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  Feeding, growth, & regeneration
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Ingestive conditioning & other food-based learning

  Ingestive conditioning & other food-based learning is considered here, while LARVAL FEEDING, ADULT FEEDING, PREY RESOURCES, ENVIRONMENTAL EFFECTS ON FEEDING, and GROWTH & REGENERATION are considered in other sections.
Research study 1

graph showing learning curve for ochre stars Pisaster ochraceusphotograph of ochre star with cutleryOne doesn’t usually consider sea stars as having the capability to learn.  Their nervous system is decentralised, a brain or even any kind of substantial ganglionic cluster is absent, and their sensory and motor capabilities are built on a radial plan.  Nonetheless, experiments done by a researcher at the University of California, Santa Barbara suggest that sea stars Pisaster ochraceus can readily learn to associate light with presence of food.  The experiments involve 8 medium-sized sea stars held in individual black-plastic-covered tanks.  A 40W light above each tank is used  as the conditioning stimulus.  Food consists of sliced-open mussels.  On sensing a mussel (the unconditioned stimulus) dropped into its tank, a sea star crawls from its usual resting location at the top of the tank to retrieve the mussel from the bottom, then crawls back to the top to eat it.  This is the unconditioned response.  Training consists of 8 trials at 2d intervals over a 16d period.  Each trial involves the light (the conditioned stimulus) being switched on for 15min at the same time the mussel is dropped into the tank.  If a sea star does not respond in 15min, it is scored as a non-response.  At the end of the 16d training period the author predicts that, like Pavlov’s dogs that after training salivate on exposure to a conditioning stimulus sound, the sea stars will crawl to the bottom of the aquarium in a conditioned response to the light being switched on.  And this is what happens (see graph).  For the subsequent 10 tests up to Day 36, all with the light being turned on, all 8 individuals exhibit conditioned responses where they crawl to the bottom in “expectation” of food.  They have learned to associate the light with food.  Note that after Day 38, when the sea stars are fed only after a delay of 12h, their responses decline fairly quickly to zero.  What “memory” of the association they have is quickly lost.  The design of the experiment seems somewhat convoluted, and the attempt to use each individual as its own control (not clearly explained by the author) seems inappropriate, but perhaps the work will stimulate someone else to repeat it with an improved design.  Landenberger 1966 Anim Behav 14: 414.

NOTE  during these 10 tests, the author still gives food to the subjects but now does so immediately at the end of the 15min conditioned-stimulus period (light switched on).  This is described as a “control”, but a control for what is not clear, and how this might affect the learning process is not known.  Even Pavlov’s dogs had to eat, but this was unlikely to have happened so close in time to the preceding test session.  In many respects, the behaviour from Day 38-42 seems to be a clearer example of learning, because the sea stars are not fed until 12h after the end of the test

NOTE  to date this seems not to have happened, at least not for P.  ochraceus or other west-coast species.  Similar training experiments have been done on some Atlantic-coast asteroids

Research study 2

graphs showing ingestive conditioning in 2 species of Pisaster sea starsIngestive conditioning is the modification of an animal’s behaviour by past experiences in eating certain foods, and, as such, can be considered a type of learning.  An individual is conditioned to prefer or avoid a food depending upon what type of experience it remembers from past encounter with it.  In tests of ingestive conditioning in sea stars by the same researcher featured in Research Study 1 above, Pisaster spp. fed only on Tegula for 3mo show a greater preference for these snails when presented later in cafeteria-style tests along with chitons and mussels.  When mussels are presented with other prey species to P. giganteus and P. ochraceus, the proportion of mussels ingested increases over time.  Note in the graphs that while both species are consuming almost 100% mussels by the end of 5wk, the learning curve for P. giganteus is much steeper than that for P. ochraceus.  This may be explained by the latter species having had more recent experience eating mussels in the field prior to being brought into the laboratory.  Landenberger 1968 Ecology 49: 1062.

NOTE  in humans and other vertebrates, ingestive conditioning results from feedback signals being generated from a meal, and leads to that food type being either sought out or avoided for later meals. On the positive side such signals could include a satisfying fullness, extra energy, feeling of well-being, and so on, while on the negative side they could include feelings of nausea, bloatedness, continued hunger, and the like.  Sensory input used by vertebrates to identify favoured foodstuffs include vision, smell, and taste.  Vertebrates develop search images for foodstuffs, such that other equally good foods may be bypassed in the search for foods that have the most recent familiarity.  Invertebrates such as sea stars also develop search "images" for favoured foods such as mussels, but use chemotactile ("taste/touch" cues. This topic is considered in more detail elsewhere in the ODYSSEY: LEARN ABOUT NUDIBRANCHS & RELATIVES: FOODS & FEEDING: INGESTIVE CONDITIONING