Propulsion in Cubomedusae: Mechanisms and Utility

Evolutionary constraints which limit the forces produced during bell contractions of medusae affect the overall medusan morphospace such that jet propulsion is limited to only small medusae. Cubomedusae, which often possess large prolate bells and are thought to swim via jet propulsion, appear to violate the theoretical constraints which determine the medusan morphospace. To examine propulsion by cubomedusae, we quantified size related changes in wake dynamics, bell shape, swimming and turning kinematics of two species of cubomedusae, Chironex fleckeri and Chiropsella bronzie. During growth, these cubomedusae transitioned from using jet propulsion at smaller sizes to a rowing-jetting hybrid mode of propulsion at larger sizes. Simple modifications in the flexibility and kinematics of their velarium appeared to be sufficient to alter their propulsive mode. Turning occurs during both bell contraction and expansion and is achieved by generating asymmetric vortex structures during both stages of the swimming cycle. Swimming characteristics were considered in conjunction with the unique foraging strategy used by cubomedusae.     

Behavioural response of the scyphozoan jellyfish Aurelia aurita (L.) upon contact with the predatory jellyfish Cyanea capillata (L.)

Underwater manipulative experiments were carried out in situ to investigate the sensibility of the jellyfish Amelia aurita (L.) to contact with the tentacles of Cyanea capillata (L), commonly known as a predator on A. aurita. Movements of individual medusae touched by tentacles of C. capillata and other objects were video‐recorded during SCUBA dives. The behavioural variable studied was change in swim pulse frequency. The results showed that A. aurita was highly susceptible to the tentacles of C. capillata and responded with an increased swim pulse frequency when touched at the umbrellar margin but not at the central exumbrella. Contact with other objects also induced a behavioural response in A. aurita.     

Structure and function of the locomotory system ofPolyorchis montereyensis (Cnidaria,Hydrozoa)

Summary 1. The structure and function of the locomotory system of the anthomedusanPolyorchis montereyensis Skogsberg were studied in detail. Anatomical investigations were carried out primarily on fresh or formalin fixed specimens; histology was done on specimens fixed in Bouin's fluid. Functional analyses were based largely on photography and cinematography.2. Swimming inP. montereyensis involves the alternating antagonistic action of the subumbrellar swimming muscles and the elastic mesoglea.3. The swimming muscle consists of striated contractile elements arranged circularly in four discontinuous subumbrellar sheets and a sheet on the subumbrellar side of the velum. There is also a sheet of radially arranged fibers on the exumbrellar side of the velum. The four subumbrellar sheets are anchored to the bell along the perand interradii.4. The mesogleal skeleton consists of five components: (a) the matrix of the bell mesoglea, (b) optically visible fibers that traverse the bell from gastrodermal lamella to exumbrella, (c) the basement membrane or supporting lamella, (d) a system of joints, and (e) the velar mesoglea.5. The morphology, orientation, and distribution of the mesogleal fibers suggest that their major role is maintaining the radial integrity of the bell during deformation. The amount of stretch in a region of the bell wall during contraction is inversely proportional to the number of fibers per unit area there. In regions of the bell which are not deformed during contraction fibers are sparse or absent.6. Mesogleal volume remains constant during swimming. Locally the mesoglea is subjected to forces of stretch and compression, but the critical element in narrowing the bell involves bending or folding the mesoglea around a series of structural joints. The fulcrum of these joints is anchored to the exumbrella by concentrations of mesogleal fibers. The joints consist of eight adradial regions of highly deformable mesoglea lacking visible fibers. The regions are triangular in cross section and are separated from the remainder of the mesoglea (98–99 % of the total) by the gastrodermal lamella. A circular apical joint is also present.7. Sequential changes in shape and position of the bell relative to a fixed grid during contraction and recovery were measured in order to determine such parameters of swimming as rate of contraction, rate of expulsion of water, change in bell velocity during contraction and recovery, momentum, etc.8. The function of the velum was determined by cinematographic analysis of swimming animals both before and after removal of the velum. In normal swimming the velum serves mainly to constrict the aperture of the bell, thus increasing the velocity of expelled water, and hence increasing the force driving the medusa foward. Medusae swam with a greatly decreased velocity after velum removal.9. Turning is accomplished primarily by asymmetrical contraction of the exumbrellar velar radial muscles, whereby the velar aperture is displaced to one side, water is expelled obliquely, and the bell turns toward that same side. The ability to turn was lost after velum removal.10. Studies of the relationship between individual size and the various parameters of swimming inP. montereyensis show that: (a) the duration of the contraction phase of the swimming beat is roughly proportional to the square root of the subumbrellar circumference (or bell height); (b) smaller individuals swim faster relative to their bell height than do larger ones; (c) the velum is relatively better developed in small animals and plays a proportionately more important role during swimming.

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Struktur und Funktion des lokomotorischen Systems vonPolyorchis montereyensis (Cnidaria, Hydrozoa)

Kurzfassung Mit Hilfe histologischer und kinematographischer Methoden wurden Bau und Funktion des lokomotorischen Systems der AnthomedusePolyorchis montereyensis Skogsberg analysiert. Die Schwimmbewegungen resultieren aus der antagonistischen Wirkung der Muskulatur der Subumbrella und der elastischen Mesogloea. Struktur, Anordnung, Verteilung und Verankerung der Muskelzellen werden beschrieben. Die Körperschicht der Mesogloea besteht aus 5 Komponenten mit Skelettfunktion: der Matrix der Schirmmesogloea, einem System von Muskelfasern, der Stützlamelle, acht adradialen Verbindungssträngen von stark deformierbarer, fibrillenloser Mesogloea und der Mesogloea des Velums. Die Fribrillen und Verbindungsstränge der Mesogloea bewirken, daß die Schirmglocke während der Kontraktion deformiert werden kann. Die Funktion des Velums sowie die kontinuierlichen Veränderungen der Glocke in bezug auf Form und Lage während des Schwimmvorgangs, insbesondere die Geschwindigkeit der Kontraktion, des Wasserausstoßes und der Fortbewegung bei Individuen verschiedener Größe, wurden eingehend untersucht. Kleinere Medusen schwimmen relativ schneller als größere, was hauptsächlich auf stärkere Kontraktionen des Velums zurückzuführen ist.

     

Kinematic properties of the jellyfish Aurelia sp.

A new, relatively simple method for determining the kinematic properties of jellyfish is presented. The bell movement of the scyphomedusa (Aurelia sp.) during its pulsation cycle was analysed using computer-aided visualization. Sequences of video images of individual Aurelia in a large aquarium were taken using a standard video camera. The images were then processed to obtain time series of the relative positions of selected points on the surface of the medusa’s bell. The duration of the bell relaxation was longer than that of the bell contraction, thereby confirming published results. In addition, the area of the exumbrellar surface of Aurelia increased during bell relaxation by more than 1.3-times that of the exumbrellar surface area during the maximum contraction of the bell. The volume change during the bell pulsation cycle was also measured using the same visualization method. Significant changes, of up to 50%, in the subumbrellar cavity volume were revealed while, in contrast, the volume between the exumbrellar and subumbrellar surfaces generally remained unchanged during the entire pulsation cycle of the bell. Comparison of the time series of the exumbrellar surface area and of the subumbrellar cavity volume indicated that the change of volume takes place before the change of the surface area of the bell. Guest editors: K. A. Pitt & J. E. Purcell Jellyfish Blooms: Causes, Consequences, and Recent Advances     

Reynolds number limits for jet propulsion: a numerical study of simplified jellyfish

The Scallop theorem states that reciprocal methods of locomotion, such as jet propulsion or paddling, will not work in Stokes flow (Reynolds number=0). In nature the effective limit of jet propulsion is still in the range where inertial forces are significant. It appears that almost all animals that use jet propulsion swim at Reynolds numbers (Re) of about 5 or more. Juvenile squid and octopods hatch from the egg already swimming in this inertial regime. Juvenile jellyfish, or ephyrae, break off from polyps swimming at Re greater than 5. Many other organisms, such as scallops, rarely swim at Re less than 100. The limitations of jet propulsion at intermediate Re is explored here using the immersed boundary method to solve the 2D Navier-Stokes equations coupled to the motion of a simplified jellyfish. The contraction and expansion kinematics are prescribed, but the forward and backward swimming motions of the idealized jellyfish are emergent properties determined by the resulting fluid dynamics. Simulations are performed for both an oblate bell shape using a paddling mode of swimming and a prolate bell shape using jet propulsion. Average forward velocities and work put into the system are calculated for Re between 1 and 320. The results show that forward velocities rapidly decay with decreasing Re for all bell shapes when Re<10. Similarly, the work required to generate the pulsing motion increases significantly for Re<10. When compared to actual organisms, the swimming velocities and vortex separation patterns for the model prolate agree with those observed in Nemopsis bachei. The forward swimming velocities of the model oblate jellyfish after two pulse cycles are comparable to those reported for Aurelia aurita, but discrepancies are observed in the vortex dynamics between when the 2D model oblate jellyfish and the organism. This discrepancy is likely due to a combination of the differences between the 3D reality of the jellyfish and the 2D simplification, as well as the rigidity of the time varying geometry imposed by the idealized model.     

Contrast and rate of light intensity decrease control directional swimming in the box jellyfish Tripedalia cystophora (Cnidaria, Cubomedusae)

Box jellyfish respond to visual stimuli by changing the dynamics and frequency of bell contractions. In this study, we determined how the contrast and the dimming time of a simple visual stimulus affected bell contraction dynamics in the box jellyfish Tripedalia cystophora. Animals were tethered in an experimental chamber where the vertical walls formed the light stimuli. Two neighbouring walls were darkened and the contraction of the bell was monitored by high-speed video. We found that (1) bell contraction frequency increased with increasing contrast and decreasing dimming time. Furthermore, (2) when increasing the contrast and decreasing the dimming time pulses with an off-centred opening had a better defined direction and (3) the number of centred pulses decreased. Only weak effects were found on the relative diameter of the contracted bell and no correlation was found for the duration of bell contraction. Our observations show that visual stimuli modulate swim speed in T. cystophora by changing the swim pulse frequency. Furthermore, the direction of swimming is better defined when the animal perceives a high-contrast, or fast dimming, stimulus.     

GALVANIC STIMULATION OF LUMINESCENCE IN PELAGIA NOCTILUCA

1. Pelagia noctiluca responds to galvanic stimulation by a luminescent glow at the anode. If placed near the cathode a secondary glow occurs also on the cathodal side. 2. The luminescent slime of Pelagia when subjected to the galvanic current glows around the cathode. This is referred partly to the movement of hydrogen bubbles, but in the main to the alkali formed at the cathode. 3. The cause of galvanic stimulation in Pelagia is ionic. (1) Anodal stimulation is referred to the blocking of positive ions by the tissue on that side. (2) Cathodal stimulation, when the animal lies near the cathode, is due to the diffusion of alkali outward from a region of high concentration (the cathode). 4. Only the margin of the bell is excited to luminescence by the galvanic current. It is therefore concluded that nervous elements are the seat of excitation. 5. Luminescence is not a result of muscular contraction, since K ion causes relaxation of musculature but a continuous luminescent glow in Pelagia. The galvanic current causes pulsations of the bell (contraction and relaxation of the musculature) but a continuous glow.     

Do blind cavefish have behavioral specializations for active flow-sensing?

Blind cavefish use a form of active sensing in which burst-coast swimming motions generate flow signals detected by the lateral line. To determine if blind cavefish have evolved behavioral specializations for active flow-sensing, including the ability to regulate flow signal production through lateral line feedback, the swimming kinematics of blind and sighted morphs of Astyanax were compared before and after 24?h of familiarization with a novel, dark environment and with and without lateral line functionality. Although both morphs showed little difference in the vast majority of kinematic parameters measured, blind morphs differed significantly from sighted morphs in having a much higher incidence of swim cycle sequences devoid of sharp turns. Both lateral line deprivation and familiarization with the arena led to significant declines in this number for blind, but not sighted morphs. These findings suggest that swimming kinematics are largely conserved, but that blind morphs have nevertheless evolved enhanced abilities to use lateral line feedback when linking swim cycles into continuous, straight trajectories for exploratory purposes. This behavioral specialization can best be understood in terms of the intermittent and short-range limitations of active flow-sensing and the challenges they pose for spatial orientation and navigation.     

The presence and distribution of Antho-RFamide-like material in scyphomedusae

Summary The nervous systems of the scyphomedusae Chrysaora hysoscella, Cyanea capillata and Cyanea lamarckii (Phylum Cnidaria) were stained using an antiserum against the anthozoan neuropeptide Antho-RFamide. Staining was widespread in all three species. In Chrysaora, the antiserum revealed nerve nets in the subumrella and exumbrella ectoderm, in both faces of the oral lobes, and in the endoderm lining the subumbrella and exumbrella surfaces of the gastric cavity. The most prominent staining occurred in a dense plexus of neurons in the ectoderm at the base of the tentacles. This nerve net sent projections into the subumbrella ectoderm. For the most part, staining in the two species of Cyanea was similar to that in Chrysaora, with a few exceptions. These include the presence, in Cyanea, of an obvious tentacular nerve tract and nerve nets associated with clusters of cnidocytes in the tentacles. Radioimmunoassays of extracts from Chrysaora and Cyanea lamarkii revealed that both species contain large amounts of Antho-RFamide-like material (up to 55 nmol/animal). The results indicate that Antho-RFamide-like neuropeptides are widespread in scyphomedusae.     

The energy density of jellyfish: Estimates from bomb-calorimetry and proximate-composition

Two techniques are described to calculate energy densities for the bell, gonad and oral arm tissues of three scyphozoan jellyfish (Cyanea capillata, Rhizostoma octopus and Chrysaora hysoscella). First, bomb-calorimetry was used, a technique that is readily available and inexpensive. However, the reliability of this technique for gelatinous material is contentious. Second, further analysis involving the more labour intensive proximate-composition analysis (protein, fat and carbohydrate) was carried out on two species (C. capillata and R. octopus). These proximate data were subsequently converted to energy densities. The two techniques (bomb-calorimetry and proximate-composition) gave very similar estimates of energy density. Differences in energy density were found both amongst different species and between different tissues of the same species. Mean (± S.D.) energy density estimates for whole animals from bomb-calorimetry were 0.18 ± 0.05, 0.11 ± 0.04, and 0.10 ± 0.03 kJ g wet mass^− 1 for C. capillata, R. octopus, and C. hysoscella respectively. The implications of these low energy densities for species feeding on jellyfish are discussed.     

Scaling of ctenes and consequences for swimming performance in the ctenophore Pleurobrachia bachei

Ctenophores coordinate large macrociliary structures called ctenes to propel themselves through the water. The morphology and kinematics of the ctenes mediate swimming performance. We investigated morphological and kinematic factors affecting swimming performance in free‐swimming ctenophores (Pleurobrachia bachei) using high speed videography. Our morphological results showed that the relationship between body size and ctene morphology and arrangement in P. bachei were well described using linear (i.e., isometric) relationships, which suggests functional limitations of ctenes that vary among individuals of different sizes. Our kinematic results showed that isometric constraints on swimming performance can potentially be overcome by alterations in kinematics: (a) swimming speed in P. bachei increased with ctene beat frequency over a range of body lengths, and (b) the separation of ctenes into clumps of cilia allowed the ctene to increase in width during the effective stroke and decrease in width during recovery. Separation increases the surface area of the ctene during the effective stroke, likely increasing the thrust produced. The finding that ctenes are not monoliths and instead are separated into clumps of cilia has not been previously described, and we subsequently observed this trait in three other ctenophore species: Euplokamis dunlapae, Bolinopsis infundibulum, and Beroe mitrata. Flexibility in function may be a necessary corollary to isometric development of the ctenes as propulsive structures.     

Kinematics of swimming in two burrowing anguilliform fishes

Anguilliform or eel-like fishes are typically bottom dwellers, some of which are specialized burrowers. Although specializations for burrowing are predicted to affect the kinematics of swimming, it remains unknown to what extent this is actually the case. Here we examine swimming kinematics and efficiency of two burrowing anguilliform species, Pisodonophis boro and Heteroconger hassi, with different degrees of specialization for burrowing. Our data suggest that differences in the swimming kinematics may indeed be related to the differences in burrowing specialization and style between both species. The resemblance between the swimming kinematics of P. boro and previously published data for Anguilla anguilla and Anguilla rostrata may be linked with the relatively limited burrowing specialization of P. boro and suggests an overall stereotypy in anguilliform forward-swimming patterns. The body of H. hassi, in contrast, is more specialized for tail-first burrowing and backward swimming bears a striking resemblance to the backward burrowing motions observed in this species. These motions differ significantly from backward swimming in Anguilla and in P. boro. The kinematics of forward swimming are, however, comparable across species. Thus, our data suggest that specializations for burrowing may affect swimming kinematics in anguilliform fishes, but also that forward swimming and burrowing are not necessarily incompatible. Future studies comparing the kinematics and mechanics of burrowing in these and other anguilliform fishes are needed to better understand how specializations for burrowing constrain backward swimming in H. hassi.     

Rainbow trout display a developmental shift in red muscle kinetics, swimming kinematics and myosin heavy chain isoform

Both activation and relaxation times of rainbow trout Oncorhynchus mykiss red muscle were shorter in parr than in older juveniles. Furthermore, parr red muscle had a faster maximum shortening velocity than that of older fish, as estimated with the force-clamp technique. Parr swam with higher tailbeat frequencies and lower tailbeat amplitude than did older fish across a range of length-specific steady swimming speeds. The developmental shift in contraction kinetics of red muscle and steady swimming kinematics was associated with a reduction from two or three myosin heavy chain isoforms in parr to one in older juveniles. This transition provides a mechanism to explain the variations in muscle contraction kinetics and swimming performance.     

Comparing the local dynamic stability of trunk movements between varsity athletes with and without non-specific low back pain

The local dynamic stability of trunk movements, quantified using the maximum Lyapunov exponent (λ_max), can provide important information on the neuromuscular control of spine stability during movement tasks. Although previous research has displayed the promise of this technique, all studies were completed with healthy participants. Therefore the goal of this study was to compare the dynamic stability of spine kinematics and trunk muscle activations, as well as antagonistic muscle co-contraction, between athletes with and without low back pain (LBP). Twenty interuniversity varsity athletes (10 LBP, 10 healthy controls) were recruited to participate in the study. Each participant completed a repetitive trunk flexion task at 15 cycles per minute, both symmetrically and asymmetrically, while trunk kinematics and muscular activity (EMG) were monitored. The local dynamic stability of low back EMG was significantly higher (lower λ_max) in healthy individuals (p=0.002), whereas the dynamic stability of kinematics, the dynamic stability of full trunk system EMG, and the amount of antagonistic co-contraction were significantly higher when moving asymmetrically (p<0.05 for all variables). Although non-significant, kinematic and trunk system EMG stability also tended to be impaired in LBP participants, whereas they also tended to co-contract their antagonist muscles more. This study provides evidence that Lyapunov analyses of kinematic and muscle activation data can provide insight into the neuromuscular control of spine stability in back pain participants. Future research will repeat these protocols in patients with higher levels of pain, with hopes of developing a tool to assess impairment and treatment effectiveness in clinical and workplace settings.     

A comparative analysis of the locomotory systems of medusoid Cnidaria

Summary 1. The locomotory systems of forty-two genera of hydro- and scyphomedusae, representing nine orders, were surveyed, to serve as a basis for generalizations about medusan locomotion. The more pertinent of these have been discussed. The comparisons are based on previous studies ofPolyorchis montereyensis andCyanea capillata (Gladfelter 1972a, b).2. The bell ranges from broad and flat to long, narrow and pointed; the mesoglea ranges from soft to rigid. Radial mesogleal fibers are distributed in the regions of the bell deformed during contraction, and are most densely arrayed along joint apices. A variety of joint systems is present, from none or temporary creases, to permanent exumbrellar creases, to elaborate systems in most Anthomedusae and Scyphomedusae: a classification of mesogleal skeletons was erected.3. The contractile elements of the swimming muscle vary from widely spaced cord-like elements in weak swimmers to tightly packed ribbon-like processes with aligned striations in small strong swimmers, to thick cord-like elements in closely spaced, highly convoluted array arranged on larger mesogleal folds. Radial swimming muscles are present in flat forms.4. The velum (Hydromedusae) or its analog, the velarium (Cubomedusae) is important in forms with strongly arched bells, where it functions as a nozzle during straight swimming and turning. It is less important in broader bells, which, however, can turn in a smaller radius than tall forms.

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Eine vergleichende Analyse der lokomotorischen Systeme von Cnidarier-Medusen

Kurzfassung Auf der Grundlage einer funktionell-morphologischen Analyse des lokomotorischen Systems bei Hydro- und Scyphomedusen wurde der Versuch unternommen, den Mechanismus ihrer Schwimmbewegungen allgemein zu charakterisieren. An Vertretern von insgesamt 42 Gattungen wurden der Bau und die funktionelle Variabilität des Schirmes, der Mesogloea, der Fibrillen der Mesogloea, der kontraktilen Elemente der Muskulatur und des Velums bzw. des Velariums untersucht und verglichen sowie eine Klassifizierung der Medusen nach der Struktur der Mesogloea und der Art der Fortbewegung vorgenommen.

     

A comparative analysis of parvalbumin expression in pinfish (Lagodon rhomboides) and toadfish (Opsanus sp.)

This study examines the role of a myoplasmic protein, parvalbumin, in enhancing muscle relaxation by fishes. Parvalbumin is thought to bind free Ca²⁺ during muscle contraction, thereby reducing intracellular [Ca²⁺] in muscle and speeding muscle relaxation by reducing Ca²⁺ availability to the troponin complex. We hypothesized that parvalbumin expression is ubiquitously expressed in fish muscle and that its expression levels and role in muscle relaxation would depend on the activity level and the thermal environment of a given fish species. Muscle contractile properties and patterns of parvalbumin expression were examined in pinfish (Lagodon rhomboides) and two species of toadfish (gulf toadfish, Opsanus beta, and oyster toadfish, Opsanus tau). Unlike another sparid (sheepshead), the active swimming pinfish does not express parvalbumin in its slow-twitch red muscle. However, both sheepshead and pinfish have relatively high levels of parvalbumin in their myotomal white muscle. Gulf toadfish from the Gulf of Mexico expressed higher levels of parvalbumin and had faster muscle relaxation rates than oyster toadfish from more northern latitudes. The faster muscle of gulf toadfish also expressed relatively more of one parvalbumin isoform, suggesting differences in the binding properties of the two isoforms observed in toadfish swimming muscle. Parvalbumin expression and its role in muscle relaxation appear to vary widely in fishes. There are many control points involved in the calcium transient of contracting muscle, leading to a variety of species-specific solutions to the modulation of muscle relaxation.     

Importance of mechanics and kinematics in determining the stiffness contribution of the vertebral column during body-caudal-fin swimming in fishes

Whole-body stiffness in fishes has important consequences for swimming mode, speed and efficiency, but the contribution of vertebral column stiffness to whole-body stiffness is unclear. In our opinion, this lack of clarity is due in part to the lack of studies that have measured both in vitro mechanical properties of the vertebral column as well as in vivo vertebral kinematics in the same species. Some lack of clarity may also come from real variation in the mechanical role of the vertebral column across species. Previous studies, based on either mechanics or kinematics alone, suggest species-specific variation in vertebral column locomotor function that ranges from highly stiff regimes that contribute greatly to whole-body stiffness, and potentially act as a spring, to highly compliant regimes that only prohibit excessive flexion of the intervertebral joints. We review data collected in combined investigations of both mechanics and kinematics of three species, Myxine glutinosa, Acipenser transmontanus, and Morone saxatilis, to illustrate how mechanical testing within the context of the in vivo kinematics more clearly distinguishes the role of the vertebral column in each species. In addition, we identify species for which kinematic data are available, but mechanical data are lacking. We encourage further investigation of these species to fill these mechanical data gaps. Finally, we hope these future combined analyses will identify certain morphological, mechanical, or kinematic parameters that might be associated with certain vertebral column functional regimes with respect to body stiffness.     

Ontogenetic changes in the bell morphology and kinematics and swimming behavior of rowing medusae: the special case of the limnomedusa Liriope tetraphylla

Swimming animals may experience significant changes in the Reynolds number (Re) of their surrounding fluid flows throughout ontogeny. Many medusae experience Re environments with significant viscous forces as small juveniles but inertially dominated Re environments as adults. These different environments may affect their propulsive strategies. In particular, rowing, a propulsive strategy with ecological advantages for large adults, may be constrained by viscosity for small juvenile medusae. We examined changes in the bell morphology and swimming kinematics of the limnomedusa Liriope tetraphylla at different stages of development. L. tetraphylla maintained an oblate bell (fineness ratio ≈ 0.5-0.6), large velar aperture ratio (R(v) ≈ 0.5-0.8), and rapid bell kinematics throughout development. These traits enabled it to use rowing propulsion at all stages except the very smallest sizes observed (diameter = 0.14 cm). During the juvenile stage, very rapid bell kinematics served to increase Re sufficiently for rowing propulsion. Other taxa that use rowing propulsion as adults, such as leptomedusae and scyphomedusae, typically utilize different propulsive strategies as small juveniles to function in low Re environments. We compared the performance values of the different propulsive modes observed among juvenile medusae.     

Field observations of four Aurelia labiata jellyfish behaviours: swimming down in response to low salinity pre-empted swimming up in response to touch,but animal and plant materials were captured equally

Jellyfish live in complex environments and must continually make behavioural choices. In field observations, adult Aurelia labiata were confronted with a conflict between swimming up elicited by touch of the manubrium and swimming down elicited by low salinity. Following a touch, downward-swimming medusae (1.5–2.0 m deep) turned and swam to within 0.5 m of the surface when the salinity in the top 1.5 m of the water column was greater than 20 ppt but medusae uniformly refused to swim up into the top 1.25 m when the salinity was less than 20 ppt even after being touched three times. The central nervous system of A. labiata appears to have neural circuitry that specifies their response when medusae encounter stimuli that elicit incompatible behaviours. Upward-swimming adult medusae had animal, vegetable or cellulose (paper) material dispersed ahead of them. Medusae captured each material on the bell margin and transported it to a gastric pouch. Medusae displayed only minor behavioural differences in the process. Having sensory, neural and muscular systems organized to capture and pass to the stomach, a huge variety of materials allows medusae to survive in different seasons and environments.