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  Defenses
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  Fast crawling & swimming
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Defenses of nudibranchs and their relatives include fast crawling & swimming, considered in this section, and
CAMOUFLAGE (CRYPSIS),
MUCOUS COATINGS,
CERATAL AUTOTOMY,
NUTRITIONAL CONTENT,
SPICULES,
NEMATOCYSTS
,
VACUOLATED SKIN WITH PROTECTIVE SPINDLES
,
ACID SECRETIONS
,
INK & OPALINE SECRETIONS
,
SECONDARY METABOLITES,
ALARM PHEROMONES
,
APOSEMATIC (WARNING) COLORATION & BATESIAN MIMICRY, and
NAVANAX: A SPECIAL CASE STUDY, considered in other sections. 

 

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Research study 1

 

photograph of adult aeolid nudibranch Dendronotus iris swimmingSwimming in Dendronotus iris can readily be invoked by mechanical stimulus. It begins with an initial 45o bending of the anterior part of the body.  This produces a powerful propulsive stroke by the head, and creates on the stretched side of the body wall a large twisted muscular wave that passes posteriorad.  The locomotory mode, best described as side-to-side thrashing, is effected by the sweep of these waves.  Studies at Friday Harbor Laboratories, Washington on a 14cm-long individual shows that side-stroke frequency is about 45 . min-1.  Kjerschow Agersborg 1922 Biol Bull 42: 257.

NOTE formerly Dendronotus giganteus

 

 

Adult-sized Dendronotus iris swimming
with head facing the viewer and foot tightly
pressed together in a wedge shape 0.5X

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photograph of adult aeolid nudibranch Dendronotus iris swimming taken from a video

CLICK HERE to see a video of Dendronotus iris swimming. More than in most nudibranchs that swim, the swimming action of Dendronotus seems to be uncoordinated thrashings. However, like most other nudibranch species that thrash in this manner, rising off the surface is the primary transport function, and water currents do the rest.

NOTE  the video replays automatically

 
Research study 2
  Most or all species of Dendronotus swim but, once again, it is not clear if they do so to escape predators. Dendronotids  have no obvious defenses.  Observations of missing cerata on D. iris suggest that fishes may take bites out of them.  Also, rough handling may lead to broken-off cerata, which may then thrash about in a kind of swimming.  Could this motion act to decoy fishes, allowing the intended prey to swim or crawl away?  The swimming is more a side-to-side thrashing which relies on water currents to effect lateral transport.  It is inefficient, but effective.  The author reiterates a description of a Dendronotus swimming at the surface in 130-meter deep water about a km offshore.  Robilliard 1970 Veliger 12: 433; see also Farmer 1970 Veliger 13: 73 for different modes of swimming in opisthobranchs.
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Research study 3
 

drawings showing responses of sea hare Aplysia californica to cutaneous stimulation, such as tail-pinch or electrical shockOn cutaneous stimulation in the anterior region, sea hares Aplysia californica will turn and crawl rapidly away from the site of “attack”.  Interestingly, later application of stimuli often leads to the same turning-away response regardless of where the stimulus is applied. Walters & Erickson 1986 J Comp Physiol A 159: 339.

NOTE  includes pinching with forceps, electrical shock, and application of NaCl crystals

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Research study 4
 

photographic series showing simulated response of a sea hare Aplysia californica to its tail being pinched with forcepsAvoidance behaviour of Aplysia californica to adverse stimuli such as pinching the tail with forceps includes tail-withdrawal and galloping locomotion, simulated in these photographs. Leonard & Lukowiak 1986 Behaviour 98: 320. 

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Research study 5
 

photograph of Pleurobranchaea californica courtesy Kevin Lee, Fullerton, CaliforniaThe notaspidean opisthobranch Pleurobranchaea californica swims to escape mechanical disturbance and potential predators.  It does so by an episode of alternating dorsal and ventral body flexions, similar to that seen in Tritonia diomedea ( Research Study 7 below).  The behaviour is initiated by a central pattern generator consisting of only 7 neurones located in the cerebropleural ganglia, and over-rides all other behaviours.  Nothing is known of the role that swimming plays in the everyday activities of PleurobranchaeaJing & Gillette 1999 J Neurophys 81: 654. Photograph courtesy Kevin Lee, Fullerton, California diverKevin.

 
Research study 6
 

series of drawing showing swimming movements in the nudibranch Melibe leoninaAt least 47 world species of gastropods1 swim, including many nudibranchs.  Most swim in response to disturbance or the presence of predators. The hooded nudibranch Melibe leonina schematic showing swimming direction of nudibranch Melibe leonina in relation to the long axis of its footswims by lateral flexions of its body, with the head and tail nearly touching one another. The foot is generally folded in on itself during swimming (see drawings on Left).

Sculling forces generated by the flapping tail propel the animal ventrally, so the direction of swimming is actually at right angles relative to the long-axis of the foot (see data for 7 individuals above Right).

histogram showing % individuals of the nudibranch Melibe leonina swimming in response to different stimuliStudies at Friday Harbor Laboratories, Washington show that Melibe swims spontaneously, as well as in response to different types of aversive stimuli.  Such stimuli include tail-pinching with forceps, application of salt (KCl2), and contact with several graph showing amplitude and frequency of swimming contractions of the nudibranch Melibe leoninaputative predators3 including the sunflower star Pycnopodia helianthoides (see histogram on Left).

Amplitude and frequency of the body movements is consistent regardless of how the swimming bout is initiated, with each complete cycle lasting 2-4 seconds (see graph on Right, showing frequency of body contractions of a tethered individual). 

An individual will spontaneously swim for a few seconds or as long as 25min.  Long, experimentally forced swims may be interrupted by regular periods of “resting” at the surface, where the cerata are spread parallel to the surface of the water and the body lies motionless.  An early investigator describes this behaviour, accompanied by sinking, as “death feigning”, perhaps a form of defense.  It may also be an energy-savings strategy that allows an individual to remain in the water column for longer periods, perhaps to rest or feed.  In addition to a predator-avoidance strategy, swimming in Melibe may allow sampling of new habitats and finding mates.  Lawrence & Watson 2002 Biol Bull 203: 144: for reference to “feigned death” see Kjerschow-Agersborg 1921 Am Nat 55: 222; for information on neural components of swimming see Watson et al. 2001 Amer Zool 41: 1026; see also Watson et al. 2002 Biol Bull 203: 152 and Thompson & Watson 2005 J Exp Biol 208: 1347.

NOTE1 3 of the commonest modes of swimming are included in this total: 1) flapping of mantle or foot (21 species), 2) dorso-ventral undulation (5 species), and 3) lateral flexion (18 species).  The last is the most common type used by aeolid and dendronotid nudibranchs

NOTE2 1ml of 1M KCl solution applied to the head

NOTE3 other sea stars, including the blood star Henricia levisuscula and Pisaster spp., several crabs species, and an anemone are also tested, but only contact with Pycnopodia induces swimming

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photograph of a nudibranch Melibe leonina swimming taken from a video

CLICK HERE to see a video of Melibe leonina swimming. Note how the edges of the foot are pressed tightly together during the swimming bout.

NOTE  the video replays automatically

 
Research study 7
 

photograph of nudibranch Tritonia diomedea courtesy Russ Wyeth and Owen WoodwardPhysical contact with sunflower stars Pycnopodia helianthoides will induce swimming in the nudibranch Tritonia diomedea.  The swimming is non-directional and tends generally to move the nudibranch downst-ream of the predator.  In one instance, recorded by underwater video camera in studies at Vargus Island, British Columbia, 2 Tritonia respectively crawl and swim downstream after being exposed to an odour plume of a single sunflower star caged upstream (see schematic on Right). The conformation of these field escape and other behaviours with ones observed in the laboratory give confidence to the authors that their previously published laboratory observations are accurate.  Wyeth & Willows 2006 Biol Bull 210: 81; photograph courtesy Russ Wyeth and Owen Woodward.

NOTE the spread of the odour plume is assumed to be identical to the spread of fluoresceine dye emitted from sources manually placed amongst the arms of the sea star. The dye alone does not cause Tritonia to swim 

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photograph of nudibranch Tritonia diomedea about to swim from contact with a sunflower star Pycnopodia helianthoides taken from a video

CLICK HERE to see a video of Tritonia diomedea swimming to escape a sunflower star Pycnopodia helianthoides. Note the relatively short distance traversed by the nudibranch in about 1min of thrashing/swimming. Video courtesy Russ Wyeth and Owen Woodward.

NOTE  the video replays automatically

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