Foods & feeding
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  Preferred foods & feeding ecology: genera H-M
 

This section continues with information on diets and feeding ecology of west-coast nudibranch genera H-M. Information on diets and feeding ecology of west-coast genera is provided below for GENERA H-M and in 5 other sections:
PREFERRED FOODS FEEDING ECOLOGY & GROWTH: GENERA A-C
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PREFERRED FOODS FEEDING ECOLOGY & GROWTH: GENERA D-G,
PREFERRED FOODS FEEDING ECOLOGY & GROWTH: GENERA N-P
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PREFERRED FOODS FEEDING ECOLOGY & GROWTH: GENERA R-T,
PREFERRED FOODS FEEDING ECOLOGY & GROWTH: SACOGLOSSANS, and INGESTIVE CONDITIONING.

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Haminoea

 

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Research study1

 

photograph of Haminoea virescens courtesy  Jan Kocian
In the Puget Sound region of Washington Haminoea callidegenita eats benthic diatoms and detritus. Gibson & Chia 1995 Mar Ecol Prog Ser 121: 139.

 

 

 

Shown here the related Haminoea virescens.
Photo courtesy Jan Kocian and seaslugforum

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Hermissenda

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Research study 1
 

photograph of aeolid nudibranch Hermissenda crassicornis crawling amongst several colonial tunicates
photograph of an aeolid nudibranch Hermissenda crassicornis crawling on a solitary tunicate Cliona sp.Hermissenda crassicornis
in the field usually eat various species of hydroids, but in the laboratory individuals do well on diets of solitary tunicates Ciona spp., small anemones Metridium spp., and even bits of miscellaneous invertebrates and fish-food pellets. Harrigan & Alkon 1978 Biol Bull 154: 430; Avila 1998 J Moll Stud 64: 215.

 


Hermissenda crassicornis
crawling
among several species of colonial
tunicates 0.8X

Hermissenda crassicornis crawling on a
solitary tunicate, possibly Ciona sp. 1X

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Research study 2
 

close-up photograph of aeolid nudibranch Hermissenda crassicornis showing features of head regionBehavioral studies combined with intracellular-recordings of nerve-receptor pathways in the mouth/tentacles and rhinophores in Hermissenda crassicornis at Woods Hole, Massachusetts confirm earlier findings that the oral tentacles and mouth are highly sensitive to palatable foods, while the rhinophores possess little or no "taste" function.  Alkon et al. 1978 J Gen Physiol 71: 177.

NOTE  the author of this earlier study suggests that the oral tentacles are sensitive to “gustatory” stimuli, that is, close-in contact with potential food items, while the rhinophores are responsive not to “distance stimuli” but to certain types of tactile stimuli.  In fact, the author concludes that Hermissenda has no distance perception of food, only contact perception via the oral tentacles.  This doesn’t make much sense in view of the location of the rhinophores on top of the head and their “layered” morphology.  Current thinking of rhinophore function in nudibranchs is that they are for long-distance chemical perception. Kjerschow Agersborg 1925 Acta Zool 6: 167

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Research study 3
  photograph of aeolid Hermissenda crassicornisHermissenda crassicornis have for some time been cultured at Woods Hole, Massachusetts in the “Hermissenda Resource Facility” to supply specimens for researchers.  Usually, hydroids such as Tubularia crocea, a component of the species' natural field diet, are supplied as food, but they are not always available.  Fish pellets have been tried with some success, but they create unwanted water fouling.  Tests of live invertebrates including tunicates, mussels, and sea anemones indicate that while alternative prey may be eaten, only Tubularia and sea anemones (Metridium senile and Haliplanells luciae) promote growth. In the laboratory, Hermissenda lose body mass on diets of tunicates (Ciona intestinalis) and mussels (Mytilus edulis).  Interestingly, the best laboratory diet with respect to body-mass gain is the sea anemone M. senile.  Avila & Kuzirian 1995 Biol Bull 189: 237.
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Research study 4
 

drawing of gut system of nudibranch Hermissenda crassicornisphotograph of an aeolid nudibranch Hermissenda crassicornis crawling on kelpAnalyses of gut contents of Hermissenda crassicornis collected at 3 sites in mid-northern California discloses a highly varied diet, but with some degree of selectivity.  Surprisingly, for a species that is usually thought to specialise on hydroids, preferred food in laboratory tests is the colonial tunicate Aplidium solidum.  In the field, the same tunicate species is also generally favoured and, while hydroids are frequently eaten, they are always done so in limited quantity.  Sea anemones are consumed only rarely in the field, with only the smallest polyps being eaten.  Other prey items in the guts of field animals are copepods, polychaetes foraminiferans, and sometimes remains of aeolids.  As for the restricted diet of hydroids found in both laboratory and field animals, the authors speculate that it may be based upon a need to consume a minimum number of hydroids to obtain sufficient nematocysts, but not so many as to cause damage to their guts.  Further research may be needed on this subject.  Megina et al. 2007 Cah Biol Mar 48: 1.

NOTE  commonly eaten hydroids at one of the sites are Obelia longissima (91%) and Plumularia sp. (9%)

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Hopkinsia (now Okenia)

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Research study 1
 

photograph of nudibranch Okenia rosacea courtesy Kevin Lee,  Fullerton, California
In the San Diego region of California Okenia (Hopkinsia) rosacea feeds esclusively on bryozoans, most notably Eurystomella bilabiata. McBeth 1971 Veliger 14: 158.

 


Okenia rosacea 1.5X. Photo courtesy Kevin Lee,
Fullerton, California diverkevin

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Janolus

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Research study 1.1
 

graph showing growth of veligers of the nudibranch Janolus fuscusgraph showing growth of juvenile nudibranchs Janolus fuscusCulture of larvae of Janolus fuscus at the Oregon Institute of Marine Biology, Charleston provides data on growth of veligers (see graph on Left), and of juveniles collected from the field (graph on Right).  Food for the larvae is a mixed diet of flagellates Isochrysis galbana and Rhodomonas lens.  Adult food is a diet of arborescent bryozoans Bugula pacifica. Wolf & Young 2012 Biol Bull 222 (2): 137.

NOTE for a photograph of this species go to SETTLEMENT & METAMORPHOSIS

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Melibe

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Research study 1
 

photograph of nudibranch Melibe leonina courtesy Charles Seabourne, Malibu.drawing of internal morphology of digestive system of Melibe leoninaIn Puget Sound, Washington Melibe leonina is often found crawling on the blades of eelgrass Zostera sp.  The most distinctive feature of Melibe is its oral hood; a highly moveable expansion of tissue extending over and around the mouth and bearing a double row of tentacles along its periphery. 

drawing of ceras of Melibe leonina showing details of digestive-gland ducts

The hood is used to capture small planktonic crustaceans by a sieving/straining action. Compression of the hood moves the prey to the fleshy oral lips and mouth. A radula or other form of masticatory apparatus is lacking, although there is a kind of gizzard with stomach-plates (see drawing upper Right). The liquified food passes from the stomach into the many branches of the digestive gland within the cerata (see drawing lower Leftt). An intestine leads to the anus. Kjerschow Agersborg 1921 The Amer Nat 55: 222; Kjerschow Agersborg 1923 Q J Microsc Sci  67: 507. Photo courtesy Charles Seabourne, Malibu.

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Research study 2
 

histogram of dietary preferences of large vs. small Melibe leonina
photograph of Melibe leonina courtesy Kevin Lee, Fullerton, California.A more recent study on Melibe leonina in kelp beds in Monterey, California provides further information on the feeding process.  During the summer and autumn, many dozens of Melibe can be found living on a single Macrocystis kelp plant.  Dye injections into the cerata to permit identification by the authors show that individual nudibranchs routinely swim from plant to plant.  Larger individuals tend to hold their hoods open and upward.  When a planktonic object impinges on the hood, it closes, and the tentacles interlock to prevent escape.  Further contraction of the hood forces water out and moves prey items backwards to the mouth area where they are ingested.  During feeding the hood may be swept from a vertical to a horizontal position, as though straining the water for food.  Smaller individuals behave differently when feeding.  They press their open hoods onto the substratum and appear to feed on surface-inhabiting organisms such as copepods and bivalve spat.

The histograms show the percentage makeup of the diets of large (>10cm body length) and small (<10cm) Melibe.  Note that while small individuals eat principally surface-dwelling copepods (harpacticoids) and some pelagic copepods, large individuals eat a preponderance of swimming copepods, ostracods, and larvae (zoeae, megalopae, veligers).  Ajeska & Nybakken 1976 Veliger 19: 19. Photo courtesy Kevin Lee, Fullerton, California diverkevin.

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Research study 3
 

schematic showing time course of feeding cycle in Melibe leonina
Three feeding modes are described for Melibe leonina: 1) bottom grazing, 2) hood extending into the water column, and 3) if the water is calm (or perhaps only in aquarium conditions), upside-down floating from the surface tension with the hood extended.  A study on feeding using animals collected in San Juan Islands, Washington adds some detail to what is known about the feeding process.  As the hood compresses (squeezes), it is tilted slightly upwards, perhaps aiding in the movement of food towards the ingestive opening at the back.  The filtering is done mostly by the posterior inner tentacles which, when clogged with prey, are positioned inside the mouth.  The mouth closes around them and the lips strip off the prey.  Prey that is too large and/or covered with sharp processes may be regurgitated.

The time-course of the cycle (at 6-12oC) is shown in the schematic. Note that a complete cycle takes about 30sec, of which about 12sec (or 40%) is spent with the hood open.  The authors note that the feeding cycle is characterised by phases of proportional duration, suggesting the presence of a central-pattern generator, perhaps in the buccal or cerebral ganglia.
 
graph showing frequency of feeding cycles in relation to concentration of barnacle-larvae prey in Melibe leoninaAnother feeding experiment is conducted in which live natural prey (mostly barnacle larvae filtered from seawater) is offered to aquarium-housed Melibe leonina, with concentrations being increased every 10min.  Hood closures, or feeding cycles, are counted at the end of each 10-min period from a videotape record.  The first 10min represents a “no food” baseline.  The baseline feeding frequency of 0.5 cycles . min-1 is maintained up to a  prey concentration of about 700 organisms . liter-1.  After that, frequency increases dramatically, and then more slowly, to a maximum (in the experiment) of about 3 cycles . min-1 at a prey concentration of 4900 organisms . liter-1.  This is 6 times the rate in the absence of prey.  The authors note that incomplete feeding cycles (cycles that terminate just before hood closure) are more common at low prey densities. Watson & Trimarchi 1992 Mar Behav Physiol 19: 183.

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Research study 4
 

drawing of a juvenile Melibe leonina showing details of buccal ganglia in relation to the digestive system

Research on the role of the nervous system in feeding in Melibe leonina shows that the buccal ganglia are involved, but not in food capture; rather, in the transfer of food through the esophagus.  The ganglia are located on either side of the esophagus just posterior to the mouth, and link by nerve commisures to the more dorsally located cerebral ganglia (not shown in the diagram).  Two motor nerves project from each ganglion, one innervating the posterior esophageal region, the other, the anterior esophageal region close to the mouth. The anterior motor nerves innervate a pair of salivary glands.

histogram comparing numbers of brine-shrimp prey ingested by Melibe leonina with buccal ganglia removedRemoval of the buccal ganglia has no significant effect on the ability of Melibe to catch and ingest brine shrimp Artemia salina (see graph on Right). Note in the graph that sham-operated and ganglia-removed specimens exhibit a similar ability to catch and eat Artemia over a 5-d post-operative period, but with rates slightly decreased from control rates presumably owing to residual trauma from the surgery. 

So, what role do the ganglia play? After 5d, examination of the gut system shows that the prey Artemia are clogged in the esophagus and do not pass through into the stomach as they do in control and sham-operated animals (see graph on lower Left).   In control histogram showing feedin activity of the nudibranch Melibe leonina with buccal ganglia removedand sham-operated animals, 83 and 70%, respectively, of the prey eaten has moved into the stomach, while in ganglia-removed animals only 40% has moved into the stomach.  The authors conclude that the buccal ganglia are not involved in prey capture, but appear to be responsible for controlling the transfer of food from mouth to stomach.  They further speculate that, while swallowing is regulated by the buccal ganglia, the rhythmic movements of the hood may be controlled by a central-pattern generator located possibly in the cerebral ganglia.  Trimarchi & Watson 1992 Mar Behav Physiol 19: 195.

NOTE  sham-operated animals are incised in the same manner as ganglia-removed animals, but the ganglia are left intact.  The authors note that 12h after surgery the animals appear to behave normally (hood closures resemble those exhibited by control animals); however, by 5d after surgery most of the operated animals are dead

 
Research study 5
 


Studies on Melibe leonina collected from eel-grass beds in San Juan Island, Washington confirm that hood closures take place even in the absence of prey, and that frequency of closure is proportional to concentration of prey.  The prey used in these lab experiments is live brine shrimp Artemia salina. The researchers compare closure frequencies under treatments of live Artemia (all treatments use a concentration of 1500 particles . ml-1), frozen Artemia, seawater conditioned with live Artemia (“smell”), plastic beads1, combination of beads and “smell”2, and seawater alone (control).  Individual Melibe are exposed to a treatment and hood closures are then monitored for a 20-min period.  Note in the histogram3 that: 1) hood closures occur spontaneously (see control), 2) closures increase significantly4 in the presence of either “smell” or beads, 3) there is no significant difference in closure frequencies between treatments where particles are involved (beads or frozen Artemia), and 4) the most effective treatment is live Artemia.  The authors note that “smell” alone usually leads to about 70% incomplete closures.  As to why live prey is more effective than dead prey, the authors surmise that an additional stimulus must be provided, perhaps vibrations, from the swimming brine shrimp.  Watson & Chester 1993 Veliger 36: 311.

NOTE1  the beads are plastic SEPHADEX, 350µm in diameter

NOTE2  the beads are soaked overnight in Artemia-conditioned seawater

NOTE3  for some reason, perhaps to make their data more robust for statistical analysis, the authors log-transform the data.  This makes the histogram essentially unreadable.  However, some actual rates of hood closure for interested readers (taken from another table in the paper) are: control = 0.2 . min-1 (i.e., once every 5min), smell = 0.4 . min-1, beads = 0.5 . min-1, smell + beads = 0.6 . min-1, frozen Artemia = 0.6 . min-1, live Artemia = 1.2 . min-1

NOTE4  the horizontal lines on the histogram indicate statistically homogenous subgroups

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