Foods, feeding, & growth
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  Preferred foods, feeding ecology, & growth: genera N-P

This section continues with information on diets and feeding ecology of west-coast nudibranch & relatives genera N-P. Information on diets and feeding ecology of other west-coast genera can be found in other sections:

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Research study 1

photograph of the cephalaspidean Navanax inermis courtesy Kevin Lee, Fullerton, California.schematic showing tracking fidelity of the cephalaspidean Navanax inermis to a mucous trail of a prey Bulla gouldiana
The large (to 12cm) cephalaspidean Navanax inermis (photo above Left) eats many types of opisthobranchs.  An early report of its feeding behaviour in San Diego, California indicates that it locates its prey by contact chemoreception of the mucus-impregnated trail of its prey. Apparently, it is incapable of distance chemoreception. 

To test this the author guides a crawling prey Bulla sp. (another cephalaspidean: photo below Left) in an aquarium in the laboratory in such as way that it leaves a mucous track in the shape of an open reverse-9 shape on the surface of the sand (see drawing).  The Bulla is allowed to rest at the end of its trail. A Navanax is then photograph of the cephalaspidean Bulla gouldiana courtesy Kevin Lee, Fullerton, California. placed at the start of the trail. It follows the trail faithfully until it meets the Bulla and eats it.  Note that in its passage the predator passes within about 1-2cm of the resting Bulla, yet doesn’t sense it.  Paine 1963 Veliger 6: 1. Photos courtesy Kevin Lee, Fullerton, California diverkevin.

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Research study 2
  photograph of a cephalaspidean Haminoea virescens courtesy Jan Kocianphotograph of an anaspidean opisthobranch Phyllaplysia taylori
On sand/mudflats at Newport Beach, California Navanax inermis lives with 3 other gastropod species: another cephalaspidean Haminoea virescens, the anaspidean Phyllaplysia taylori, and a neogastropod Nassarius sp. (shown here Nassarius fossatus).  Of these, only Haminoea is eaten, perhaps because the eelgrass location of Phyllaplysia and the shell of Nassarius make them undesirable as prey.  By guiding these and other potential gastropod prey in predetermined directions in aquarium tanks, the authors further test the tracking ability of Navanax.  Some circular tracks are created, many straight ones, and some in which one species is substituted halfway along for another.  Known prey such as Hermissenda crassicornis and Bulla gouldiana are photograph of a snail Nassarius fossatus courtesy Dave Cowles, Walla Walla University, Washingtontracked with 100% fidelity.  Nassarius is not tracked at all.  If Nassarius is substituted for Hermissenda at a mid-way point, the trail-following Navanax will neither follow the new Nassarius trail, nor will it eat this snail if guided into contact with it.  The authors consider this discriminatory tracking behaviour to be an energy-savings strategy – why follow a trail that leads to an unpalatable prey?  The authors confirm that Navanax uses contact chemoreception to locate and follow trails.  Blair & Seapy 1972 Veliger 15: 119. Shown here the related Haminoea virescens. Photo of Haminoea virescens courtesy Jan Kocian and seaslugforum. Photo of Nassarius fossatus courtesy Dave Cowles, Walla Walla University, Washington
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Research study 3

drawing of head region of cephalaspidean Navanax inermis showing details of morphology
photograph of the cephalaspidean Navanax inermis showing detail of head region courtesy Kevin Lee, Fullerton, CaliforniaThe use of contact chemoreception in prey-location by Navanax inermis is confirmed in another study in southern California in which the authors tether Navanax and watch which way the they attempt to turn when stimulated on various parts of the front of the body.  There are 3 structures at the front of the body that potentially mediate predatory responses (see drawing upper Right).  These are the anterior lateral folds, tentacles, and the pharyngeal lips of the pharynx. The lips are generally hidden, but become exposed when the pharynx is protracted (extended). 

Stimuli used include cotton swabs, both clean (control) and soaked in mucus of Hermissenda crassicornis, and live Hermissenda or pieces of Hermissenda. Application of mucus or fresh pieces of Hermissenda to one or other lateral folds stimulates Navanax to turn in that direction (96%) versus a lesser response to control swabs (28%).  photograph of phalliform sensory organ of the cephalaspean Navanax inermis courtesy courtesy Kevin Lee, Fullerton, California Contact appears to be necessary, as mucus-soaked swabs held near the lateral folds elicit no response. In comparison with the 96% value for the lateral folds, application of mucous swabs to the tentacles produces only 25% turning.  Application of live Hermissenda (or fresh pieces) to the tentacles causes 100% turning.  When live prey is touched to the pharyngeal lips, the pharynx begins to suck.  The authors conclude that sensors on the anterior lateral folds are responsible for trail recognition and that they respond only to contact, not to water-borne, stimuli.  Thus, when Navanax encounters a prey trail it turns onto the trail and any deviation from the trail is auto-corrected by asymmetrical stimulation of the lateral folds. 

The sensory units on the lateral folds and tentacles appear to be small ciliary tufts that the authors call phalliform organs.  There are about 1000 of these 0.5mm-sized units scattered over the combined surfaces of the lateral folds and tentacles.  The tufts are retractable (for protection?) and each is associated with a ganglion within the base of the tuft.  The study raises many good research questions, including the chemical identity of the stimulus, how it is recognised by the ciliary tufts, and how Navanax identifies the direction of the trail when it first encounters it.  Murray & Lewis 1974 Veliger 17: 156. Photo of head of Navanax inermis courtesy Kevin Lee, Fullerton, California diverkevin..

NOTE  the difference noted here is statistically significant, but should be viewed with caution as repeated tests are made on a few individual Navanax and the data analysed as though they are independent (58 tests on 6 test specimens)

NOTE  lit. “penis shape” L.

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Research study 4

photograph of the cephalaspidean Bull gouldiana courtesy Kevin Lee, Fullerton, California.Despite all of these confirmations that distance chemoreceptive ability is lacking in Navanax inermis, a later laboratory study suggests that there may, in fact, be such an ability.  The authors use a T-maze to assess the ability of Navanax to locate prey Bulla goldiana, another cehalaspidean, from a distance.  The results are not overwhelming, but a significant proportion of Navanax (62%) select the prey-containing arm of the maze over the empty arm (38%).  In addition, when exposed to seawater bearing the scent of the prey, Navanax exhibits increased locomotory movements.  Susswein et al. 1982 Mar Behav Physiol 8: 231.

NOTE  another research group reports the discovery of 2 identical polypropionates in Navanax inermis and Bulla gouldiana, thus providing a possible chemical basis for the predator-prey relationship.  Spinella et al. 1993 Tetrahedron 49: 3203.

NOTE  the authors use 3 runs of 18 specimens to yield 54 data points, so pseudoreplication is involved; therefore, the authors’ interpretation of their results should be viewed with caution

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Research study 5

graph showing relative sizes of cephalaspids Navanax inermis preying on anaspid sea hares Aplysia californicaIn Santa Catalina Island, California the opisthobranch-eating cephalaspid Navanax inermis attacks and consumes sea hares Aplysia californica.  In the field, Navanax prefers Aplysia of lengths greater than about 2cm.  However, at lengths greater than about 10cm, Aplysia reaches a refuge in size.  If the predator attempts to swallow a larger-sized prey, it is usually rejected.  In some cases, a large Aplysia may get stuck in Navanax's gullet for several minutes or hours, leading to partial digestion.  Survival of these damaged Aplysia often depends upon the extent to which gas exchange via the ctenidia has been impeded. Laboratory experiments indicate that the probability of successful attack by Navanax on Aplysia depends upon their relative sizes (see graph on Left). Note in the graph that if the predator is more than twice the length of the prey, then the probability of success is 60-100%.  As the two get closer in size, however, attack success falls off to 20% or less.  If cover, such as tufts of red algae Plocamium cartilagineum is provided in the test aquaria, then encounter frequencies decrease significantly, especially for the smaller size-classes of prey.  Pennings 1990 Mar Ecol Progr Ser 62: 95.

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Research study 1

This genus of shelled snail seems an unusual inclusion in the nudibranch section of the ODYSSEY, but its unusual feeding behaviour and photograph of parasitic snail Odostomia columbiana crawling on its host snail Trichotropis cancellatareasonably common occurrence on the west coast warrents some attention.  An early study by a researcher at Friday Harbor Laboratories describes the ectoparasitic feeding behaviour of Odostomia columbiana on a littorine relative Trichotropis cancellata.  The parasite crawls freely over the surface of its host and from time to time uses its long, evaginable proboscis armed with a stylet to slice into and then suck body fluids from the host’s soft parts.  In Orcas Island, Washington, T. cancellata is about 40% infested with the parasite, prompting the investigator to question whether the thorny periostracum could be the attractive factor in the relationship.  Results from a series of choice experiments using host snails with fully removed, half removed, and intact periostraca suggest that the periostracum may indeed be attractive to the snail (see illustrations below).  Although it is not clear the extent to which the parasite frequents its host’s periostracum in its daily behaviour, benefits in doing so could include protection from being dislodged by waves or strong currents, and perhaps from predators.  Clark 1972 The Veliger 14 (1): 54.

NOTE  it is a member of the subdivision “Lower Heterobranchia” that, along with Pulmonates (snails and slugs) and Opisthobranchia, make up the large clade Heterobranchia.  It is a member of Family Pyramidellidae

NOTE  this species itseself feeds parasitically on the food streams of various tubeworms, a behaviour described elsewhere in the ODYSSEY

A single Odostomia columbiana crawls on it host snailTrichotropis cancellata 2X

Below: results of choice experiments testing whether the host's periostracum may be the attractive factor to O. columbiana (an experiment missing from the study might be one testing whether isolated fragments of the periostracum are themselves attractive to the parasite):
photograph 1 in a series of periostracum-attraction experiments photograph 2 in a series of periostracum-attraction experiments photograph 3 in a series of periostracum-attraction experiments
CONTROL experiment testing one intact Trichotropis against another yields non-significant results Trichotropis with intact periostracum is highly preferred over one with half its periostracum stripped clean Trichotropis with intact periostracum is highly preferred over one with all periostracum stripped clean
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Research study 0

An early report on the biology of the sacoglossan Olea hansineensis at Friday Harbor Laboratories, Washington notes, in reference to its feeding, the absence of jaws and radula.  There is no description of gut contents of field specimens, but the author reports that laboratory-held individuals come to the water surface and crawl there upside-down where they "constantly nip" at microscopic organisms.  Agersborg 1923 The Nautilus 36 (4): 133.

NOTE  the author provides a description of this new species and, rather endearingly, adds that the new genus name honours her sister “who for a number of years was a constant source of inspiration to me in my scientific studies”, with the species name honouring “my first, the noblest, and the greatest of all my teachers, my mother”.  The author herself received a Master of Science degree in 1916 from the University of Washington, and went on to a productive career in marine biological research, with emphasis on study of opisthobranchs

Research study 1

photograph of sacoglossan Olea hansineensisOther sacoglossan opisthobranchs are suckers of plant juices, gaining entry by means of a single-toothed radula that slices open the plant’s epithelial cells.  It is therefore surprising to find through studies at Friday Harbor Laboratories, Washington that Olea hansineensis is carnivorous, subsisting on eggs of other opisthobranchs and that a radula is apparently absent. An individual in the laboratory will sometimes float upside-down from the water surface and wave its head back and forth, likely directionally sensing food.  When feeding, an Olea will push its head into an egg mass primarily by ciliary power, and suck in individual eggs using its muscular pharyngeal pump.  Eggs are sucked in at a prodigious rate, up to 20 per minute, and are often consumed in order from an egg string.  An individual well rest for a time between meals.  Crane 1972 The Veliger 14 (1): 57. Photograph courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

lateral section of body of sacoglossan Olea hansineensis showing eggs in stomachNOTE  in the field these include Haminoea virescens, Aglaja domedea, and Gasteropteron pacificum, but in the laboratory eggs of other opisthobranch species are readily eaten

Lateral section of O. hansineesis following a meal of Gasteropteron eggs. The eggs
are packed into the midgut and some have been moved into the cerata for digestion

Research study 2

drawing of radula of sacoglossan Olea hansineensisA later study at Friday Harbor Laboratories, Washington reveals that Olea hansineensis does, indeed, possess a radula but, although similar in size and structure to that found in other species of sacoglossans, it is unusually small, only about one-twentieth of a mm in length.  It consists of 5 teeth rather loosely strung together in a membraneous radular sac (see drawing).  The entire assembly is morphologically rather odd and its location in the pharynx relative to the proboscis is not made clear.  Its small size suggests that it might be used just for it a brief period post-settlement.  Still, it is difficult to understand the possible functionality attributed to it by the author, such as teeth 1 & 2 “might serve for general grubbing around”, and teeth 3 & 4 “appear capable of piercing young algal growth”.  The muscle arrangement that would enable such fine movements of the floppy radular sac and differential use of the teeth is not described. The author suggests that a follow-up study would be useful and the reader would tend to agree.  Gascoigne 1974 The Veliger 17 (3): 313.

NOTE  the author includes a description of the reproductive system, but is not considered here

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Research study 1
  drawing of nudibranch Onchidoris fusca crawling on its barnacle preyOnchidoris fusca preferentially feeds on barnacles in the mid-intertidal area of the shore.  It penetrates the soft membranes between the shell-plates of the barnacle using its radula.  Once an opening is made, the nudibranch inserts its proboscis and feeds on the soft internal tissues.  The proboscis is highly extensible and can reach to all parts of the prey.
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Research study 1

photograph of dorid nudibranch Peltodoris nobilisIn the San Diego area of California Peltodoris (Anisodoris) nobilis eats several species of sponges, most notably Myxilla agennesMcBeth 1971 Veliger 14: 158.

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Research study 2

photograph of dorid nudibranch Doris montereyensisRadular characteristics such as number of cusp rows, number of cusps per row, and length of cusps are often used in taxonomic studies to differentiate species.  However, a study at Friday Harbor Laboratories, Washington of these features in Peltodoris (Anisodoris) nobilis (photo above in Research Study 1) and Doris montereyensis (photo on Right), two large-sized spongivorous nudibranchs, show that while all 3 features change significantly with increasing live mass, as expected, the 2 species cannot be differentiated on the basis of radular characteristics.  The authors argue that radular characteristics are of little taxonomic use in this group of molluscs; specifically, regression statistics of radular characters would be of limited utility in delineating species unless live mass is taken into account, and then only for number of cusp rows and length of cusps.  As for the latter, the data1 do show a significant difference in the relationship of increasing cusp size to increasing live mass in the two species, with Doris2 maintaining relatively shorter cusps with increasing body size.  This is interpreted as a progressive “fine-tuning” of the species’ resource-utilisation abilities.  Doris feeds on sponges with little or no skeletal organisation.  Thus, an increase in food intake with increasing body size can be realised with a simple increase in number of rows and number of cusps per row, with the cusps remaining relatively short3.  In contrast, Anisodoris feeds on sponges with highly organised and robust skeletons.  Here, a longer relative tooth size with increasing body size will maximise the amount of prey ingested per unit time.  Bloom & Bloom 1977 J Moll Stud 43: 296.

NOTE1 the authors have chosen to present all their data as tabulated regression statistics, which is fine for the science part of the study, but falls short on clarity of presentation

NOTE2 cusp size in Peltodoris increases with body mass at about 4 times the rate of increase in Doris

NOTE3 the authors provide an apt analogy of boosting the size of a “broom” while maintaining a constant “bristle” size

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Research study 3

In a study at the Bamfield Marine Sciences Centre, British Columbia a researcher claims that “Peltodoris nobilis… consumed multiple (sponge) species… ” in Barkley Sound, but a check of the tabulated data shows that of 24 individuals whose diets can be identified at 3 sites, all 24 consume a single sponge species Hymeniacidon ungodon; hardly “multiple species” as claimed.   Penney 2013 J Moll Stud 79: 64.

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Research study 1

map of northern California showing distribution of the aeolid nudibranch Phidian hiltoni at various times dating from 1904 to presentThe aeolid nudibranch Phidiana hiltoni in California usually preys on hydroids, but recent work by researchers at the Marine Sciences Institute, Santa Barbara shows that it will also eat several species of nudibranchs.  This is shown directly by diet analyses and laboratory feeding trials, and indirectly by effects on nudibranch populations in areas of northrn California into which P. hiltoni is spreading.  The movement northwards from the species’ previously most northerly limit of distribution is shown on the maps above dating from 1904.  Through feeding tests in the laboratory the researchers determine which other nudibranch species are vulnerable to predation by Phidiana, then they correlate this vulnerability with the extent to which these other nudibranch species have increased or decreased in numbers after the arrival of P. hiltoni (see histogram below Right). Of 40 nudibranch species observed, 15 are found to be vulnerable (including P. hiltoni itself) and these have shown a statistically significant two-thirds decline in overall population numbers over the 3-decade span between assessments.  In comparison, 25 non-vulnerable species exhibit no significant overall change in numbers.  Only P. hiltoni has significantly increased in number.  The authors conclude that the incursion of P. hiltoni has likely caused the decline, perhaps through a combination of predation and competition for hydroid prey.  The causes for range expansion of P. hiltoni may relate to warming coastal water temperatures in that part of California.  Goddard et al. 2011 Mar Biol 158: 1095. Photograph courtesy Kevin Lee, Fullerton, California diverkevin.

photograph of aeolid nudibranch Phidiana hiltoni courtesy Kevin Lee, Fullerton, Californiahistogram assessing effects of incursion of the nudibranch Phidian hiltoni into areas of California previously not occupied abundances of other nudibranch speciesNOTE1   overall, 20 taxa are present in feces of 62 field-collected P. hiltoni.  Eight of these taxa are hydroids, and these represent over 80% of the prey records.  Genus Plumularia is the most common hydroid prey

NOTE2  population numbers of nudibranchs are assessed by timed counts during 1969-1975 and again in 2007-2009.  Literature reports yield distributional data for P. hiltoni for dates outside of the authors’ own collections, during the period 1904-2010

NOTE3  while this is true for overall nudibranch numbers, only 2 species, Hermissenda crassicornis and Cuthona albocrusta, show significant decreases in population numbers. It also should be noted that 4 non-vulnerable species also decline significantly in numbers during the same period

Phidiana hiltoni 6X

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Research study 1

drawing of anaspidean Phyllaplysis taylori showing location of anusdrawing of digestive system of the anaspidean Phyllaplysia tayloriPhyllaplysia taylori inhabits eelgrass plants Zostera marina and eats mainly diatoms growing on the blades of the eelgrass. Diatoms have a tough, siliceous outer covering and are generally abrasive to soft tissues.  Phyllaplysia’s radula is broad with evenly spaced shovel-shaped cusps. The food is scraped up with the radula and passed through a pair of jaws made of numerous tightly packed small rods, and is moved into the esophagus. From the esophagus the food goes to a small crop. Paired salivary glands provide mucus for lubrication and possibly some enzymes for preliminary digestion. 

From the crop the food goes to a gizzard-like stomach, which is a large muscular sac lined with intermeshing chitinous teeth that grind the diatoms to a paste (see drawing on Right).  The food then moves through the posterior gizzard and is strained through a dozen or so long spines that project from the wall of the gizzard. Final digestion and absorption occur in the digestive gland.

Studies on Phyllaplysia from Elkhorn Slough, California in which stomach teeth are stained by brief exposure to a radioactive label show that 4% of each tooth is replaced by growth every day.  Replacement of a tooth in 25d far exceeds the normal wear from a diet of diatoms and the author suggests that the excess is worn off by the milling action of the intermeshing motion of the teeth.  Beeman 1969 Biol Bull 136: 141; drawing of Phyllaplysia from Beeman 1970 Veliger 13: 1; drawing of gut system from McCauley 1960 Proc Cal Acad Sci 29: 549.photograph of radular cusps of the anaspidean Phyllaplysia taylori

NOTE an injection of 3µc of H3-thymidine per gram live body mass.  This produces a distinct labelled band at the base of each stomach tooth within 10h.  The subsequent movement of the band up to the grinding tip
of the tooth is followed autoradiographically over the next 25d

Radular cusps of Phyllaplysia taylori

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Research study 1

photograph of copulating notaspidean opisthobranchs Pleurobranchaea californica courtesey John Butler, La Jolla, California and NOAA
The large notaspidean Pleurobranchaea californica is an active predator and scavenger.  It finds its food through perception of odours and will follow a scent down a chemical gradient. Tests using application of squid juice to various parts of the body show that the sensitive areas to food odours are concentrated on the anterior parts of the oral veil and foot, and the tips of the rhinophores (indicated by purple dots on the drawings).  Pleurobranchaea can readily track prey located in the arms of T-mazes.  Lee et al. 1974 Behav Biol 12: 297. Photo courtesey John Butler, La Jolla, California and NOAA

Mating Pleurobranchaea californica at 265m
depth off the coast of San Diego, California.

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Research study 2

Pleurobranchaea californica has been the subject of many learning and behavioural studies.  Its repertoire of behaviours, including crawling, egg laying, copulation, swimming, and feeding are organised hierarchically, with feeding occupying a relatively dominant position.  Feeding involves quick eversion of a proboscis that grasps, ingests, and swallows food consisting of various living and dead organisms.  The dominance of feeding behaviour is illustrated when an indivual is flipped onto its back.  Normally, righting occurs within a few seconds but, if pieces of raw squid are fed to an inverted indivual, it remains on its back until feeding is completed.  There is plasticity in this dominance hierarchy.  For example, when an individual is laying eggs, feeding is suppressed, thought by the authors to be possibly an adaptation to reduce the risk of an individual consuming its own eggs.  If hemolymph from  an egg-laying individual is injected into a non-laying one, both egg-laying and suppressed-feeding behaviours are induced, suggesting that the behaviours are hormonally regulated.  The researchers are able to modify behaviours through a form of learning known as classical conditioning, as follows. 


photograph of a cephalaspidiean Pleurobranchaea californica about to undergo learning tests
Pleurobrancheae about to undergo conditioning.

cephalaspidean Pleurobranchia californica being touched with a probe
A touch on its oral veil with a probe normally elicits...
cephalspidean Pleurobranchaea californica withdraws from touch of probe
... withdrawal. This is a conditioned stimulus (CS). If the...

Cephalaspidean Pleurobranchaea californica undergoing classical conditioning
conditioned stimulus is combined with a food stimulus (squid juice on the rod), and touched to the oral veil (the unconditioned stimulus or US) and repeated many times...

Cephalaspidean Pleurobranchaea californica undergoing classical conditioning
... over a short period of time, an individual acquires a classically conditioned feeding response to the CS alone. Thus, the individual is now trained to lunge and bite at a...
Cephalaspidean Pleurobranchaea californica undergoing classical conditioning
...clean probe. These individuals can later be trained not to exhibit this feeding response by application of a mild electrical shock to the oral veil when they exhibit it.

Later, the same individual can be re-trained to exhibit the classical conditioning response more quickly than can naïve animals, signifying memory retention.  From these and many other pioneering experiments in the mid 1960s-early 1970s on Pleurobranchaea and other opisthobranch species, has arisen a large research discipline on brain, behaviour, and neurophysiology of learning that continues to present day.  Davis et al. 1974 Am Zool 14: 1037; see also Davis et al. 1977 J Comp Physiol 117: 99.

NOTE  for example, 20 over a 1h period

NOTEa plethora of learning studies exists for several west-coast species of opisthobranchs, including Pleurobranchis californica, Hermissenda crassicornis, Tritonia spp.,and Aplysia californica.  As this is such a huge subject, only selected papers especially those emphasising field studies will be included in the ODYSSEY

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