Question

1-a)List the defining characteristics of the Chondrichthyes. b) Draw a simplified phylogenetic tree for cartilaginous fishes and outline the characteristics of the three main types.

2- Describe the main evolutionary steps involved in the conquest of landmasses by vertebrate organisms during the Devonian period.

Answer 1

Answer 1 (a) Defining Characteristic of the Chondrichthyes:

• Endoskeleton of soft and flexible cartilage.
• lack the air-filled swim bladder which most of the bony fishes have. Therefore the Chondrichthyes must swim continuously to stay afloat.
• The Chondrichthyes  is characterized by placoid scales (also called dermal denticles, or skin teeth), with a structure similar to teeth consisting of an outer enamel layer, dentine, and an inner pulp cavity.
• 5-7 pairs of gills, not covered by operculum.
• Mouth position is central unlike bony fishes where it’s dorsal.
• All chondrichthyans have internal fertilization unlike the bony fishes.
• Chondrichthyan males have a pelvic clasper, a specialized organ used in mating.

Answer 1 (b) Simplified Phylogenetic Tree for Cartileginous Fishes as below :

The class is divided into two subclasses: Elasmobranchii (sharks, rays, skates, and sawfish) and Holocephali (chimaeras, sometimes called ghost sharks, which are sometimes separated into their own class).

Characteristics of three main types are as below :

Sharks (types of Elasmobranchs) have these characteristics:

• Fusiform Body Shape. (Cylindrical/rounded with tapering at each end) which makes swimming easier by reducing drag and minimising effort.
• Countershading- usually sharks have different colours on their dorsal and ventral sides. When a shark is darker on the dorsal side and lighter on the ventral side it is termed as countershading.
• Pectoral fins are not fused to the head.
• Heterocercal Tail.

Skates (types of Elasmobranchs) have following characteristics:

• Flattened body shape suited for bottom living.
• Enlarged Pectoral Fin attached to head.
• lack of anal fin.

• Mermaid's purse : Skates produce their young in an egg case called a mermaid's purse.These egg cases have distinct characteristics that are individualized to each species

• Electric Organ : The electric organ (EO) is a characteristic exclusive to aquatic species. Among the Chondrichthyes, the only species to possess electric organs are the electric ray and the skate.

Holocephali (a subclass of Chondrichthyes) have following characteristics:

• Deep water fishes with large head and eyes. e.g Ratfish/ Chimaeras
• Gill slits covered by a flap of skin called operculum.
• No scales instead skin is found.
• Caudal fin developed into a rat like tail.
• Accessory clasping organs in males.