Aquatic locomotion

**Aquatic locomotion** are the different strategies for an animal to move around in water.

## Stability

When moving through a medium like water or air, an animal needs to remain stable along three axes of rotation. Firstly, the animal needs to control roll, or side-to-side teetering, which can be achieved by evolving fins that stick out perpendicular to the body. If positioned near the front or back end of the body, these fins also help maintain pitch, or rotating forwards or backwards. Finally, yaw, or turning left and right, can be adjusted with dorsal or ventral fins, which typically occur posterior to the animal’s centre of gravity.

## Anguilliform swimming

Anguilliform swimming is a form a locomotion where undulations pass along the entire length of the body to generate thrust. It's a fairly unspecialized form of locomotion, and is most efficient at slow speeds. It's the most basic form of aquatic locomotion.

### Morphology

Anguilliform swimmers tend to have elongated bodies of fairly uniform width, so that all parts of the body displace an equal amount of water. Their bodies also tend to be flattened in the direction of undulation to increase surface area and therefore give them greater leverage against the water, essentially turning the whole body into one big fin.

### Comparison with Sub-carangiform swimming

The Anguilliform swimming requires slightly less muscular energy to maintain than Sub-carangiform swimming.

### Examples

On Earth, eels, lampreys, and some species of knifefish use this form of locomotion.

On TIRA 292 B, it's a form of swimming used by early Acanthopods and Orthorhachids.

## Sub-carangiform swimming

Sub-carangiform swimmers use only the rear two-thirds of their bodies to generate thrust, keeping the anterior third comparatively still. This form of swimming increases speed by concentrating the lateral movements towards the posterior end of the body.

This form of swimming grants great speed. This is essential for escaping predators and chasing down prey, so if a clade of acanthopod were to evolve this form of swimming, they’ll have a distinct edge in competing for pelagic niches. And that's what happened with Hadrorhachia, a sub-carangiform swimmer.

### Morphology

Because the majority of the work of displacing water is done the tail and hind-portion of the body, the fins tend to primarily occur along these regions, while the main body is usually shorter, stiffer, and rounder than in anguilliform swimmers.

### Comparison with Anguilliform swimming

Subcarangiform swimming grants greater speed than Anguilliform swimming.

## Carangiform swimming

In carangiform swimming, thrust is generated by rapidly oscillating only the posterior third of the body, whereas sub-carangiform swimming involves around two thirds of the animal’s body length.

### Morphology

This specialization in swimming style is strongly correlated with a change in body shape: the smaller the portion of the body dedicated to propulsion, the more streamlined the rest of the body becomes, favoring a fusiform or pointed-oval-shaped body, and a muscular tail bearing a large, crescent-shaped or lunate caudal fin. This provides a further increase in speed, but comes at the cost of maneuverability.

### Comparison with Anguilliform swimming

Unlike anguilliform swimmers, which can execute sharp turns by adjusting the position of their elongated bodies, carangiform swimmers have comparatively little control over their direction of movement.

### Movement

To help them steer and maintain stability, carangiform swimmers usually rely on their fins, which in the case of Teleopterids,evolved from the limbs that line their bodies, which have long since lost their role in propulsion.

## Thunniform swimming

The final and fastest variant of body-driven swimming is thunniform swimming, wherein virtually all the propulsive force is provided by the tail fin, seen in many large predatory species like tuna, barracuda, and some sharks.

### Morphology

This form of swimming is also correlated will an elongated, torpedo-like body that allows for rapid acceleration, which will give these teleopterids a substantial edge competing for the niches of macropredators.

## Median/paired fin swimming

Median/paired fin swimming is an alternative method of swimming commonly found in many reef-dwelling achieving propulsion by beating the fins back and forth, keeping the body and tail comparatively motionless. It's generally slower than any of the various forms of body-driven swimming, but the chief benefit that it provides is a much more precise control of movement, which is essential when it comes to navigating the complex terrain of the reefs.

Among all the methods of swimming so far, it's the only one that doesn't involve the body and caudal fin in the production of thrust.

### Examples

On earth, manta rays and ocean sunfish. On TIRA 292B, remipterids.

## See also

* Countershading
* Aquatic feeding mechanisms