From a side view, the Trivalve's body resembles a magatama, and it is mostly covered in a type of colorful exoskeleton. The creature has six visible legs, with four large ones supporting its weight, and two small ones right next to two pedipalp-like extremities. It has three eyes arranged in a vertical line, two small eyes for depth perception and a large complex eye between these two. Under the eyes is a beak-shaped mouth pointing downwards.
The Trivalve species appears to have two morphs that vary from each other in coloration. One morph has a shell that is pale orange on the top and darker orange on the sides and underside, with the fins sharing the same coloration. The other morph is pale blue in colour, with irregularly-shaped white stripes radiating outwards from the centre of the body.
The orange-shelled Trivalve has bright yellow eyes, whilst the blue-shelled Trivalve has slightly darker, more orange eyes.
The body of the Trivalve looks very similar to the shell of nautiluses, the most basal of living cephalopods. The name Trivalve is inspired by the class Bivalvia, possibly suggesting some comparable anatomical features. The creature's ability to swim upside-down seems to be inspired by horseshoe crabs.