Fish Locomotion

Class: Actinopterygii

Order: Perciformes

Family: Scombridae

Scombridae is the family of the mackerels, tunas, and bonitos.

Scombrids have two dorsal fins, each of which can be depressed into grooves in the back, and a series of finlets between the rear dorsal fin and anal fin and the tail. The base of the tail is slender, and the caudal fin strongly divided. Primarily swift predators of open seas; some of the smaller species strain zooplankton through their gill rakers.

Example of a Scombridae. This one’s a mackerel.

Mode of locomotion is Thunniform:

Most fishes move by generating an undulating wave along their bodies along their bodies that pushes water backwards but thunniform swimmers isolate this movement to the crescent moon-shaped tail (lunate tail) which is much taller narrower and stiffer than that of other fish. Scombrids swim by restricting lateral undulations to the most caudal body segments, but maintain their sizeable red-muscle mass in the mid-body region.2

Thunniform swimmers such as the Scombrids have a number of ecomorphological adapting for their mode of locomotion. The hallmarks of thunniforms are a teardrop share and a stiff bodied swimming style.1. . They have from 5 to 11 small separate non-depressible sail like finlets in a row running from the dorsal and anal fins out on onto the pununcle. These fins serve to reflect water along the peduncles to reduce drag and so increase swimming speed. The extended anal may also help to maintain stability. The body remains stiff while the caudal peduncle and tail move to generate thrust. The angle of the caudal fin is changed during each phase of the tail beat so to keep maximum thrust at all times. This means there is a concentration of muscle in the mid section of the fish. The dark colour in the diagram below on the left hand side indicates red muscle. The caudal fin displays high aspect ratio. “The symmetrical morphology of the homocercal tail has led to the assumption that, during steady swimming, forces are generated only in the horizontal plane and that no is lift produced”. 9

Three sources of lift traditionally identified for scombrids are the pectoral fins, body and caudal keels. Also tuna swim constantly.

Note: this mode of locomotion is also referred to in the literature as carangiform; the difference between carangiform and thunniform seems to refer to the amount of undulation occurring in the posterior end of the body. Most of the literature I looked at used thunniform to refer the mode of locomotion at the other extreme from anguilliform so this is the mode of locomotion I used for talking about Scombrids.

Relative amount of body used for propulsion for carangiform (left) and thunniform (right)

Class: Actinopterygii

Order: Anguilliformes

The order Aguilliformes consists of the eels, both freshwater and marine.

Most eels prefer to dwell in shallow waters, hide at the bottom layer of the ocean, sometimes in holes. They are long and thin and the caudal fin is totally absent, their fins are always spineless. The back and anal fins are long, usually connecting with the tail fin. The belly and chest fins are absent. The shoulder girdle is separate from the skull. The scales are cycloid or absent. Most eels are predators and so need to be able to swim quickly.

Mode of locomotion is Anguilliform: Anguilliform is a purely undulatory mode of swimming, in which most or all of the body participates. The side-to-side amplitude of the wave is relatively large along the whole body, and it increases toward the tail.

Each wave is generated by contractions of the eel’s muscles in a few anterior segments on one side of the spine while the muscles on the other side of the spine relax and slightly stretched. The resulting bending of the body towards the side of the body that is contracted passes backwards as the wave of muscle contraction moves towards the posterior. While this is happening the anterior muscles, which have been contracted relax, their partners on the other side of the spine contract and the eel bends in the