Sea worm
From Halopedia, the Halo wiki
Sea worms are viewable creatures that can be seen in the shallow waters of the Silent Cartographer. They are very small, and wriggle around near the surface of the water. You can see these worms if you walk into the ocean around this level, until you are submerged in water but still in shallow water. They are harmless to the player, and they do not interact with your character at all. Repeated firing of weapons, or the detonation of grenades while underwater amongst the worms, will not kill them. This is most likely due to game mechanics. In actuality, these creatures are most likely fragile, as most worms of Earth are. It is still possible however, that the worms are completely immune to phisical damage.
One explanation for this is perhaps the worms are extra-dimensinal creatures from another universe. Since they would not obey the laws of phisics as we know them, they might not be damaged by things such as high-velocity metalic materials (human bullets and shrapnel), or superheated ionized gases and liquid/solid incendiary materials (Covenant plasma technology and napalm/other heat producing substances used in combat). If the worms are phisicaly tangible, their bodies could be used in the production of super-stengthened materials possible usedul in things such as body armor and bullet-proof vests.
The enigmatic and highly elusive, passive, and mysterious sea worms of Halo are, and may remain so forever, a biological and scientific oddity of the galactic pool of life. Sea worms continue to enthral those who view them in the wild, but since none have ever been captured and put in captivity, they simplt remain a mystery to scientists and military super soldiers all over the galaxy.
