Ecology & behaviour
  For convenience, this section is separated by species, Aplysia californica, Corambe steinbergae, Melibe leonina, Onchidoris bilamellata, and Tritonia diomedeaA final short section deals with how to mark a nudibranch using stains and inks.
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Aplysia californica
Much research work has been published on the neurophysiology of learning and behaviour in sea hares in the laboratory, and is not included here.  Presented below are a few field studies on learning and behaviour of Aplysia, and one involving field-collected animals.

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Research study 1
 

photograph of a crawling sea hare Aplysia californica, courtesy Kevin Lee, Fullerton, CaliforniaA study based at the Scripps Institution of Oceanography, La Jolla but done at locations from southern California to northern Baja California employs snorkeling, tidepool wading, and SCUBA to monitor field behaviour of Aplysia californica.  Seawater temperatures during the 2mo study during Jul-Aug range around 20oC.  Results show that the most common behaviour is feeding, with several hours per day being devoted to it.  Most common foods are red algae of the genera Laurencia and Gigartina.  Satiation is not uncommon in these field animals.  During exposure to air in the intertidal zone, most commonly no more than 3-5h, individuals are inactive; otherwise, they locomote over distances of 10m or more per day.  An interesting observation by the researchers is that sea hares being buffeted in a surge channel have noticeably “sticky feet”, perhaps an adaptation to maintain more solid foothold.  The sea hares typically aggregate into groups, and copulation is commonly observed.  The authors report no incidences of defensive withdrawals or spontaneous inking.  In over 70h of observations and despite sometimes continual contact of sea hares with anemones Anthopleura elegantissima and A. xanthogrammica, no incidence of predation is recorded. No observations are made of nighttime behaviour.  This research represents the first detailed study of field behaviour in A. californicaKupfermann & Carew 1974 Behav Biol 12: 317. Photograph courtesy Kevin Lee, Fullerton, California diverKevin.

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Research study 2
 

graph comparing siphon-withdrawal responses in sea hares from calm- and turbulent-water habitatsHabituation is a learned behavior in which an animal exhibits a decreased response with repetition of the same stimulus.  In nature it serves to decrease time and energy expenditure in responding to harmless stimuli.  While well known in laboratory studies of neurophysiology of learning in sea hares Aplysia californica, its demonstration in field animals is rare.  However, one such demonstration, near La Jolla, California, shows that the typical defensive-withdrawal response of Aplysia to physical stimulation is more weakly expressed in individuals living in turbulent environments versus ones living in calm-water habitats. The same pattern is true for the inking response. Note in the graph that habituation is also faster in animals from wave-turbulent environments than from calm areas.  Carew & Kupfermann 1974 Behav Biol 12: 339.

NOTE  on contact with a strange object a sea hare will rear back quickly, scrunching its head back into its body, and pulling in its siphon

NOTE  the stimulus used for the defensive-withdrawal part of the study is a squirt of seawater from a syringe towards the siphon, with the magnitude of response being measured by the duration that the contracted siphon remains hidden between the parapodia.  The stimulus used for inking is a pin inserted into a parapodium

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Research study 3
 

photographs showing head-withdrawal response in a sea hare Aplysia california Common defensive behaviour of Aplysia californica includes head-withdrawal, as would be exhibited with a touch to the head region or perhaps a water jet from a syringe as described in Research Study 2 above. Leonard & Lukowiak 1986 Behaviour 98: 320.

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Research study 4
 

histogram comparing retention capabilities for habituation in young and old sea hares Aplysia californicahistrograms comparing retention abilities of sea hares Aplysia californica of different ages to sensitisation trainingAs noted above, a common behaviour of Aplysia californica to physical stimulus is a pulling back of the head, combined with withdrawal of the gill and siphon.  A question asked by researchers at Columbia University and The New York State Psychiatric Institute is how aging might affect the response, specifically, whether the siphon-withdrawal reflex would be retained in memory as long, and that habituation and sensitisation would occur equally well, in old individuals as in young ones.  Experiments involve field-collected animals and consist of tests on small (130g live mass) and large (1200-1400g) individuals collected in the same area and at the same time. Treatments involve directing water-jets of standardised duration (0.8sec) and intensity onto the siphon area, and measuring responses in duration of siphon withdrawal (sec).  In tests involving sensitisation, individuals are habituated to the stimulus by subjecting them to 4 sessions of repeated stimulation as before, then sensitised by accompanying a last jet stimulation with a relatively strong electrical shock.  Individuals so treated exhibit a stronger (sensitised) response to the jet, one that includes withdrawal, but with additional “balling up” of the body and release of ink.  Comparison of young and old individuals show that aging 1) significantly impairs the long-term retention of habituation (see graph on Left), and 2) inhibits acquisition of sensitisation of the reflex (see graph on Right)  The authors note that in several respects what they find for Aplysia is similar to that found for various vertebrates including humans.  Bailey et al. 1983 Behavioral Neural Biol 38: 70.

NOTE  other than being in California the collection site is not given.  Results for experiments involving laboratory-cultured animals are similar to those of field-collected ones and are not considered here

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  Corambe (Doridella) steinbergae
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Research study 1
 

In areas of southern California 2 dorid nudibranchs Corambe steinbergae and  Corambe pacifica co-occur episodically on kelp Macrocystis pyrifera where photograph of dorid nudibranch Corambe steinbergae feeding on Membranipora sp., courtesy Bill Rudman, Sea Slug Forum, Australiathey feed exclusively on bryozoans Membranipora membranacea.  Settlement of both nudibranch species occurs before peak abundance of their prey, thus ensuring that sufficient food is available to satisfy the nutritional needs of the older, larger stages.  The potential for exploitative food competition exists, but may be somewhat lessened by a longer developmental time of C. pacifica (26d for pacifica vs. 17d for steinbergae after larval settlement).  Corambe pacifica is half again larger than C. steinbergae and eats 6-fold more bryozoan zooids per day.  Coexistence of the 2 species is also enabled by their differential ability to exploit prey patchiness. When the prey is in low abundance, no reproduction of the dorids is possible. When the prey is in limited abundance, some reproduction is possible, and where it is over-abundant, maximum reproduction is possible.  Corambe steinbergae is more reproductively “efficient” than C. pacifica, while the latter is 2-fold  more fecund than the formerYoshioka 1986 Mar Ecol Progr Ser 33: 81.  Photograph courtesy Bill Rudman, Sea Slug Forum, Australia seaslugforum.

NOTE none of this makes much sense as presented here, and the reader is encouraged to read the original version

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  Melibe leonina & Gasteropteron pacificum
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Research study 1
 

photograph of gastropod Gasteropteron pacificumDispersal in benthic marine invertebrates is thought to involve mainly: 1) planktonic egg capsules/larvae, 2) rafting of attached eggs or post-metamorphic individuals, and 3) crawling.  However, several years of daily/weekly observation of Melibe leonina and Gasteropteron pacificum from floating docks in San Juan Island, Washington suggest to one researcher that swimming could be added to the list.  The author’s data suggest that swimming (by reproductively mature individuals) is highly seasonal, with Melibe being most commonly seen in September-March and Gasteropteron in September-February.  While both species have planktonic veligers, the author suggests that swimming by adults may be an important dispersal mechanism in both species.  Mills 1994 p. 313 In, Reproduction and development of marine invertebrates (Wilson et al., eds.) The Johns Hopkins University Press, Baltimore.

NOTE although it is clear from this account and from the video below that Gasteropteron is a capable swimmer, it is more commonly benthic and has conventional crawling abilities.  Apparently, it can form a temporary siphon by rolling the posterior margin of the cephalic shield into a tube, which suggests that it may sometimes burrow. Bertsch 1969 Veliger 11: 431.

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photograph of the opisthobranch Gasteropteron pacificum swimming taken from a video

CLICK HERE to see a video of Gasteropteron pacificum swimming. Gasteropteron is thought to swim hydrodynamically by flapping its broad parapodia. However, note the completeness of each flapping stroke, to the extent that the parapodia appear to form propulsive tubes. Perhaps the notion of jet propulsion should be examined for this species as it has been for the sea hare Aplysia brasiliana.

NOTE  the video replays automatically

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Research study 2
  Studies at Hopkins Marine Station, Pacific Grove, California on Melibe leonina reveal 6 canonical behaviours divided between 2 major modes of feeding and resting. The 6 behaviours are:
 
drawings illustrating one of 6 main behaviours of Melibe leonina
FEEDING: oral hood extends & contracts.
drawings illustrating the "hood open" behaviour of Melibe leonina
HOOD OPEN: hood, cerata, & rhinophores extended
drawing illustrating alert behaviour in Melibe leonina
ALERT: hood partially closed & oral tentacles tucked in
 
drawing illustrating roaming behaviour in Melibe leonina
ROAMING: crawling with hood partially closed
drawing illustrating another type of roaming behaviour in Melibe leonina
ROAMING 2: crawling with hood partially closed
drawing illustrating resting behaviour in Melibe leonina
RESTING: hood closed, cerata & rhinophores folded in
 

photograph of Melibe leonina courtesy Charles Seaborn, Malibu, CaliforniaTransitions within these 2 modes are more likely to occur than transitions between modes.  Interestingly, the amount of time spent in the feeding mode is positively correlated with body size, but the average length of a feeding bout is independent of size.  Thus, body size regulates the probability of an individual entering the feeding mode, but does not influence the basic feeding pattern.  Individuals in the presence of food are almost continually in the feeding mode.  Other results show that transitions from one behaviour to another occur infrequently.  For example, an individual in the feeding mode has about an 80% probability of staying in the feeding mode and a 20% chance of transition to the resting mode.  Conversely, there is about a 20% probability of staying in the resting mode vs. 80% transition to the feeding mode.  The authors present data for transition probabilities between the 6 main behaviours, both starting and ending, for replicate sets of observations (36 for each replicate set), but are not included here.  One significant observation is that the probability of a given transition between behaviours is independent, but with the proviso noted above that transitions are most likely to occur with a mode.  Schivell et al. 1997 Biol Bull 192: 418. Photograph courtesy Charles Seaborn, Malibu, California.

NOTE  this means they are reduced to the simplest and most significant form possible

NOTE  other behaviours observed but too infrequent to include, are 1) resting locomotion: crawling but in the resting posture, 2) crumpling: an alarm response characterised by contraction of most of the body wall musculature, 3) swimming, and 4) copulation/egg-laying

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  Onchidoris bilamellata
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Research study 1
 

photograph of nudibranch Onchidoris bilamellata courtesy Paul Young, Massachusettshistograms showing time spent by nudibranchs Onchidoris bilamellata in different combinations of dark/light and smooth/rough substrataMost laboratory studies on habitat choice in marine invertebrates involve 1-factor tests, such as light or substratum.  But when an animal makes a choice in the field for habitats, mates, food, and so on, it must have to assess multiple factors simultaneously.  For this type of investigation we turn to research on Onchidoris bilamellata1 in New Brunswick that uses a multifactorial design to test effects of light intensity and substratum texture2 on microhabitat selection.  The advantage of a multifactorial3 approach is that the effect of each factor on the other can be assessed independently, and the relative importance of several factors can be assessed. 

Results show that Onchidoris prefers darkness over light, regardless of texture (as the authors predicted), and rough over smooth substratum, but only when they are in the dark (half of what the authors predicted). In the light, they exhibit no preference for textures.  Thus, preference for one factor, light intensity, dominates over preferences for another factor, texture.  The authors evaluate their methodology in comparison with other experimental approaches, and offer useful recommendations for multifactorial, multilevel designs.  Barbeau et al. 2004 J Exp Mar Biol Ecol 307: 1. Photo courtesy Paul Young, Massachusetts and seaslugforum.

NOTE1 O. bilamellata is one of several opisthobranch species that has a circum-boreal distribution, being found on both Pacific and Atlantic coasts. However, it is surprising that there have been so few equivalent ecological studies done on west-coast nudibranchs

NOTE2 texture is either smooth or rough, the latter created by engraving 0.5 x 0.5mm pits in the Plexiglas bottom of test aquaria (about 90 pits . cm-2).  The Plexiglas sheets are cleaned carefully after each test to remove mucus-trail residues.  Each of 16 test aquaria carries a Plexiglas bottom divided into halves and treated to be smooth/smooth, smooth/rough, rough/smooth, or rough/rough. Matched sets of 4 of these are exposed to 4 conditions of light: light/light, light/dark, dark/light, and dark/dark.  This creates 32 combinations of light and texture, of which half are duplicates (differing only in which ends of the aquaria are used). Only half are used for data collection (see last NOTE below for further explanation).  Each test involves a nudibranch being placed in an aquarium in seawater and monitored for 30min.  Its choice is measured by the time it spends in a certain half of the aquarium

NOTE3 each of the 16 aquaria actually has 2 combinations.  For example, the one highlighted in light blue is one-half LIGHT/SMOOTH and one-half light/rough, but only the combination indicated in capital letters is used in the analyses.  A bit of noodling through the array will show show that the complementary combinations of DARK/SMOOTH, LIGHT/ROUGH, and DARK/ROUGH are represented elsewhere in the array.  This unique “doubled” design ensures that all data are independent.  Thus, observation of animals selecting A over B is independent of the observation of animals selecting B over A, unlike in most other choice studies where selections of A and B are observed in the same experimental unit (thus, selection for A affects an animal’s selection for B).  Note also that the 4 corners of the array represent CONTROLS used to assess for positional biases (in a perfect world each of these bars would read 15min). The design of the experiment is excellent

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Tritonia diomedea

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Research study 1
 

photograph of nudibranch Tritonia diomedea courtesy Russ Wyeth & Owen Williams, St. Francis Xavier University, Nova Scotia
schematic of orientation of nudibranch Tritonia diomedea in a natural magnetic fieldStudies at Friday Harbor Laboratories, Washington reveal that the nudibranch Tritonia diomedea is able to perceive the direction of the earth’s magnetic field.  In one test, for example, animals placed for 90min in a circular tank filled with seawater mostly move to the east side of the tank (see diagram above Right). 

 

schematic showing orientation of nudibranch Tritonia diomedea in a cancelled-out magnetic fieldIf, however, the horizontal component of the magnetic field is now canceled out experimentally with a magnetic coil system, the orientation is lost (see diagram lower Right). Lohmann & Willows1987 Science 235: 331. Photograph courtesy Russ Wyeth & Owen Williams, St. Francis Xavier University, Nova Scotia.

 

 
Research study 2
 

graph showing angle of orientation of nudibranch Tritonia deomedea in relation to day of lunar monthAs unusual as this orientating behaviour of Tritonia is, it is made even more interesting by having a lunar component.  Further tests done on Tritonia diomedea at Friday Harbor Laboratories, Washington, indicate that an eastward movement as seen in the preceding graph would be more likely to occur during a waxing moon (i.e., mean angle of orientation of 88o). In comparison, at the start of the lunar cycle on a new moon, individuals have a tendency to move to the south (i.e., mean angle of about 180o). While cautioning the reader that their data could be a result of a laboratory artifact not yet known, the authors note that were field animals to respond similarly they would move in a slow counter-clockwise circle over the course of a lunar month.  What function might be served by this behaviour is not known.  Lohmann & Willows 1987 Science 235: 331.

NOTE  the behaviour is not tested in moonlight directly; rather, the modulation of the magnetic response observed during different phases of the moon is thought to be based on a lunar-rhythm pacemaker system located somewhere in the nervous system

 
Research study 3
  diagram of central nervous system of nudibranch Tritonia diomedea
drawing of nudibranch Tritonia diomedea dissected to show nerves thought to be responsible for magnetic orientationMore recent studies on Tritonia diomedea show that certain specific neurones in the brain respond to changes in magnetic field.   At least 4 pedal neurones and associated nerves have been identified that respond with increased electrical activity to experimentally applied earth-strength magnetic fields (see drawing of cerebral and pedal ganglia on Left). The neurones are large and their branches, 2 of which are shown in the drawing on Right, radiate out into the foot and outer body-wall regions. Note that there is little or no overlap between areas of the foot innervated by these branches (labelled LPdN1-2). The authors suggest that these neurones release excitatory peptides that in turn control beating rates and direction of beating of the cilia used by Tritonia to crawl.  Wang et al. 2003 J Exp Biol 206: 381; see also Wang et al. 2004 J Exp Biol 207: 1043.
 
Research study 4
 

schematics showing orientation of nudibranch Tritonia diomedea to currents and to magnetic fieldIn a unique study of natural orientation and behaviour in Tritonia diomedea, an underwater video camera is used in time-lapse mode to record crawling behaviour in the field in relation to current direction and magnetic field.  The study is done in areas of Puget Sound, Washington and Vargas Island, British Columbia on mud bottoms where one of Tritonia’s principal prey, sea pens Ptilosarcus gurneyii, is abundant.  schematic showing orientation of nudibranch Tritonia diomedea to mates and food, and similar orientations after mating and feedingTritonia that have not recently eaten tend to crawl upstream but show no significant magnetic orientation (see figure upper Left).

A strong orientation to upstream food and mates is shown in the Right-hand figure, but this is lost after feeding or mating (see schematic showing orientation of nudibranch Tritonia diomedea to predatory seastars and the likediagram on Right. 

 

If a predatory sea star, Pycnopodia helianthoides, is placed in a 30cm-upstream location (touch by the seastar causes Tritonia to swim), Tritonia crawls downstream (see figure lower Left).   In other treatments, designated "controls" by the authors, where a dive-glove is placed 30cm upstream, or a Pycnopodia is positioned 30cm downstream, crawling orientation is random.  Not surprisingly, field behaviour of T. diomedea differs in several respects from laboratory behaviour, most notably in lack of consistent magnetosensory orientation.  The authors suggest that current flow may supersede magnetic response in these field situations.  Wyeth et al. 2006 Biol Bull 210: 97; also Wyeth & Willows 2006 Biol Bull 210: 81.

NOTE  known as a positive rheotaxis (lit. “current arrangement” G.).  The authors determine current direction by a combination of watching direction of movement of “snow” (visible particulates) and fluorescine dye in the video record

NOTE there is difficulty in interpreting the "control" experiments. First, they seem to be not really controls; rather, they represent 2 additional treatments, one with a downstream predator and the other with an inanimate upstream object. Apart from this, it is not clear why crawling direction is not upstream in both of these circumstances. Second, the data for the 2 "control" experiments should not have been combined (the authors remark that they do this for convenience of data presentation, but perhaps it was done to "bulk up" the few data points obtained).  Finally, the design of the rubber-glove “control” treatment is a bit unusual in that if a positive upstream crawling orientation (i.e., towards the glove) were to have occurred, it would have been difficult to interpret

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  How to mark a nudibranch
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Research study 1
 

Mark/recapture studies on nudibranchs, as on other animals, require that individuals be tagged with something that is durable, conspicuous, and harmless.  Tests using looped cords and injected spots of the vital dye Methylene Blue with Rostanga pulchra at Hopkins Marine Station, Pacific Grove, California prove unsuccessful.  The cords pull out and the dye spots fade too quickly.  However,  a technique of notching the mantle edge with scissor cuts proves more useful.  In one experiment, 29 individuals are marked and released.  Twenty-four are recovered the next day, 15 the following day, 15 on the 3rd day, 4 on the 11th day, and 1 after 37d.  The method can be used on at least 7 locations on the mantle, thus permitting the use of a simple coding system.  Anderson 1973 Veliger 161: 121.

NOTE  the recovery statistics seem unimpressive, but keep in mind that they do demonstrate the potential longevity of a notched mantle edge.  However, if the purpose of the test was to assess the durability of the marks, then rather than releasing the marked animals en masse into the environment as was done, with risk of most them wandering off, it might have been better to cage them in some way. Also, how can the author otherwise be sure that the marked individuals have not just died from their wounds?

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Research study 2
 

Another study using the nudibranch Doriopsilla albopunctata at Moss Landing, California describes a potentially useful marking technique involved subcutaneous injections of black tattoo ink.  The ink is applied on the dorsum part of the animal in up to 3 marks per individual, permitting a simple code to be developed.  The marks last for up to 6wk.  Kiest 1990 Veliger 33: 416.

NOTE  similar injections of red food colouring and the vital stain Janus Green prove not useful because of their short durability in seawater (<4d)

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