Scales may be of two types, cycloid or ctenoid. Cycloid scales are smooth and ctenoid have comb like teeth or spines on one edge, many fish have a combination of both types of scales. Scales are covered with a thin film of mucus which in addition to acting as a lubricant, forms a defence against parasites and a protective coating for wounds. M ailed catfish have a double row of large interlocking bony plates or scutes along each side of the body instead of scales. Some species have no scales but have toughened skin reinforced with cartilage, which affords some protection.
S pines are the most common form of weapon for protection or attack.
They may be extensions to fins, form venomous integral parts of fins or may be mounted as knives elsewhere on the body. Erectile spines are seen below the eye on Loaches, and Triggerfish have a locking mechanism which fixes fins in a rigid erect position. N ot all defense systems are mechanical. Other forms include 'Chemical warfare', the possession of poisonous flesh or the excretion of danger signal substances to warn other members of the same species. A fish may rely on its close relationship with others for safety, the shoaling technique is an example of safety in numbers.