MARINE biologists in California have found evidence that the decomposing organic matter that constantly sinks to the ocean depths is colonized by a surprising variety of microscopic animals that appear to hitchhike rides part of the way down.

The discovery suggests that oceanic ecosystems are more complicated than has been thought. It may lead scientists to reassess the volume of organic material, called detritus, that ultimately reaches the ocean bottom after the hitchhikers get off.

Marine scientists have long known about the ''rain of detritus,'' or ''marine snow,'' but have thought that it was composed only of dying organisms, their wastes and bacteria responsible for decomposition.

As part of an extensive ocean study sponsored by the National Science Foundation, two researchers from the University of California at Santa Cruz, sampled detritus at sites throughout the eastern Pacific. Traps Placed in Sea

Dr. Mary Silver, a professor of marine sciences, and Marcia Gowing, a marine researcher, placed cylindrical traps at various depths from 50 to 2,000 meters. The traps were filled with formalin, a chemical that instantly kills and preserves microscopic organisms.

The researchers returned two to four weeks later to collect what was in the traps. At first, what they found did not appear any different from detritus as it had traditionally been described. Not until the detritus was examined with scanning and electron microscopes did they find evidence of what Dr. Silver called ''a huge array of organisms.''

Many were protozoans, single-celled animals larger and more structurally complex than bacteria. Dr. Silver believes that the protozoans may migrate onto the detritus for short periods to feed on both the bacteria and the decomposing material, then hop off and float back to the surface to avoid being killed by increasing cold and pressure. Although this needs further investigation, she said, it is likely that ''a great deal of what gets on, gets off.''

Bacteria, unlike protozoans, move freely only over short distances. Once attached to detritus, they stay attached all the way down, scientists have thought. By assuming that everything attached to the detritus sank, scientists may have overestimated the volume of organic material that ends up on the floor of the sea, Dr. Silver said.

photos of one-cell marine animals (Marcia Gowing)