title for learn-about section of A SNAIL'S ODYSSEY
  Locomotion & behaviour
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Research study 1
 

Diel vertical migration is a common feature in the behaviour of many hydroid species.  Most species undergo trips of 50m or less, some up to 10 times further; others, not at all.  Observation of 7 species of hydromedusae from San Juan Islands, Washington in a 2-m tall experimental tank shows that most species undertake diel vertical migrations and they do so by swimming.  Light, or rather lack of it, appears appears to be the photograph of hydromedusa Aequorea victoria courtesy Chris Gunn, North Island Explorerproximal factor initiating the migrations, even in species lacking recognised photoreceptors.  The author notes a correlation between the extent and timing of vertical migration with spawning times, and suggests that at least one function of migration could be to place a mass of reproductively mature individuals in close proximity at the surface during spawning times.  The author provides observational data on behaviour of each of the 7 species.  Mills 1983 J Plankton Res 5: 619. Photograph courtesy Chris Gunn, North Island Explorer, Campbell River, British Columbia.

NOTE  these include Aequorea victoria, Bougainvillia principis, Gonionemus vertens, Mitrocoma cellularia, Clytia gregaria, Polyorchis penicillatus, and Stomotoca atra.  Field observations in Saanich Inlet, British Columbia allow Aglantha digitale to be added to this list.  Arai & Fulton 1973 J Fish Res Bd Can 30: 550.

NOTE  an example of a hydromedusan species lacking ocelli is Aequorea victoria, one of the most abundant hydromedusans in inland waters of Washington and British Columbia.  The author notes that this species is harvested in quantity annually to provide stocks of aequorin, a calcium-activated luninescent protein used in biomedical research

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Research study 2
 

photograph of hydromedusan Gonionemus vertens courtesy Dave Cowles, Walla Walla University, Washingtongraph showing oxygen-consumption costs of swimming hydromedusae Gonionemus vertens and Stomotoca atraCost-analyses of swimming in 2 species of hydromedusae Gonioneumus vertens and Stomotoca atra in Friday Harbor Laboratories, Washington uses individuals tethered to a "force platform" allowing simultaneous measurement of swimming and oxygen consumption. graph showing comparative costs of transport in in animals using different modes of propulsion, including pulsing in hydromedusansLocomotory rates are determined from the swim-beat frequency of these tethered individuals and correlative velocity is obtained from freely swimming medusae.  Both species swim in bouts of 5-10 bell contractions, each bout lasting up to 3sec, followed by periods of quiescence lasting 10-90sec depending upon species.  Swimming speeds when pulsing are comparable for the 2 species, ranging around 5cm . sec-1.  Note that oxygen upake of the 2 species is also similar (graph upper Right).

Swimming costs in the medusae are surprisingly high, almost one order of magnitude greater than for vertebrate swimmers of equivalent body mass (see graph on Right). The high cost is attributed to a combination of energy expended in periodic accelerations of the animal’s mass and some mass of fluid around it, and energy dissipated in bell deformations and recovery strokes.  The author concludes that the high cost of locomotion in these medusae is a general consequence of swimming with a discontinuous production of thrust.  Daniel 1985 J Exp Biol 119: 149. Photograph courtesy Dave Cowles, Walla Walla University, Washington rosario.wallawalla.edu.

NOTE  locomotory cost or cost of transport is determined as a dimensionless ratio calculated from the rate of energy expenditure divided by the product of an individual’s mass and speed.  It is commonly used when comparing costs of different modes of locomotion.  For example, crawling, as by snails, is most costly, followed by running, flying, and swimming in that order

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Research study 3
 

photograph of hydromedusan Polyorchis penicillatus courtesy Dave Cowles, Walla Walla University, Washingtongraph showing swim-pulse frequency of the hydromedusan Polyorchis penicillatus swimming at fast speedsMany species of hydromedusae, including Polyorchis penicillatus, undertake vertical migrations, both daily (die)l, and during development, and the question arises as to what responses to light might be controlling these behaviours.  Studies at the Bamfield Marine Sciences Centre British Columbia using Polyorchis medusae suspended on glass-tubing supports in a vertical tank show that light plays a strong role in swimming. Specifically, pulse frequencies of swimming are generally constant in constant light conditions, but they increase in direct proportion to rates of decrease in light intensity.  This causes upward swimming at dusk.  In contrast, slowly increasing light intensity causes inhibition in swimming. As the medusa is negatively buoyant, this behaviour contributes to sinking at dawn.  The medusae respond to single, short-duration, shadows usually with a single swimming pulse.  Maximum response to shadows occurs at 450-550nm, in the blue-green part of the visible spectrum, suggesting that the shadow response may not be to predators, but is more likely a part of nighttime upward movement in the water column.  The author notes that photic responses of P. penicillatus are age-related, and therefore may be involved with ontogenetic changes in feeding behaviour. Swim-pulse frequency is known to slow with increasing size/age in this species. Arkett 1985 Biol Bull 169: 297. Photograph courtesy Dave Cowles, Walla Walla University, WA rosario.wallawalla. edu.

NOTE  in this vertical tank, a medusa is tethered by a 1mm-diameter glass rod inserted horizontally through the bell and fastened to either side of the tank.  This permits “normal” swimming pulses but prevents contact of the animal with the side walls of the container, a circumstance that the author suggests may have biased other studies of light effects on free-swimming medusae.  Physical contact can lead to “crumpling” or infolding of the body and cessation of swimming, and to increased excitation of the tentacles making contact.  The tethering also ensures that the bell margin bearing the light-sensitive ocelli remains in constant orientation to a light source

NOTE  these data represent fast swimming in P. penicillatus.  Normal “maintenance” swimming is much slower, at about 5-15 pulses . min-1 depending upon size

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Research study 4
 

photograph of hydromedusan Polyorchis penicillatusdrawings showing bell morphology of the hydromedusan Polyorchis penicillatus with respect to locomotory meansLocomotion of a medusa, whether scypho- or hydromedusa, is by jet propulsion.  Most of the bell of a hydromedusan consists of transparent, non-cellular mesoglea that is traversed by numerous radially arranged fibers (see drawing).  Circular contractile tissues of the subumbrellar surface power the jetting cycle.  Contraction of these tissue reduces the diameter of the bell and forces water out in a jet.  During contraction the bell does not change in length, but the mesoglea deforms non-uniformly around the circumference of the bell and some folding occurs.  The recovery part of the propulsive cycle occurs passively, powered by potential energy stored during the bell’s deformation. No energy is used to re-expand the bell.  Studies at the Bamfield Marine Sciences Centre, British Columbia on the hydromedusan Polyorchis penicillatus show that most, if not all, of the stored potential energy is stored as strain energy in the radial fibers of the mesoglea. In the first studies of their kind, the authors determine the mechanical energy generated during the jet cycle of a hydromedusan.  DeMont & Gosline 1988 J Exp Biol 134: 313, DeMont & Gosline 1988 J Exp Biol 134: 333.

NOTE it is common for authors to refer to these contractile tissues as "muscles"; however, true muscles of mesodermal origin are not found in cnidaria and, so, another term "contractile tissues" is used here

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photograph of hydromedusan Polyorchis penicillatus swimming taken from a video

CLICK HERE to see a video of a hydromedusan Polyorchis penicillatus swimming.

NOTE the video replays automatically

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Research study 5
 

drawings of different hydromedusae used in a study of adaptive design for locomotion in the San Juan Islands, Washingtongraph showing accelerations in swimming of several hydromedusans collected in the waters around San Juan Islands, WashingtonStudies of locomotion in hydromedusae at Friday Harbor Laboratories, Washington reveal that the jetting mode of propulsion is finely tuned to bell streamlining and velar structure.  The 6 species selected for comparison co-occur in the waters around San Juan Islands and represent a range of sizes, bell morphologies, and swimming habits (see drawings on Left). Results show that swimming performance, measured as acceleration and velocity, correlates well with bell streamlining and size/shape of velar aperture.  In fact, the better developed the velum of a species, the better correspondence there is between jet-drawings showing flow patterns around swimming hydromedusae from a study conducted in the San Juan Islands, Washingtonthrust production and acceleration.  For example, accelerations in the prolate species range generally from 20-45cm . sec-2, as compared with 5-10cm . sec-2 in the oblate species (see graph upper Right).

Medusae with prolate shapes, well-developed vela, and good jet propulsion, such as Aglantha digitale, Sarsia sp., and Proboscidactyla flavicirrata, swim with much less drag-generated vortices at the bell margins than do species with oblate shapes, such as Aequorea victoria, Mitrocoma cellularia, and Cyatia gregaria (Phialidium gregarium). Note in the drawings on the lower Left that flow patterns in the prolate medusae show a strong jet component with maximum flow velocity located directly behind the velar aperture.  Prolate species tend to be ambush predators, sitting motionless with tentacles extended to capture actively swimming prey.  These prolate species can swim quite quickly, and do so primarily to escape from their own predators and/or to re-position themselves.  In comparison, the oblate medusae have more diffuse jets, swim more slowly and more continuously, and shed stronger vortices at the bell margin.  Note the more pronounced vortices travelling through the extended tentacles of the oblate forms.  These vortices may aid in feeding, as suspended prey entrained in the current vortices will be brought into direct contact with the tentacles trailing in the medusa’s wake.   Colin & Costello 2002 J Exp Biol 205: 427.

 

NOTE  measured as the ratio of diameter of velar aperture to diameter of the bell.  The better “jetters” have a narrow velar aperture relative to the entire velar area, while the poorer jetters have a large velar aperture relative to the entire velar area.  These latter species are termed “rowers” by the authors, because as they contract their bells during swimming, the margin of the bell appears to act as a paddle, thus contributing to the propulsive stroke

NOTE  prolate medusae are taller than they are wide = more bell-shaped, while oblate medusae are wider than they are tall = upside-down bowl-shaped

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Research study 6
 

drawings of hydromedusae used in study of adaptive design of bell and velum for most effective swimmingA companion paper to Research Study 5 above assesses laboratory foraging behaviour and time-budgeting in 8 hydromedusae species collected in waters around the Friday Harbor Laboratories, Washington.  The “cruising-predatory” species with oblate shapes spend a greater proportion of time swimming (74-92%) than the “ambush-predatory” species with prolate shapes( 20-47%). The former medusae have their tentacles extended almost continuously, whether swimming or drifting, while the latter contract their tentacles when swimming, and only extend them while drifting.  In both cases, as remarked on by the authors, the respective behaviours appear to optimise foraging effectiveness.  Colin et al. 2003 Mar Ecol Progr Ser 253: 305.

NOTE  these 8 represent 5 of the 6 species used in the previous study, with Proboscidactyla flavicirrata being omitted, and with Leuckartiara sp. and Stomotoca atra (both with prolate form) and Eutonia indicans (oblate form) being added

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