HYDRODYNAMICS OF COPEPODS: A REVIEW HOUSHUO JIANG1 and THOMAS R. OSBORN2,3 1Department of Applied Ocean Physics and Engineering, MS 12 Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA E-mail:[email protected] 2Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, MD 21218, USA 3Center for Environmental and Applied Fluid Mechanics, The Johns Hopkins University, Baltimore, MD 21218, USA (Received 1 October 2002; accepted 19 September 2003) Abstract. This paper reviews the hydrodynamics of copepods, guided by results obtained from recent theoretical and numerical studies of this topic to highlight the key concepts. First, we briefly summarize observational studies of the water flows (e.g., the feeding cur- rents) created by copepods at their body scale. It is noticed that the water flows at individ- ual copepod scale not only determine the net currents going around and through a copepod’s hair-bearing appendages but also set up a laminar flow field around the copepod. This laminar flow field interacts constantly with environmental background flows. Theoreti- cally, we explain the creation of the laminar flow field in terms of the fact that a free-swim- ming copepod is a self-propelled body. This explanation is able to relate the various flow fields created by copepods to their complex swimming behaviors, and relevant results obtained from numerical simulations are summarized. Finally, we review the role of hydro- dynamics in facilitating chemoreception and mechanoreception in copepods. As a conclu- sion, both past and current research suggests that the fluid mechanical phenomena occurring at copepod body scale play an important role in copepod feeding, sensing, swarming, mating, and predator avoidance. Keywords: chemoreception, copepod, feeding current, hydrodynamics, mechanoreception, numerical simulation, self-propelled 1. Introduction Calanoid copepods live in water environments. Water flows are created, whenever the copepods feed and/or swim by rapid beating of their anten- nae, mandibular palps, maxillules and maxillae (i.e., the cephalic append- ages), elicit an escape reaction resulting from the combined actions of the antennules and swimming legs, or even sink freely through the water col- umn by stopping all the activities of the appendages. (This is because most of them are negatively buoyant.) The water flows so created are crucial for the survival of the copepods, as they have to engage in various survival tasks of feeding, predator avoidance, and mating in the three-dimensional water environments (e.g., Yen and Strickler, 1996; Yen, 2000). Surveys in Geophysics 25: 339–370, 2004. Ó 2004 Kluwer Academic Publishers. Printed in the Netherlands. 340 HOUSHUO JIANG AND THOMAS R. OSBORN The study of the water flows created by copepods and associated inter- actions with environmental background flows forms the scope of the hydrodynamics of copepods. Generally, the studies of the hydrodynamics of copepods may be divided into three research directions: (1) the water flows at a copepod’s appendage scale, i.e., the water flows around and through a copepod’s hair-bearing appendages, (2) the water flows at the scale size of an individual copepod (e.g., the feeding current created by a copepod hovering in the water column, the flow field around a swimming copepod, and the vortical flow structure shed in the wake of a copepod eliciting an escape reaction), and (3) the interaction between the water flows created by a copepod and the environmental background flows sur- rounding it (e.g., the manner in which the small-scale turbulence around a copepod erodes the laminar feeding current created by the copepod). Important questions are the following: (i) How does a copepod alter its feeding current by adjusting its body orientation and the forcing which it applies to the adjacent water, in response to a turbulent eddy or turbu- lence-induced shear of a scale comparable with the size of the copepod? (ii) In general, how does a copepod change its swimming behavior and the- refore change the flow field around its body in response to the small-scale turbulence? (iii) How does the interaction between the flows created by a copepod and the environmental background flows affect the copepod’s sen- sory mechanisms such as mechanoreception and chemoreception? The study of the water flows at a copepod’s appendage scale has been reviewed by Jørgensen (1983), LaBarbera (1984) and Shimeta and Jumars (1991), mainly focusing on the mechanisms of suspension feeding. In addi- tion, Childress et al. (1987) utilized a number of highly simplified models of appendage motion, such as the movement of Stokeslets, spheres or stalks, to set up an average scanning current in Stokes flow in a suitable far-field formulation; they discussed the possible applications of these mod- els in understanding the feeding efficiency and strategies of small organisms such as copepods. Some more recent studies have discovered that different aspects of morphology and behavior are important in determining the leak- iness of a hair-bearing appendage at different Reynolds numbers, and have provided insights about the function of arrays of hair-like olfactory anten- nae (e.g., Koehl, 1992, 1995, 1996; Loudon et al., 1994). As to the study of the interaction between the water flows created by a copepod and the environmental background flows surrounding it, experi- mental studies have documented the behavioral responses of copepods to laboratory-generated turbulence (e.g., Costello et al., 1990; Marrase´ et al., 1990; Saiz and Alcaraz, 1992; Hwang et al., 1994; Caparroy et al., 1998). Marrase´ et al. (1990) revealed the difference in the flow field around a teth- ered copepod under two different background flow conditions: a non-tur- bulent condition and a turbulent condition. Kiørboe and Saiz (1995) HYDRODYNAMICS OF COPEPODS: A REVIEW 341 provided a simple analysis of the erosion of copepod feeding current by the small-scale turbulence. Osborn (1996) proposed a conceptual model of the interaction between a copepod’s feeding current and the surrounding small-scale turbulence, in which copepod feeding is considered as turbulent diffusion of the food in towards the region where the feeding current serves to capture the food well before it is identified. Although there is a large lit- erature on the effect of the small-scale turbulence upon the encounter rate between predators and prey (see the reviews in Dower et al. (1997), and Lewis and Pedley (2000)), only some speculations about the interactions between the copepod-created water flows and the environmental back- ground flows can be found from the up-to-date literature (e.g., Strickler, 1985; Granata and Dickey, 1991; Yamazaki, 1993; Yamazaki and Squires, 1996; Strickler et al., 1997). Studies of the interactions in a more dynamic way are still needed. That is, future studies are needed to investigate the spatial and temporal variations of the flow field around a copepod under the influence of environmental background flows at suitable spatial and temporal scales. Since one of these two research directions has been reviewed extensively by other researchers and the other has not yet been investigated extensively, we choose not to include them in this review. The main purpose of this paper is to review (from the viewpoint of hydrodynamics) the study of the water flows at individual copepod scale. The content includes (1) descriptions of the flow fields obtained from observational studies, (2) understanding the creation of the flow fields based on the evidence from both observations and theoretical analyses, (3) numerical simulations of the flow fields, and (4) effects of the flow fields on copepod sensory mechanisms. As has been pointed out at the very beginning, the water flows at individual copepod scale play an important role in copepod feeding, predator avoidance, and mating. It is noteworthy that the water flows at individual copepod scale not only determine the net water flows going around and through a copepod’s hair-bearing append- ages but also set up a laminar flow field around the copepod; the created laminar flow field is constantly under the influence of environmental back- ground flows. 2. Observations of the flow field at individual copepod scale More than twenty years ago, a high-speed microcinematographic technique based on a Schlieren optical pathway was used to observe the world of zooplankton such as copepods (Strickler, 1977, 1985; Alcaraz et al., 1980). In some earlier applications of this technique (e.g., Strickler and Bal, 1973; Strickler, 1975a, b, 1977; Kerfoot et al., 1980), the ‘‘footprints’’ created by free-swimming copepods were registered on film. In fact, these ‘‘footprints’’ 342 HOUSHUO JIANG AND THOMAS R. OSBORN have visualized the hydrodynamic disturbances created by the copepods at their body scale. The idea underlying this technique is direct visual obser- vation, which now has been widely used by researchers to study the feed- ing, swimming, breeding, and predator–prey interactions of zooplankton, and their interactions with environmental conditions. On the other hand, this technique directly contributed to an important finding – many cala- noid copepods create feeding currents – and made possible the quantifica- tion of the feeding currents. By analyzing the images taken on the film or later on the digital videotapes, researchers were able to measure the flow field (e.g., the feeding currents) created by copepods. 2.1. For a tethered copepod Many laboratory experiments used tethered copepods and measured the feeding currents created by them (e.g., Koehl and Strickler, 1981; Vander- ploeg and Paffenho¨ fer, 1985; Paffenho¨ fer and Lewis, 1990; Yen and Fields, 1992; Fields and Yen, 1993; Yen and Strickler, 1996; Fields and Yen, 1997; van Duren et al., 1998). Obviously, ‘‘tethering’’ made the data gath- ering much easier. Here, we adopt the descriptions by Fields and Yen (1993) of their measurement of the flow field around a tethered Pleuro- mamma xiphias (3.5 mm prosome length) to show the three-dimensional flow structure of the feeding current created by a tethered copepod (Figure 1).