Cinematographic Analysis of the Feeding Mechanism of the Pelagic Tunicate Dol/Olum Nat/Onal/S

BULLETIN OF MARINE SCIENCE, 43(3): 404-412, 1988

CINEMATOGRAPHIC ANALYSIS OF THE FEEDING MECHANISM OF THE PELAGIC TUNICATE DOL/OLUM NAT/ONAL/S

Don Deibel and Gustav-Adolf Paffenhofer

ABSTRACT Doliolids are common and abundant pelagic tunicates that form extensive blooms in continental shelf waters by reproducing asexually. They feed at high rates using a fine mucous filter and deplete large areas of the ocean ofa wide size range of particles. We have developed methods for collecting and handling living doliolids that allow us to investigate the feasibility of observing thcir feeding mechanism using high-speed cinematography. These direct observations are needed to understand the factors that regulate feeding rate and food particle selection and to evaluate fluid mechanical constraints on the feeding process. We filmed the manipulation and ingestion of nanoplankton, the synchronous arrest of the gill cilia, and rapid "escape" contractions of the circumferential muscles. The gill cilia ceased beating in < 100 ms when a large or noxious particle touched the mouth. Some large particles (e.g., Ceratium spp.) were not rejected but were wrapped in a mucous cocoon and ingested. Escape contractions were rapid (duration of ca. 50 ms), and usually were preceded by ciliary arrest. Several observations, including a high flow velocity at the surface of the filter and low retention efficiency of small particles, suggest that the pore size of the filter may be relatively large- a characteristic that would be adaptive to the known neritic habitat of doliolids. We conclude that high-speed cinematography can be used to study the fluid mechanics of feeding, and details of particle selection and ingestion by transparent, gelatinous pelagic tunicates.

Doliolids are common and abundant pelagic tunicates inhabiting primarily continental shelf waters. They are transparent zooplankters that feed on a wide size range of suspended particles (from bacteria to diatoms) and detritus using a fine mucous filter. Doliolids have a complex life cycle, including obligatory al- ternation of sexual and polymorphic asexual generations. Asexual reproduction permits rapid multiplication and population growth in response to favorable conditions, often resulting in blooms covering hundreds of square kilometers (DeDecker, 1973; Atkinson et al., 1978; Deibel, 1985; Paffenh6fer and Lee, 1987). Within blooms, population density may be greater than I,OOO'm-3 (Deibel, 1985; Paffenh6fer and Lee, 1987),which, coupled with high feeding rates (Deibel, 1982b), results in depletion of a wide size range of particles from large areas of the ocean (Deibel, 1985). As a part of our program to understand the ecology of doliolids (Deibel, 1982a; 1982b; 1985; Paffenh6fer and Lee, 1987), we are interested in the feeding mechanism and how it relates to the selection and ingestion of food particles and how it affects the amount of energy needed to process water for feeding. Direct observation of the feeding mechanism provides a basis for understanding how feeding rate depends on body size, and on food particle size and abundance. Despite the ecological importance of doliolids, the feeding mechanism is poorly understood, owing to the difficulties in capturing and handling them and the technical demands of observing small, transparent, rapidly moving structures, such as cilia. We have developed techniques for capturing and handling doliolids (Deibel, 1982a; 1982b), which allow us to explore cinematographic techniques of observation. In this paper we report preliminary observations of the feeding mechanism and behavior of Dalia/urn natianalis using high-speed cinematography (HSC). The application

404 DEIBEL AND PAFFENHOFER: FEEDING MECHANISM OF DOLIOLUM 405 of HSC to copepod feeding has resulted in new information on important ecological processes, on a spatial scale of individual setules and food particles, and a temporal scale of milliseconds (see references in Price et aI., 1983). Although our ultimate aim is to quantify feeding kinematics and behaviors, our first goal was to investigate the feasibility of using HSC to observe the feeding apparatus of a transparent pelagic tunicate. General characteristics of the feeding mechanism of Dalia/urn rnulleri were described over 60 years ago by Fedele (1921). Fedele's account is the definitive description of doliolid feeding, and has much in common with other descriptions at that time of ascidian feeding by MacGinitie (1939) and Orton (1913). Although Fedele (1921) worked with D. rnulleri. we have observed D. natianalis, which is similar (Fig. 1). The anatomical nomenclature of pelagic tunicates has not been standardized, so we have followed the conventions of Berrill (1950). Dalia/urn natianalis is barrel shaped and surrounded by 8 muscle bands (Fig. 1). Muscle bands I and 8 control the opening of the anterior (branchial) and posterior (atrial) apertures, respectively, by regulating cuspate valves. The interior of the test is largely hollow, with a flattened gut and transverse gill septum per- forated by numerous slits. Food particles are collected by the pharyngeal mucous filter. The filter is secreted by the endostyle, and is drawn upward along the walls of the pharyngeal cavity by the peripharyngeal bands. The gill cilia pump water without assistance from the test muscles. This is fundamentally different from the feeding mechanism of salps, which swim continuously to feed using rhythmic contractions of their test muscles. The feeding current of doliolids flows into the test carrying suspended food particles through the branchial aperture. Water is drawn sequentially through the mucous filter, through the gill slits, and into the atrial cavity. The water leaves the test through the atrial aperture. Food particles are trapped on the filter. At the dorsal midline of the pharyngeal cavity the filter is wrapped into a mucous cord which extends from the ciliated funnel (at the anterior end of the cord) to the mouth (at the posterior end of the cord: Fig. 1). During active feeding the filter moves upward along the pharyngeal walls and is continuously wrapped into the cord, which rotates as it is drawn into the mouth. Food particles trapped in the filter are thus ingested.

METHODS

D. nationalis Borgert were collected 16-100 km southeast of Savannah, Georgia, U.S.A., on 20 and 27 May 1985, by slowly towing a 0.4 x 0.4 m Tucker Trawl (llO-/Lm mesh with a closed codend cup) in near-surface waters. At the various collection sites water temperature ranged from 24.1-26.4°C and salinity from 33-380/00.After each tow doliolids were sorted into I-liter or 4-liter glass jars containing unfiltered surface water that had been pumped onboard. Five films were made of four phorozooids ranging in chamber length from 2.0-3.3 mm. In the laboratory, doliolids were maintained in glass jars on a plankton wheel rotating at about I rpm. Two algal species were offered as food, Isochrysis galbana (4.5 /Lm diameter), and Thalassiosira weissflogii (12 /Lm diameter). Algae were used when in log-phase growth, and were raised in FIlO medium under a 12:12 light-dark photoperiod. Doliolids were preconditioned for 3-5 days on the same concentration of food as that used during filming (i.e., 0.15 mm' .1-1 of each alga for a total concentration of 0.30 mm' .1-1). Food concentration was monitored daily with a Coulter Counter TAIl and was maintained near the target concentration. Details of the filming apparatus and technique are described by Alcaraz et at. (1980), and Price et al. (1983). Traditionally, copepods are tethered to a hair for filming using cyanoacrylate glue. This technique cannot be used for doliolids. Prior to filming, doliolids were transferred to an optical cuvette containing 400 ml of glass-fiber filtered seawater (at 20°C) to which algae had been added as described above, and were tethered either to a fine, glass capillary tube by suction or to a fine, plastic capillary tube using petroleum jelly. The tube was attached to a Brinkmann micromanipulator for positioning doliolids during filming. Tethering doliolids for movies is not likely to introduce artifact because they 406 BULLETIN OF MARINE SCIENCE, VOL. 43, NO.3, 1988

~".; ,," ,'" "

1mm

Figure 1. Line drawing of a phorozooid of Dolia/urn natianalis. The drawing was traced from a cine film with structural details added after examining a preserved specimen. This individual is 3.32 mm long. It is oriented approximately in lateral view, B = brain; T = tunic; MC = a schematic representation of the mucous cord; PC = pharyngeal cavity; CF = ciliated funnel; BA = branchial aperture; V = valve; PB = peripharyngeal bands; EN = endostyle; GS = gill septum; MB = muscle band; MO = mouth; ES = esophagus; ST = stomach; VP = ventral peduncle; AA = atrial aperture; AC = atrial cavity; I = intestine; A = anus. Inset: a close-up view of several gill slits (SL) showing the cilia (C). Other labels are defined above. The nomenclature follows Berrill (1950), do not swim to feed as do salps and most copepods, but rather are motionless in the water column and use only their gill cilia to pump water. Five films were made with portions of each exposed at 30, 60, 125, or 250 frames's-1 (fps), Most of the films were made using a phase contrast condenser to simulate dark-field illumination so that transparent structures were visible, We used Eastman Ektachrome 7250 newsfilm (ASA 400). The films were viewed frame-by-frame using a LW photo-optical data analyzer (model 224-A). Images were projected onto paper and structures and particle trajectories plotted by hand. Phytoplankton cells in the plane of focus were used to establish length scales, and stationary structural features of the doliolid test were used to establish frame-to-frame justification marks. Length measurements were made on the projected images with vernier calipers to the nearest 100 /.tm. Velocities of cilia or phytoplankton cells were determined by measuring their displacement from frame to frame for as long as they remained in the focal plane.

RESULTS Feeding Mechanism ofDoliolum nationalis.-In general, the major features of the feeding mechanism were as described by Fedele (1921; see Introduction). However, we were able to observe and record several aspects of the feeding behavior which are of ecological importance. The mucous filter lines most of the pharyngeal cavity, and all of the water that is pumped into the branchial aperture must pass through the filter before it is expelled via the atrial aperture. At times, the doliolids seemed to cease production of the filter while maintaining the flow DEIBEL AND PAFFENHOFER: FEEDING MECHANISM OF DOL/OLUM 407

of water, as particles that were normally trapped in the filter periodically were observed to flow unimpeded through the gill slits and out of the test via the atrial aperture. This suggeststhat the gillcilia are controlled independently of the ciliated secretory cells of the endostyle. There was no sign of a dorsal groove, which in ascidians extends from the ciliated funnel to the mouth, and contains cilia for wrapping the mucous cord. In addition, we did not observe cilia lining the walls of the pharyngeal cavity, which in ascidiansassist in transporting the filter. Thus, the mechanism by which the mucous filter is manipulated in the pharyngeal cavity of D. nationalis is stil1unknown. When the mucous cord was within the focal plane it was possible to measure the time taken for individual Thalassiosira weissjlogii cells trapped within the cord to travel a known distance across the pharyngeal cavity. The resultant translation velocity of the mucous cord contains information on how rapidly the mucous filter is produced and ingested, and can be compared to the velocity of the inflowing water current to estimate the resistance to flow at the surface of the filter, which is needed to determine the amount of energy required to pump water through it (Deibel and Powell, 1987). The mean (±SD) translation velocity was 36.4 ± 10.6 ~m's-I (N = 7), based on net displacement which does not include rotation of the cord. Ciliary Arrest. -Before these HSC investigations, one of us (D.D.) observed Do- liolum nationalis ingesting large Ceratium spp. cells by cyclic reversal of the mucous cord, moving th~ cells first to the mouth, then backward to the ciliated funnel, then returning to the mouth, and back again. Up to six cycles of reversal were observed before some of the cells were ingested. Each reversal of the mucous cord was preceded by synchronous arrest of all the gill cilia (Fig. I). The mucous cord seemed to crush the spines on Ceratium, making the cells small enough to be ingested even though initially they were much larger than the mouth. HSC enabled us to record details of these ciliary arrest events. During arrest, the cilia appeared to lie flat along the edges of the gill slits, but they may have flicked through the slits to resume beating on the opposite side, creating a reversal of the water current. Arrest was synchronous over the entire gill septum. We measured the duration of this event in three cases at 60 fps. The mean (±SD) time from beginning of arrest to the absence of movement was 72.2 ± 9.6 ms. Only one event was filmed which included the resumption of ciliary activity following arrest. The cilia remained motionless for 3.7 s, after which 1.9 s elapsed before all resumed beating. We were not able to record reversal of the mucous cord (such as during the ingestion of Ceratium spp.), probably because we did not offer D. nationalis any extremely large or spiny particles during filming. Reynolds Number. - The Reynolds number (Re) is a dimensionless index whose magnitude indicates the relative importance of viscous and inertial forces in fluid flows, and is given by Re = Dsd/v, where D is the density of seawater at 20°C 3 and 330/00(1.025g' cm- ), s the flowvelocity pastthe cilium (cm 'S-I), d the diameter of the cilium (cm), and v the kinematic viscosity of seawater at 20°C and 33%0 (1.1'10-2 g,cm-I·s-I: Koehl and Strickler, 1981). For a cilium, s = fl, where fis the beat frequency (cycles's-I), and Ithe length of the cilium (cm: Holwill, 1974), so that Re = Dfld/v. A value of Re ~ I indicates that viscous forces dominate flow, while a value:» 1 indicates that inertial forces dominate flow. Because of insufficient magnification, "d" could not be determined from the films, but was estimated to be 130 ± 20 nm (±SD, N = 11) from transmission electron micrographs (TEM) of the gill cilia of Dolioletta gegenbauri (Deibel and Dickson, unpublished). This diameter is similar to values for the cilia of a wide 408 BULLETIN OF MARINE SCIENCE, VOL. 43, NO.3, 1988 variety of invertebrates (100 nm: Holwill, 1974). The length of the gill cilia can be measured from the films (ca. 50 ,um), which agrees with values for many invertebrates (10-100 ,um: Holwill, 1974). The mean (±SD) beat frequency estimated from the films was 11.2 ± 1.15 cycles·s-I (N = 22). Assuming no shrinkage of the cilia during preparation for TEM, the resulting Re was 6.8' 10-5, indicating predominance of viscous forces and laminar flow patterns. Filtration Rate. - Filtration rate (i.e., the total volume of water pumped per unit time) was calculated by multiplying the flow velocity (estimated by plotting the trajectories of phytoplankton cells frame-by-frame) by the cross-sectional area of the branchial aperture. If a particle had a long trajectory, the mean velocity was calculated using only instantaneous velocities that were within 15% of the maximum. Filtration rate ranged from 58.5-143 ml·d-1 (Table 1), which does not take into account time spent not pumping, e.g., during ciliary arrest. Escape Response. -Periodically, doliolids utilize their circumferential muscles to execute rapid but controlled movement by jet propulsion (Bone and Trueman, 1984). They can move in either the anterior or posterior direction. Before the fluid mechanics and energetics of this behavior can be quantified, its kinematics must be described. We found that each escape "event" can be divided into three phases; (1) ciliary arrest, (2) muscle contraction, and (3) inflation of the test. Sometimes contraction of the muscles was preceded by ciliary arrest, but on other occasions ciliary arrest occurred later, during contraction. Ciliary arrest was observed in a single case to last for 50 ms, followed by rapid contraction of the muscles taking only 57.9 ± 8.7 ms (±SD, N = 10). Inflation was much slower (191 ± 79 ms, ±SD, N = 10). Occasionally, up to four consecutive escape contractions occurred only 53.4 ± 14.0 ms apart (±SD, N = 5).

DISCUSSION Mucous Filter Translation. - The mean translation velocity of the mucous cord was 36.4 ,um's-l, which is similar to that of the ascidian Clavelina spp. (7-78 ,um·s-l: Flood, 1982), and the appendicularian Oikopleura vanhoeffeni (51-94 ,um' S-l: Steel and Powell, unpublished). This suggests a functional similarity between manipulation of the pharyngeal filters of three groups of tunicates, living in diverse habitats with different feeding mechanisms, but all making use of fine mucous filters to collect small particles from suspension. Knowledge of the relative velocity of the mucous filter and feeding current is needed to determine the effective pressure drop across the filter (Flood, 1982), and thus to estimate the energy required to pump water through it (i.e., the force needed to overcome resistance). The animal for which the mucous cord translation velocities were measured (2.17 mm chamber length) was used to estimate the flow velocity at the surface of the filter as follows. We assumed that the pharyngeal mucous filter can be approximated by a cone with diameter equal to the distance between the peripharyngeal bands (0.70 mm as measured from the film), and height equal to the distance between the peripharyngeal bands and the mouth (0.98 mm, also measured from the film). Using the equation for the lateral surface area of a cone (A = 1TRyR2 + h2) where R is the radius (cm) and h the height (cm), we arrived at a total surface area of 1.14.10-2 cm2. The mean filtration rate I 3 3 1 of 90.5 ml·d- (or 1.04·1O- ·cm ·s- : Table 1) then was divided by the lateral surface area, resulting in a flow velocity at the surface of the filter of 912 ,urn'S-I. This is at the upper end of the range of values for ascidians and gastropods with I ciliary pumping mechanisms (400-800 ,um's- : J0rgensen et aI., 1984), and is DEIBEL AND PAFFENHOFER: FEEDING MECHANISM OF DOL/aLUM 409

Table I. Filtration rates. Each row contains data from the trajectory ofa singlecell. The total number of positions plotted (i.e., total N) was 32 for these 5 cells. Only particle velocities within 15% of maximum velocities were used to calculate the mean (±SD). All measurements are from film B at 60 fps (frames per s).

Mean particle velocity (mm· ,-I) (N) Filtration rate (m!'d-I)

4.33 ± 0.26 (4) 113 3.34 ± 0.19 (4) 78.1 4.99 ± 0.36 (3) 143 3.92 ± 0.23 (3) 59.8 2.89 ± 0.12 (4) 58.5 x 3.83 90.5 ±SD 0.78 36.7 N 18 5

about three times higher than that estimated for Oikopleura vanhoeffeni with a muscular-ciliary pumping mechanism (300 ~m 'S-I: Deibel and Powell, 1987). However, it is much lower than the flow velocity in polychaetes with muscular pumping mechanisms (3,000 ~m's-I: Jorgensen et aI., 1984). The high flow rate at the surface of the pharyngeal filter of D. nationalis suggeststhat it has a relatively coarse mesh, which would be adaptive to the neritic habitat of doliolids and the requirement for handling relatively large particles (Deibel, 1985). The pore size of the pharyngeal filter of doliolids has not yet been determined experimentally. We could not estimate the translation velocity of the pharyngeal filter directly, because of the limited depth of field of our films. However, it has been shown that the translation velocity of the filter of ascidians is 2-3 times that of the mucous cord (Flood, 1982). Assuming a similar relationship for D. nationalis, the flow rate at the surface of the filter (912 ~m's-I) is still many times higher than is its estimated translation velocity (about 90 ~m' S-I).The ascidian Clavelina spp. is different-the velocity of flow at the surface of the pharyngeal filter is of the same order of magnitude as its translation velocity (Flood, 1982). Therefore, the energetic cost of pumping water for feeding may be relatively high for D. nationalis in comparison to Clavelina spp., if the pore sizes of the filters are similar. Ciliary Arrest. -Ciliary arrest and reverse translation of the mucous cord enable Doliolurn nationalis to ingest large or spiny cells. Galt and Mackie (1971) found that ciliary arrest and reversal by Oikopleura spp. was triggered by the contact of large or noxious particles with the lip. It is likely that ciliary arrest in doliolids is triggered by signals from sensory cells of the mouth (Fedele, 192I; Braconnot, 1971; Bone and Mackie, 1977). Thus, ciliary arrest must be important in the manipulation oflarge particles prior to ingestion. Fedele (1921) observed ciliary arrest and mucous cord reversal by Doliolurn rnul/eri attempting to ingest chains of large Chaetoceros spp. and large, spiny Ceratiurn spp. However, he noted that ciliary arrest was always followed by muscle contraction and backflushing of the pharyngeal cavity. He apparently did not observe the persistent cyclic reversals of the mucous cord culminating in ingestion of the particle as we did for D. nationalis. Our films show that ciliary arrest can occur without reverse translation of the mucous cord, although it appears that arrest must occur if reverse translation is to take place. This fact may explain why some cycles of ciliary arrest seem unrelated to either test muscle contraction or to external vibrational stimuli (Bone and Mackie, 1977). Thus the gill cilia must be controlled independently from cilia 410 BULLETIN OF MARINE SCIENCE, VOL. 43, NO.3, 1988

of the endostyle, peripharyngeal bands, and ciliated funnel. Bone and Mackie (1977) found that initial arrest of the gill cilia of D, mulleri is followed by a cycle of 10-20 additional arrest events separated by increasing intervals until ciliary beating is continuous. Although a similar sequence may occur for D. nationalis, our films were not sufficiently long to document events of this duration. Ciliary arrest may be primarily an adaptation of the feeding apparatus for manipulating large particles or particles of complex shape, and may not be a fundamental part of the escape response repertoire. MacGinitie (1939) described ciliary arrest in several ascidians, reporting that the cilia collapsed onto the surface of the stigmata. The way in which the gill cilia of D, nationalis resume beating is similar to that of ascidians (MacGinitie, 1939). The cilia begin beating synchronously at the center of each gill slit on opposing sides. Initiation of beating then spreads to each end of each gill slit. The duration of ciliary arrest of D, nationalis (50-70 ms) is similar to the duration of the ciliary arrest potentials of D. mulleri (20 ms: Bone and Mackie, 1977). These facts once again show functional similarities among the tunicates. Reynolds Number. - The metachronal beat frequency of D. nationalis (11 Hz) is similar to that of Oikopleura spp. (10-15Hz: Galt and Mackie, 1971), and is in the middle of the range of values given for bivalves, ctenophores, and protozoans (2-30 Hz: Sleigh, 1974). For D. nationalis Re < 1, indicating that flow past the gill cilia is laminar and is dominated by viscous forces. This Re is similar to that of ciliated protozoans and bivalves, but is larger than that of sponges in which 7 flow is driven by flagellate choanocytes (10- ), and is smaller than that of the 2 setae of copepods (10- : J0rgensen, 1983). Filtration Rates. - We could find no estimate of filtration rate or clearance rate for Doliolum nationalis in the literature. However, we can compare our direct estimates of filtration rate to particle clearance rates of Dolioletta gegenbauri fed Isochrysis galbana in the laboratory at the same temperature (Deibel, 1982b). For an animal 2.17 mm long, our direct estimate (90.5 ml·d-I: Table 1)is about 50% greater than the empirically-determined clearance rate of D. gegenbauri (56 mI, l d- : Deibel, 1982b). This suggests that there may be a difference between these species in filtration rate that is not related to animal size, or that the clearance rate is lower than the filtration rate due to (1) a particle retention efficiency of < 100%, (2) inclusion in the clearance rate estimates of time spent not feeding, i.e., in ciliary arrest and/or mucous cord reversal, (3) the clearance rate estimates being determined for several animals together in a grazing chamber and expressed as a mean rate. It is likely that particle retention efficiency is considerably less than 100%, since Andersen (1985) found values of <50% for Salpa fusiformis feeding on particles the size of Isochrysis galbana. There is promise in the use of HSC to determine directly the retention efficiencyof various sizes of particles by mucous net suspension feeders. Escape Response. -Anyone who has tried to capture a doliolid in a pipet is aware of their astonishingly rapid escape capabilities. Bone and Trueman (1984) dis- covered that an individual D. denticulatum 4.5 mm long can attain an instanta- I neous velocity of 25 cm's-I, or >50 body lengths·s- . They found that muscle contractions can be graded to adjust the magnitude of thrust over a wide range, and that directional control is highly coordinated, with closure of the appropriate cuspate valve by contraction of muscle band 1(for forward thrust) or muscle band 8 (for reverse thrust) preceding the contraction of the test muscles by only 2-2.5 ms. Our estimate of the duration of the contraction phase (48-67 ms) is similar DEIBEL AND PAFFENHOFER: FEEDING MECHANISM OF DOL/OLUM 411 to that of D. denticulatum (31-60 ms: Bone and Trueman, 1984). However, the duration of the inflation phase in D. nationa/is (112-400 ms, our observations) is somewhat longer than that of D. denticulatum (63-120 ms: Bone and Trueman, 1984). However, this difference may be due to the subjective definition of when the test is fully inflated. Because doliolids seem to be sensitive to the approach of a pipet, it is likely that this "escape" behavior is an aversion response to predators. However, no direct observations of this behavior have been reported from the field. The escape response resembles the "squirt" of ascidians, which is done primarily to dislodge and reject unsuitable food particles (MacGinitie, 1939). Similarly, Fedele (1921) found that D. mul/eri utilizes muscle contraction to backflush the pharyngeal cavity. However, our observations of D. nationa/is ingesting Ceratium spp. by cyclic mucous cord reversal suggest that this doliolid may manipulate food particles first, using muscle contractions to backflush the filter only as a last resort. In addition, doliolids are slightly negatively buoyant, and may use jet propulsion to swim upward to maintain vertical position (Bone and Trueman, 1984). We have demonstrated that high-speed films can be made of relatively transient behaviors oftransparent zooplankters. The films show that doliolids have unique adaptations for particle manipulation enabling them to inhabit particle-rich continental shelf waters. Questions that need to be addressed are the mechanism of particle size selection, particle manipulation behavior, and the energetics ofmuco- ciliary suspension feeding.

ACKNOWLEDGMENTS

We thank Captain J. Galt and the crew of the R/V BLUEFIN for assisting us with the collection of Dalialurn natianalis. D.D. thanks R. F. Lee for loaning him a stereomicroscope, and C. Galt for suggesting ways to tether gelatinous animals. We are grateful to the Department of Engineering, Memorial University, for the photo-optical data analyzer. L. MacInnis assisted with translation of Fedele (\921). S. H. Lee drafted the figure, and G. Hillier did the darkroom work. D.D. thanks G.-A. and E. Paffenh6fer for their hospitality during his stay in Savannah. This work was supported by National Science Foundation grant OCE85-00917 to G.-A.P., and by an Operating Grant from the Natural Sciences and Engineering Research Council of Canada to D.D. The authors thank the Skidaway Institute of Oceanography for organizing and supporting the Zooplankton Behavior Symposium. MSRL contribution no. 699, and NICOS contribution no. 145.

LITERATURE CITED

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DATEACCEPTED:April 6, 1988.

ADDRESSES:(D.D.) Marine Sciences Research Laboratory, and Newfoundland Institutefor Cold Ocean Science, Memorial University of Newfoundland, St. John's, NewfoundlandAlC 5S7, Canada; (G.-A.P.) Skidaway Institute of Oceanography, P.O. Box 13687, Savannah, Georgia 31406.

ApPENDIX: DISCUSSION AFTER DEIBEL AND PAFFENHOFER L. Madin: Do doliolids reject unwanted particles similarly to the way salps clean their filters? D. Deibel: Perhaps. Doliolids can and do backflush the pharyngeal cavity to reject unwanted particles, which some, but not all, salps do. We do not know whether salps manipulate particles by reverse translation of the mucous cord, as we have observed for doliolids. Finally, it appears that both salps and doliolids can suspend production of the mucous net while continuing to pump water, thus rejecting unwanted particles by passing them out the atrial aperture.