Time-lapse photography provides a view of the seafloor at timescales that make it possible to recognize behaviors and activity of often slow-moving abyssal fauna. Most behavioral studies have focused on mobile animals; sessile benthic fauna have largely been overlooked. We combed through 30 years of time-lapse camera data from Station M, a long-term study site on the abyssal plain off the coast of California, to survey behaviors of sponges and benthic cnidarians. The sessile fauna of the abyssal plain initially appear static, but time-lapse observations yield a new long-term perspective showing the dynamic lifestyles of these animals. Many hexactinellid sponge species rhythmically contracted and expanded back to full size, a process taking from days to weeks and which may reduce filter feeding activity by one-third to nearly one-half. In general, cnidarian behaviors occurred at shorter timescales, from hours to days, with less time spent contracted than sponges. Zoanthids had rhythmic contraction behaviors whereas the anemone Bathyphellia australis retracted its body with no apparent pattern or periodicity. Finally, while most sessile animals are anchored, some healthy sponges were seen drifting and rolling across the seafloor for several months. These observations expand on the natural history of these difficult-to-observe taxa. Furthermore, the behaviors of these sponges and cnidarians may affect the extent of nutrient cycling they mediate. [ABSTRACT FROM AUTHOR]