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  Feeding, growth, & regeneration
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Adult feeding


drawing of gut system of a representative sea star Pisaster ochraceusThe gut system of a sea star consists of mouth, 2 stomachs, pyloric ceca, rectum, rectal ceca, and an anus (see drawing on Right). Food is either consumed whole or enveloped in the extruded cardiac stomach, depending upon species. Preliminary digestion occurs in the cardiac stomach, then the food slurry is moved into the pyloric stomach and thence to the pyloric ceca (2 per arm) for final digestion and absorption. photograph of a leather star Dermasterias imbricata showing locations of anus and madreporiteUndigested food residues pass via the rectum to the anus, with possibly some further absorption in several rectal ceca that sprout from the rectum.

NOTE the function of the rectal ceca is not fully understood

The anus of a sea star is located in
the center of the aboral disc. It is
usually hard to spot, but not in leather
stars Dermasterias imbricata. 2.5X

Research study 1

photograph of a sunflower star Pycnopodia helianthoides with a red sea urchin
Feeding by stomach extrusion and external digestion is commonly thought to be a feature of all sea stars, but many species, especially the multi-armed ones, ingest prey whole.  For example, Pycnopodia helianthoides ingests sea urchins whole, as well as snails.  Digestion of the former takes a day or two, and later the test plates and other hard parts are egested from the mouth.  Moon snails Polinices spp. may be first suffocated by being wrapped in the arms.  This causes the snail to relax and be pulled out of its shell for digestion.  Observations indicate that a large moon snail can be completely consumed in a day. Kjerschow-Agersborg 1918 Biol Bull 35: 232.


By the "alert" appearance of this sunflower star
Pycnopodia helianthoides, it likely has full sense
of a nearby potential prey - but a prey that is
much too big and spiny to capture 0.4X

Research study 2

photograph of a sea star Crossaster papposus
A rose star Crossaster papposus has an expanded mouth diameter of several centimeters for ingestion of mussels and snails whole, in comparison with mouth diameters of only a few millimeters for sea stars that feed by extruding their stomachs.




Rose star Crossaster papposus 0.4X

Research study 3

photograph of close view of tube feet of sea star attached to the substratumphotograph of a sea star hunched over a prey on a rock

Hunched-up sea stars are seen commonly throughout the intertidal and subtidal regions.  What is going on?  Well, if the prey is an attached bivalve, the predator has fastened as many tubefeet as are available, and pulls upward (see photo on Right of Evasterias troschelii). 

If the prey is an unattached bivalve, the pulling action is the same, but from either side. In both methods, it appears that the arm ossicles lock into place to provide a rigid structure against which the tube feet pull.  The force is generated by the longitudinal musculature of the tube feet. An Evasterias troschelii attacking a littleneck clam Protothaca staminea can apply a combined force of almost 6kg over a several-hour period. The valves are pulled apart at a rate of about 1mm .  min-1.  Since the predator only requires a gape in the shells of about 0.1mm, it may insert its stomach relatively early in the attack.  Time to complete digestion of the prey is 7-16h depending upon water temperature, and relative sizes of predator and prey. Christensen 1957 Limnol Oceanogr 2: 180.

Research study 4

photograph of an ochre star eating a sea mussel Mytilus californianus
A 30-cm diameter Pisaster ochraceus can exert forces up to 4kg when opening a clam.  The predator does not use toxins in the process.  Feder 1955 Ecology 36: 764.




Ochre star Pisaster ochraceus attacking a sea mussel
Mytilus californianus
. When the prey is a sea mussel,
the stomach commonly everts and enters the prey via
the byssus-thread opening on first contact of the sea
star’s tube feet with the mussel How does the stomach
get in?  No-one really knows, but one possibility is that
it creeps in by ciliary action 1.2X

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photograph of a sea star with stomach extrudedWhat is the advantage of extruded-stomach feeding in sea stars? Check over the possibilities below, then CLICK HERE to see explanations.

It enables larger prey to be attacked and eaten. 

Digestion is more efficient. 

Portions of the substratum can be digested and eaten. 

Extruded stomach
of a sea star

Research study 5

photograph of ochre star Pisaster ochraceus with partially digested mussel Mytilus trossulusWhat effects do feeding and starvation have on digestive activities of sea stars Pisaster ochraceus?  This is investigated at the Shannon Point Marine Center, Washington by comparing the activity of proteolytic enzymes in the pyloric caeca of animals fed on mussels Mytilus trossulus or starved, for periods of up to 4wk.  Results show that feeding leads to increased levels of protease and typtic activities as compared with levels in starved individuals. Moreover, the finding of greater variability in trypsin activity than of protease activity in fed vs. starved individuals suggests to the authors that tryptic enzymes are the major digestive enzymes in P. ochraceus. Holzman et al. 1985 Mar Biol 90: 55. 


Ochre star Pisaster ochraceus with partially
digested mussel Mytilus trossulus 1.7X

Research study 6

graph showing relationship between size of ochre sea star Pisaster ochraceus and size of bay mussel Mytilus trossulus preferredhistograms showing preference by juvenile ochre sea stars Pisaster ochraceus for smaller bay mussels Mytilus trossulus that yield greatest profitabilityA topic that has received little research attention for west-coast sea stars is the relationship between body size of predator and that of its prey. This is investigated by researchers at the University of British Columbia for ochre stars Pisaster ochraceus and their principal prey in sheltered habitats, bay mussels Mytilus trossulus (8-57mm shell length). Results from a series of laboratory experiments assessing effects of predator and prey sizes on sea star’s prey-size preference and consumption rates show predictably that both increase with size of sea star (for data on prey sizes see graph on Left). Sea stars of all sizes eat significantly more small mussels (20mm L) than larger mussels (40-55mm L), probably owing to shorter handling times. Interestingly, the data show that while juvenile sea stars prefer the most profitable prey sizes (see histograms on Right), adults show only a non-significant pattern. Note in the graph for juveniles that the largest mussels are preferred, requiring longest handling time, but yielding greatest profitability. This type of “energy maximising” strategy is exhibited by many animal species, but why for sea stars it is of stronger selective value for the juveniles and not the adults is unclear. The authors discuss this finding from a “time minimising” point of view, that for whatever reason behavioral restraints on feeding time have greater selective value than greater consumption. Gooding & Harley 2015 Biol Bull 228: 192.

NOTE profitability is defined as maximal tissue consumption per unit handling time, the latter taking into account both manipulation and ingestion times leading to final discarding of the empty shell valves

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