Foods & feeding
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  Ingestive conditioning
 

This section continues with information on diets and feeding ecology of west-coast nudibranch & relatives relating to ingestive conditioning. Information on diets and feeding ecology of west-coast genera can be found in these sections:
PREFERRED FOODS FEEDING ECOLOGY & GROWTH: GENERA A-C,
PREFERRED FOODS FEEDING ECOLOGY & GROWTH: GENERA D-G
,
PREFERRED FOODS FEEDING ECOLOGY & GROWTH: GENERA H-M,
PREFERRED FOODS FEEDING ECOLOGY & GROWTH: GENERA N-P
,
PREFERRED FOODS FEEDING ECOLOGY & GROWTH: GENERA R-T, and
PREFERRED FOODS FEEDING ECOLOGY & GROWTH: SACOGLOSSANS.

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Research study 1
 

photograph of the aeolid nudibranch Aeolidia papillosaIf a nudibranch or other animal species is maintained for a few weeks on a diet of a single food species, it may become habituated to it and will select it over other species in laboratory preference tests.  Switch its diet to another less-preferred species and then test it again, and the original species drops to a lower position on the feeding heirarchy.  This is termed ingestive conditioning and is found in many animals including humans.  A number of studies of ingestive conditioning in Aeolidia papillosa, most in Britain but some at Friday Harbor Laboratories, Washington show that not only is the current conditioning diet an important influence on prey selection, but also the past-history diet.  Thus, individuals feeding on the same diet may not select prey in similar proportions if their dietary histories have differed. Hall et al. 1982 J Anim Ecol 51: 907; Hall & Todd 1984 J Exp Mar Biol Ecol 82: 11; Hall & Todd 1986 J Moll Stud 52: 193.

NOTE  no data are presented because the most complete of these studies is done in Scotland and uses anemones not found on the west coast of North America.  Aeolidia itself is circumboreal in distribution and is found on both Pacific and Atlantic coasts and in parts of northern Europe

NOTE  in these investigations preferences are determined by olfactory choice in a preference chamber.  Such tests are commonly done in pair-wise fashion in a Y-tube-type choice device. In comparison, in these experiments, single test Aeolidia are simultaneously presented with several prey (4 or more) in a multi-channeled preference chamber

 
Research study 2
 

photograph of the sacoglossan opisthobranch Placida dendritica courtesy Erling Svensen, Norwaygraph showing food/host choice in the sacoglossan opisthobranch Placida dendritica
In Newport Bay, Oregon the sacoglossan Placida dendritica1 feeds mainly on 3 green-algal species2, Codium setchellii, C. fragile, and Bryopsis corticulans.  Interestingly, an individual becomes fixed on a single food species early in life and stays with this species until it dies.  In a given area of the shore, then, some individuals will specialise on one algal species while other nearby individuals will feed on another. 

If animals are collected from one plant-host species, for example, Bryopsis corticulans, then given a choice of the 3 plant species in the laboratory, within 10h most individuals have crawled back onto the original host Bryopsis (see graph above). Placida’s rigid feeding specificity is not modified by hunger level, algal condition, or other factors and, if transferred to an “unfamiliar” food species being eaten by neighbouring conspecifics, an individual will frequently die of starvation rather than make the switch.  The author concludes that the extreme trophic specialisation of the short-lived3 Placida may make the cost4 of switching diets too high.  Trowbridge 1991 Ecology 72: 1880. Photo of Placida courtesy Erling Svensen, Norway and seaslugforum.


NOTE1 the green coloration of Placida and other sacoglossans owes to the ramification of the digestive gland on the upper surface of the body and into the dorsal projections. The gland is coloured by chloroplasts from the algal food incorporated into cells lining the tubules of the digestive gland. The chloroplasts continue to photosynthesise and provide energy and nutrients to the host sacoglossan

NOTE2  of the 3 food species, growth on Bryopsis may be 20X that on Codium spp.  Trowbridge 1992 Mar Biol 114: 443; see also Trowbridge 1992 Mar Ecol Progr Ser 83: 207 for information on preferential settlement onto Codium setchellii by Placida larvae

NOTE3  the author estimates life span of Placida dendritica to be 1-2mo, and sexual maturity being attained in less than 1mo

NOTE4  although radular tooth morphology is known to change in Placida on different plant-host species, the author considers the differences insufficient to explain the lack of adaptability in the present study

 
Research study 3
 

photograph of aeolid nudibranch Hermissenda crassicornisAn investigation on the effect of ingestive conditioning on subsequent prey choice in Hermissenda crassicornis, although done in Woods Hole, Massachusetts, may provide ideas for a similar study on west-coast Hermissenda. The experiments use a Y-tube apparatus containing test prey specimens in one or both arms, with Hermissenda crawling up the stem and making choices at the junction of the Y. Results show statistically significant results only for: 1) Ciona (a tunicate)-conditioned individuals selecting Ciona over BLANK, and 2) Haliplanella (an anemone)-conditioned individuals selecting Haliplanella over BLANK, and 3) Tubularia (a hydroid)-conditioned individuals selecting Tubularia over Metridium (an anemone).  There are several problems with the methodology in this study, however, and the results should be interpreted with caution.  Avila 1998 J Moll Stud 64: 215.

NOTE  in a Y-apparatus the test animal crawls up the base stem of the Y into a water current flowing from the double ends at the top.  Tests are done either with a single prey vs. no prey (BLANK) or with one prey vs. another prey in the ends of the Y. Prey, if motile, are constrained by cages or gates).  The test is scored when the predator, in this case Hermissenda, turns into one or the other of the branches of the Y
 
NOTE  the animals are raised from eggs for use in learning and behaviour studies and are maintained solely on a diet of hydroids Tubularia crocea.  Individuals to be tested are conditioned for 2wk on diets of hydroids Tubularia crocea and Pennaria sp., tunicates Ciona, mussels Mytilus edulis, and anemones Metridium senile and Haliplanella luciae.  The author provides no explanation for including mussels in the experiments (mussels are not eaten by field animals).  Some statistical problems include possible effects of “double” conditioning of test animals on T. crocea diets, missing data, use of animals more than once in an experiment and in more than one experiment, and generally too-small sample sizes possibly contributing to false assumption of non-significance in the data

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What advantage does ingestive conditioning confer on an animal? Consider these answers, then CLICK HERE for explanations.

An olfactory search image is formed, which increases foraging efficiency.

A predator's sense organs become more closely "attuned" to a speficic prey. 

Handling time is reduced . 

Digestive efficiency is increased. 

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Research study 4
  In general, if a previous food is adequate for growth it is of selective advantage for an animal to continue with it, rather than switch to a new perhaps untested diet and risk the chance that it will be inadequate or even toxic.  In the specific case of Aeolidia papillosa, one author notes that its colours in different geographical areas (California, Britain, France) tend to mimic the colours of its current sea-anemone food.  In a study in Britain, individuals fed for a day or two on the anemone Actinia equina, a predominately red-coloured species, begin to show tints of red in their cerata.  Perhaps these mimicking colours provide protection for Aeolidia from predatory gadoids (cods) and other fishes, which may view them as stinging sea anemones and photograph of an aeolid nudibranch Aeolidia papillosaconsequently avoid them.  Were this to be the case, Aeolidia would be better protected if it were to resemble sea anemones in its immediate vicinity.  Thus, ingestive conditioning would provide a means by which an individual Aeolidia could benefit by eating a single anemone species over a long time.  There are several implied “ifs” here, but it is an interesting idea and one worth investigating further on west-coast Aeolidia Edmunds 1983 J Moll Stud 49: 80.



Compare the colour of this specimen of Aeolidia papillosa
with the one shown in Research Study 1 above, both
from
the west coast of Vancouver Island, British Columbia 1.5X
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