Taxonomy of Shellfish

Unit 8 - Taxonomy of lobsters

8.1. Sub-order 2. Reptantia

• Dorso-ventrally flattened body.
• Rostrum short or absent.
• Abdomen well-developed or reduced with first segment smaller than the posterior ones.
• Antennules do not have stylocerite.
• Antennal scale reduced or absent.
• Thoracic legs strong with first pair usually forming large pincer-like claws or chelipeds.
• Pleopods often reduced and not adapted for swimming.
• The sub-order Reptantia is divided into three sections (infraorder) - Macrura, Anomura and Brachyura.

Section (Infraorder) 1. Macrura
Well-developed abdomen and extended uropods and telson form a broad tail-fan.
First and third legs similar.
Gills numerous.
It includes two sub-sections namely Palinura and Astacura.
Sub-section 1. Palinura
Rostrum small or absent.
Body depressed.
Legs rarely chelate.
Eg. Lobster (Panulirus, Thenus, Scyllarus)
This sub-section consists of commercially important lobsters. Hence it is dealt in detail.
Systematic position
Phylum : Arthropoda
Sub-phylum : Mandibulata
Class : Crustacea
Sub-class : Malacostraca
Series : Eumalacostraca
Super-order : Eucarida
Order : Decapoda
Sub-order : Reptantia
Section (Infraorder) : Macrura
Sub-section : Palinura
Super-family : Palinuroidea
Family : Palinuridae

General morphology

1. Body consists of two main parts

a) Cephalothorax (formed by the fusion of cephalon or head, with the thorax) with its appendages.

b) Abdomen with its appendages.

2. The 14 somites (or body-segments) of the cephalothrax (the first 6 forming the cephalon, the last 8 the thorax) are fused and only in a few places there are visible indications of the lines between the somites. Each somite carries one pair of appendages. These appendages are the following, Somite 1 (= ophthalmic somite) carries the eyes, that are usually movable and consist of a stalk, formed by one or two segments. Somite 2 (= antennular somite) carries the antennules, somite 3 (antennal somite) carries the antennae (or second antennae). Somites 4 to 9 (i.e., the last 3 cephalon somites and the first 3 of the thorax) carry the mouth parts, appendages which have a function with the dissection and ingestion of food. Somite 4 carries the mandibles. Somites 5 and 6 carry the maxillulae (or first maxillae) and maxillae (or second maxillae) respectively, both are flat leaf-like organs. Somites 7 to 9 (=thoracic somites 1 to 3) carry the first to third maxillipeds, the first is leaf-like the maxillae, the second and third are more leg-like, especially the third. Somites 10 to 14 (= thoracic somites 4 to 8) carry the five pairs of pereopods or true legs. The first pereopod and sometimes also the second and the third, often (but not always) end in a chela or pincer. The first leg usually is the largest of the true legs. The legs that do not have pincers are indicated as walking legs as they are mainly used for locomotion.

3. Dorsally the cephalothorax is encased by the carapace, a single shield-like cover, which extends all the way from the eyes to the last thoracic somite and sometimes projects beyond the eyes as a narrow median rostrum. Laterally, the carapace extends to the bases of the legs, enclosing the branchial chamber which is a space between the body and the carapace housing the branchia or gills and situated above the bases of all legs. In the Palinuridae, the number and arrangement of which is of taxonomic importance. In some genera of Palinuridae, the lateral margins of the antennular plate are ridge-like and swollen, forming a stridulating organ with a process on the inner margin of the antennal peduncle, which rubs over this ridge when the animal moves its antennae in a certain way; a rasping sound is produced by this organ.

4. Ventrally, the cephalothorax shows, between the basal parts of the appendages a central plate, the thoracic sternum, on which the lines between the thoracic somites are usually indicated as grooves. In the females, the sexual openings are visible on the basis (the sixth segment of the leg counting from the tip) of the third pereopods, in the males these openings are on the basis of the fifth pereopods. This difference usually is the character that most easily distinguishes male and female lobsters.

5.The abdomen consists of six separate somites (number 15 to 20), which are not fused, but movably connected with each other. Each somite is surrounded by a chitinous armour; the dorsal part is called tergite, the ventral part, sternite and the two lateral parts, pleura (singular pleuron). The combined abdominal sternites form the abdominal sternum, the combined abdominal tergites, the abdominal tergum. The pleura usually are downward directed lateral plates, covering externally the pleopods. The appendages of the first 5 abdominal somites (number 15 to 19) are the pleopods or swimmerets; they are implanted on the borderline between the sternite and the pleuron in the male. In the male, the first and second pair of pleopo may be transformed into copulation organs, the so-called copulatory stylets, which are often stiff and of characteristic shape. The other pleopods usually consist of a single-segmented peduncle carrying two leaf-like appendages at the top. The pleopods may be reduced or even entirely lacking on some somites. The sixth abdominal somite (= somite 20, being the last body segment) bears the tail fan, which consists of a pair of uropods and the unpaired telson. The uropods actually are the sixth pair of pleopods; they are rather wide and well calcified and usually about as long as the telson. The telson is a plate-like median appendage of the sixth abdominal somite and sometimes it is considered to represent the seventh abdominal somite. The tail fan, when spread out, can be used for propulsion.