Colonial architecture modulates the speed and efficiency of multi-jet swimming in salp colonies

Salps are marine pelagic tunicates with a complex life cycle including a solitary and colonial stage. Salp colonies are composed of asexually budded individuals that coordinate their swimming by multi-jet propulsion. Colonies develop into species-specific architectures with distinct zooid orientations. These distinct colonial architectures vary in how frontal area scales with the number of zooids in the colony. Here, we address how differences in frontal area drive differences in swimming speed and the relationship between swimming speed and cost of transport in salps. We (1) compared swimming speed across salp species and architectures, (2) evaluated how swimming speed scales with the number of zooids across colony in architectures, and (3) compared the metabolic cost of transport across species and how it scales with swimming speed. To measure swimming speeds, we recorded swimming salp colonies using in situ videography while SCUBA diving in the open ocean. To estimate the cost of transport, we measured the respiration rates of swimming and anesthetized salps collected in situ using jars equipped with non-invasive oxygen sensors. We found that linear colonies swim faster, which supports the idea that their differential advantage in frontal area scales with an increasing number of zooids. We also found that higher swimming speeds predict lower costs of transport in salps. These findings underscore the importance of considering propeller arrangement to optimize speed and energy efficiency in bioinspired underwater vehicle design, leveraging lessons learned from the diverse natural laboratory provided by salp diversity.