Simple SummaryCompetitive interactions among animals come in a variety of forms and may be influenced by the size and number of the individuals involved, and whether these individuals are from the same species or not. The deep sea is a food-limited environment where it can be assumed that larger or faster scavengers might have an advantage over smaller or slower ones. However, very little work on competitive relationships in deep-sea megabenthic species has been done. This study looked at four co-existing deep-sea invertebrate species of the Northwest Atlantic using time-lapse video recordings in controlled, darkened laboratory experiments. We measured food approach and consumption dynamics in interspecific and intraspecific trials with individuals of similar and differing sizes. Diverse competitive and cooperative behaviours occurred depending on the species, relative body sizes, and the number of individuals present. Surprisingly, larger or faster individuals (or species) did not always outcompete smaller or slower ones. In addition, some species changed their foraging based on competitive pressure, while others did not. Overall, this study sheds light on the feeding strategies of co-existing deep-sea invertebrates and the behavioural relations among them, providing important baseline ecological information for the comprehension of food web structures in remote and poorly understood environments.AbstractCompetitive interactions come in a variety of forms and may be modulated by the size and number of individuals involved, and/or the resources available. Here, intra- and interspecific competitive behaviours for food (i.e., foraging/food search and feeding/food ingestion) were experimentally characterized and quantified in four co-existing deep-sea benthic species. Three sea stars (Ceramaster granularis,Hippasteria phrygiana, andHenricia lisa) and one gastropod (Buccinum scalariforme) from the bathyal Northwest Atlantic were investigated using video trials in darkened laboratory conditions. A range of competitive or cooperative behaviours occurred, depending on species (conspecific or heterospecific), comparative body size, and the number of individuals involved. Contrary to expectations, small individuals (or smaller species) were not always outcompeted by larger individuals (or larger species) when foraging and feeding. Moreover, faster species did not always outcompete slower ones while scavenging. Overall, this study sheds new light on scavenging strategies of co-existing deep-sea benthic species in food-limited bathyal environments, based on complex behavioural inter- and intraspecific relationships.