Prawns (The Key Exam Question)
In university practical and theory examinations, examiners frequently ask students to distinguish between marine penaeid prawns and freshwater caridean prawns:
Figure 3: Abdominal Pleural Overlap — Penaeid vs. Caridean
| Characteristic | Penaeid Prawns (e.g., Penaeus monodon) | Caridean Prawns (e.g., Macrobrachium rosenbergii) |
|---|---|---|
| Branchial Gills | Dendrobranchiate (branching like tree boughs). | Phyllobranchiate (flat, leaf-like plates). |
| Abdominal Overlap | 2nd pleuron overlaps *only* the 3rd abdominal segment. | 2nd pleuron **overlaps both 1st and 3rd segments**. |
| Chelate Legs (Pincers) | First **3 pairs** of walking legs (pereiopods 1, 2, 3) are chelate. | Only first **2 pairs** are chelate. |
| Enlarged Fighting Leg | None; all chelipeds are slender and similar. | 2nd pereiopod is massively enlarged into an elongated fighting claw. |
| Reproduction | Free spawners; release eggs into the open ocean. | Females brood eggs on pleopods (**"in-berry"**). |
| First Larval Stage | Hatches as a free-swimming Nauplius. | Hatches as a more advanced Zoea. |
5. Commercially Important Indian Crustaceans
Figure 4: Penaeus monodon (Fabricius) — Giant Tiger Prawn, recognizable by prominent dark brown and buff transverse bands.
Figure 5: Macrobrachium rosenbergii (de Man) — Giant Freshwater Prawn (Scampi), showing massively enlarged blue 2nd chelipeds.
Figure 6: Scylla serrata (Forskål) — Giant Green Mud Crab with 9 sharp anterolateral teeth, cultured for live seafood export.
Figure 7: Artemia salina (Brine Shrimp, Class Branchiopoda) — Essential live feed for marine fish and prawn hatcheries.
6. Physiology of Moulting & Larval Metamorphosis
Hormonal Regulation of Ecdysis (निर्मोचन का अंतःस्रावी नियंत्रण):
Crustacean growth requires shedding the rigid, calcified exoskeleton.
Crustacean growth requires shedding the rigid, calcified exoskeleton.
- Inter-moult Phase: The X-organ / Sinus Gland complex in the eyestalk produces Moult-Inhibiting Hormone (MIH), keeping the Y-organ suppressed.
- Pre-moult Trigger: Environmental cues inhibit MIH. The cephalic Y-organ is disinhibited and secretes Ecdysone (20-hydroxyecdysone).
- Ecdysis: Ecdysone stimulates epidermal chitinase and protease secretion, digesting old cuticular layers and allowing the prawn to withdraw from its shell.
Figure 8: Larval Stages Sequence in Penaeid Shrimps
7. University Examination Practice Questions (With Explanations)
Q1. How many pairs of appendages are present in a typical malacostracan decapod?
A) 13 pairs B) 19 pairs C) 21 pairs D) 8 pairs
Explanation: Malacostracans possess 5 cephalic + 8 thoracic + 6 abdominal pairs = 19 pairs.
A) 13 pairs B) 19 pairs C) 21 pairs D) 8 pairs
✔ Correct Answer: B (19 pairs)
Explanation: Malacostracans possess 5 cephalic + 8 thoracic + 6 abdominal pairs = 19 pairs.
Q2. In which group of prawns does the pleuron of the 2nd abdominal segment overlap BOTH the 1st and 3rd segments?
A) Penaeidae B) Caridea C) Sergestidae D) Palinuridae
Explanation: This dual overlap is the diagnostic hallmark of the Infraorder Caridea (*Macrobrachium*).
A) Penaeidae B) Caridea C) Sergestidae D) Palinuridae
✔ Correct Answer: B (Caridea)
Explanation: This dual overlap is the diagnostic hallmark of the Infraorder Caridea (*Macrobrachium*).
Q3. Moult-Inhibiting Hormone (MIH) is synthesized by which organ in crustaceans?
A) Y-organ B) Green gland C) X-organ in the eyestalk D) Mandibular gland
Explanation: MIH is synthesized by the X-organ and stored in the sinus gland to prevent premature moulting.
A) Y-organ B) Green gland C) X-organ in the eyestalk D) Mandibular gland
✔ Correct Answer: C (X-organ in the eyestalk)
Explanation: MIH is synthesized by the X-organ and stored in the sinus gland to prevent premature moulting.
Q4. The statocyst (organ of balance and orientation) in a prawn is located in the:
A) Eye stalk B) Precoxa of the Antennule C) Telson D) Antenna scale
Explanation: The statocyst is a sand-grain-filled vesicle embedded in the basal segment of the 1st antenna (antennule).
A) Eye stalk B) Precoxa of the Antennule C) Telson D) Antenna scale
✔ Correct Answer: B (Precoxa of the Antennule)
Explanation: The statocyst is a sand-grain-filled vesicle embedded in the basal segment of the 1st antenna (antennule).
⚡ Quick Revision Highlights for Exams:
- Crustacea Diagnostic: 2 pairs of antennae, biramous appendages, calcified cuticle.
- Tagmata: Cephalothorax (13 segments) + Abdomen (6 segments) + Telson.
- 19 Pairs of Limbs: 5 Cephalic + 8 Thoracic (3 maxillipeds + 5 pereiopods) + 6 Abdominal (5 pleopods + 1 uropod).
- Penaeid vs. Caridean: 3 chelate pairs in penaeids; 2 chelate pairs in carideans (with 2nd pleuron overlapping 1st and 3rd).
- Moulting Axis: X-organ produces MIH (inhibition); drop in MIH allows Y-organ to produce Ecdysone (shedding).
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