Going with the flow: hydrodynamic cues trigger directed escapes from a stalking predator

Abstract

In the coevolution of predator and prey, different and less well-understood rules for threat assessment apply to freely suspended organisms than to substrate-dwelling ones. Particularly vulnerable are small prey carried with the bulk movement of a surrounding fluid and thus deprived of sensory information within the bow waves of approaching predators. Some planktonic prey have solved this apparent problem, however. We quantified cues generated by the slow approach of larval clownfish (Amphiprion ocellaris) that triggered a calanoid copepod (Bestiolina similis) to escape before the fish could strike. To estimate water deformation around the copepod immediately preceding its jump, we represented the body of the fish as a rigid sphere in a hydrodynamic model that we parametrized with measurements of fish size, approach speed and distance to the copepod. Copepods of various developmental stages (CII-CVI) were sensitive to the water flow caused by the live predator, at deformation rates as low as 0.04 s 21 . This rate is far lower than that predicted from experiments that used artificial predator-mimics. Additionally, copepods localized the source, with 87% of escapes directed away (greater than or equal to 908) from the predator. Thus, copepods' survival in life-threatening situations relied on their detection of small nonlinear signals within an environment of locally linear deformation.royalsocietypublishing.org/journal/rsif J. R. Soc. Interface 16: 20180776

Suction feeding performance and prey escape response interact to determine feeding success in larval fish

“…In those cases, the hydrodynamic signal available to the copepod was weaker, resulting in a late escape response and a shorter reaction distance to the approaching predator (Viitasalo et al 1998). Similar results were observed for Amphiprion ocellaris larvae feeding on the calanoid copepod Bestiolina similis (Tuttle et al 2019). In the present study we found an opposite trend: feeding success was limited to cases in which the larvae swam quickly towards the prey.…”

“…Previous studies of feeding success by larval fish have focused on their interactions with non-evasive prey (Hernández 2000; Krebs and Turingan 2003; China and Holzman 2014; China et al 2017) although several studies have reported on interactions between copepods and clownfish larvae (Jackson and Lenz 2016; Robinson et al 2019; Tuttle et al 2019). In small marine larvae that hatch from pelagic eggs, the hydrodynamic environment (denoted by Re) is the dominant factor that determines larval kinematics and prey capture performance.…”

“…The predictions of that CFD model for inert prey were the opposite: that the distance in which such prey could be captured decreased through ontogeny (Yaniv et al 2014). Observations on Amphiprion ocellaris larvae feeding on the calanoid copepod Bestiolina similis (Jackson and Lenz 2016; Tuttle et al 2019) revealed that feeding success on evasive prey increased throughout ontogeny, and that older larvae were able to capture more evasive prey and from a greater distance compared to younger larvae. The results of the present study (Fig 4A) further corroborate the CFD prediction (Yaniv et al 2014), and demonstrate that for interactions with evasive prey, Reynolds number is not the only parameter that determines strike success (Table 2).…”

“…Copepods are well-known for their ability to execute high-acceleration escape responses when sensing a hydrodynamic disturbances (Yen et al 1992; Fields and Yen 1997; Buskey et al 2002; Tuttle et al 2019). Viitasalo et al (Viitasalo et al 1998) assessed the factors that determine the success of predatory strikes by adult three-spine sticklebacks on two species of copepods.…”

“…Striking from too far would allow evasive prey long enough time (~70 ms; Fig 4B) to respond to the hydrodynamic disturbance produced by the predator. Conversely, a stealth approach followed by a fast lounge might provide the prey with little time (<10 ms) to respond to the predator (Tuttle et al 2019). Thus, despite being “noisier”, successful strikes on evasive prey might depend on striking fast to precede the prey’s escape response.…”