Polynesian tattoo symbol: guardians

The taniwha can be shape shifters, but they are often depicted like lizards or snake-like creatures with a bird's head (often like eels), or like big sharks and sometimes whales.
In western Polynesia tanifa is the name of a species of shark, and mangō taniwha is the Maori name for the great white shark.

They are considered either highly respected kaitiaki (guardians) of people and places, or dangerous, predatory beings.
On this account some depictions show the taniwha while devouring a human.

Some are said to have voyaged to New Zealand on the first canoes and therefore they act as guardians for the descendants of those families.
Sometimes an ancestor can turn into a taniwha after death, becoming the family protector.
It appears to be not by chance that the facial tattooing of the Maori chiefs, or tā moko, closely resembles the lines of the carved taniwha on the posts and rafters of the community house.

a Taniwha face and b traditional lines from Te Pehi Kupe's own moko, 1826.

Moko is also the name of a great lizard in legends throughout the western Pacific area, where crocodiles were present, and could therefore be at the origin of such stories, and one reason for the common shape of the taniwha.

This similarity in the depictions of ancestors like taniwha is also evident in the carved prow of war and burial canoes:

Whenever the Maoris found that a piece of land or a stretch of water presented dangers, or was the scene for repeated accidents, they related this to the presence of a taniwha in the area, and this meant that people should pay more attention in such places, or avoid them completely. The presence of a taniwha makes a place tapu, which has the double meaning of sacred and forbidden.
This is strictly related to the role of the Maori people toward the land: we shall respect and preserve it in order to avoid the rage of the taniwha living there.

When rivers are deviated, trees cut, this sometimes results in floods or landslides, attributed by tradition to the rage of the taniwha whose home was destroyed.
It's easy to understand that even those who don't believe in them, should pay attention to the places deemed to be the homes of such beings: the configuration of these places makes them dangerous and building there does not only go against the beliefs of a whole culture, but also against common sense.

Like in carving, taniwha are used in tattooing to represent powerful guardians, bestowing their protection upon the bearer of the tattoo.

There are several ways to represent them, but the most common is that of a snake-like creature with fish tail and bird's head, much like the manaia.
Oftentimes, only parts are depicted, like the characteristic hands or beak:

The above images show the taniwha having three fingers. It is a common depiction since three is a number with meaning in Maori lore but four fingers are commonly found as well.
The accredited reason for designing three fingers only is that they represent power over the realms of air, water and land. The upper "spur" of the hand represents the thumb.

It is also common to see several taniwha woven together, or interlaced with human figures:

This is especially true in carving, where the designs were adapted to fill all the available space.

The taniwha was incorporated centrally in this half sleeve:

This taniwha head was created by joining several other elements (manta, hammerhead shark, turtle, lizard, tiki mainly):

You can click on the images to read the full description of each tattoo.