title used in an account of west-coast marine invertebrates entitled A Snail's Odyssey
  Foods, feeding, & growth
  black dot
  Cirral-net feeding
  This section deals with cirral-net feeding, while FOODS and GROWTH & ENERGY BUDGETING are dealt with in other sections.
 
Research study 1
 

Photograph of a cirrus of a barnacle Balanus glandula showing details of its branched (biramous) structure and of its setae courtesy Arsenault et al. 2001 Proc Roy Soc Lond B 268: 2149.There are 6 pairs of cirri in a barnacle, derived from the 6 pairs of swimming appendages of the cypris larva. Only the 3 pairs of larger, posterior cirri form the filtering apparatus. Each cirrus is biramous, that is, bears 2 branches (see photograph on Right). Each segment of each branch of each cirrus bears bristles, or setae, and the net is created by overlapping setae originating from adjacent branches. In Balanus glandula there are 4-7 pairs of setae arising from each segment. The inset detail in the photograph shows a segment bearing 7 pairs of setae, only half of which can be seen photograph of a feeding barnacle Balanus nubilisbecause of the camera angle. When covered by water the cirri stroke repetitively to catch food. Food impinging on the setae is scraped off periodically by the shorter, anterior 3 pairs of cirri and passed to the mouth for ingestion.  Photograph on Right courtesy Arsenault et al. 2001 Proc Roy Soc Lond B 268: 2149.

NOTEthe cirri are commonly referred to as "legs"

NOTE not much has been reported on feeding behaviour in west-coast acorn barnacles, but a study in Japan on Semibalanus cariosus suggests that more feeding is done at night and that light has general and significant effects on activity.  Takeda et al. 1998 Crustaceana 71: 299.

Balanus nubilis feeding. Note the 12 cirral
branches of the posterior 3 pairs of cirri
that form the filtering net. Also visible
are some of the shorter, stouter branches
of the anterior 3 pairs of cirri that are
involved in food manipulation and ingestion 2X

 
 
photograph of a barnacle Balanus nubilis feeding taken from a video

CLICK HERE to see a video of a giant barnacle Balanus nubilis feeding. Note that this individual is not stroking. It may be that it is allowing a slight current to do the work, or perhaps there is little food available in the aquarium in which it is being held.

NOTE the video replays automatically

 
Research study 2
 

graph showing leg (cirrus) length in barnacles Balanus glandula in relation to wave velocityStudies on the barnacle Balanus glandula at the Bamfield Marine Sciences Centre, British Columbia show that individuals in still water produce cirri that are almost twice as long as ones produced by individuals in moving water.  The magnitude and precision of the adjustment is amazing.  Over a 3-fold range of wave exposure the average maximum velocity of  breaking waves explains over 95% of the variation in average cirrus length. Longer cirri are effective in calm water but not in high-velocity water flow, because the higher drag would cause bending or buckling.  The precision of the response suggests to the authors that barnacle cirrus length could even be used to estimate average wave exposure in different populations along the shore.  Arsenault et al. 2001 Proc Roy Soc Lond B 268: 2149.

NOTE  measured with mechanical devices, each consisting of a hollow plastic ball attached to a spring inside a plastic-pipe by monofilament line.  Drag on the ball pulls on the spring which, in turn, moves a small rubber disc along the line.  The device is calibrated by suspending known masses to the ball

 
Research study 3
 

photograph of a cirrus of a barnacle Balanus glandula showing length measurement used in study of allometry in several west-coast barnacle speciesgraph showing allometric relationship between ramus (cirrus) length and body size in 2 species of west-coast barnacles Semibalanus glandula and Pollicipes polymerusBoth the size of a cirrus, and the length and spacing of setae will affect feeding effectiveness under different flow conditions.  This is investigated in detail for 4 intertidal barnacles species Balanus glandula, Chthamalus dalli, Semibalanus cariosus, and Pollicipes polymerus at the Bamfield Marine Sciences Centre, British Columbia.  All species exhibit a negative allometric relationship between ramus (cirrus) length and body mass (log-log slopes = 0.23-0.29; data shown in the graphs only for Semibalanus cariosus and Pollicipes polymerus). What this means is that as a barnacle grows, its cirri become disproportionately smaller relative to overall size.  The authors suggest that this may allow smaller barnacles to “reach above” boundary layers, thus avoiding lower flow-velocities and particle flux associated with these layers.  The 3 acorn-barnacle species exhibit strong morphological effects of wave exposure, with calm-water individuals having cirri that are 37-80% longer and 18-25% thinner than in individuals from wave-exposed areas (data not shown here, but see similar data for cirrus length in Research Study 1 above). Also, the setae of calm-water individuals are 36-50% longer and up to 25% more closely spaced.  Similar, but less pronounced, differences are evident in goose-neck barnacles Pollicipes.  Longer cirri with longer more closely packed cirri will permit more efficient feeding in areas of low flow, while shorter, stouter cirri with shorter setae will be less vulnerable to damage in areas of high-energy flow.  Interestingly, the barnacle species exhibiting least variability in these parameters, Pollicipes polymerus, also inhabits the narrowest range of habitats (only high-energy shores), while the species exhibiting greatest variability, Balanus glandula, inhabits the widest range in habitats – exemplifying the selective advantage of increased phenotypic plasticity.  Marchinko & Palmer 2003 Zoology 106: 127.

NOTE  although goose barnacles have their own section in the ODYSSEY, for convenience they are included here along with the acorn barnacles

 
Research study 4
 

photograph of cirrus lengths of barnacleshistograms showing effect on cirrus size of conditions of water flow in barnacles Balanus glandulaIs the cirrus/seta response in barnacles mainly genetic or phenotypic?  Is the response age-dependent?  What is its time-scale?  These questions are addressed in a study on Balanus glandula at the Bamfield Marine Sciences Centre, British Columbia using field and laboratory treatments.  To test the first question, juvenile barnacles are translocated from wave-protected and wave-exposed field sites to high- and low-flow conditions in the laboratory and kept for 35d.  The histogram on the Left shows that individuals from the wave-protected site maintain long cirri in low-flow conditions in the lab, but grow short cirri in high-flow conditions (left set of 3 bars). Conversely, individuals from the wave-exposed site grow long cirri in low-flow conditions in the lab, but maintain short cirri in high-flow conditions. Similar results are obtained for ramus (cirrus) diameter and setal lengths, but these data are not shown here.

Age-dependence is tested by translocating juvenile and adult barnacles from protected and exposed shores into calm-water conditions.  Remarkably, both age groups from the wave-exposed site produce cirri over 100% longer than the original population, while those from the protected site remain unchanged.  The important implication here is that cirri can be modified at any time throughout an individual’s lifetime.  A third translocation of adults from the 2 sites into low-flow conditions shows that the transformation is quick, with the wave-exposed individuals gowing cirri (leg 6) as long as their protected-shore counterparts within a few weeks (likely requiring only 1-2 moults; see photos on Right). The author concludes that the cirrus variation is largely environmentally induced, but possibly with some genetic input.  Thus, rather than the one extreme of larvae having to settle in areas with flow conditions matching certain genetically coded requirements, they settle regardless of flow conditions and rely on phenotypic plasticity for quick adaptation.  Marchinko 2003 Evolution 57: 1281.

NOTE  high flow: 36cm . sec-1; low flow: 1cm . sec-1

 
Research study 5
 

The previous studies are done at comparatively slow flow speeds and the question arises as to the extent of this phenotypic plasticity: does it reach a limit at faster speeds?  This is investigated at the Hopkins Marine Station, California by measuring cirral leg lengths of Balanus glandula at 15 sites differing in flows from 0.5-14m . sec-1. Results show that at the 4 “slowest” sites (4 locations on the Monterey Pier: water velocities <3m . sec-1), cirral lengths decrease with velocity as predicted.  However, no effect is measured at the remaining 11 sites with faster velocities, despite a 4-fold variation in speed.  The authors conclude that the “plastic” response has an upper limit of about 2-4m . sec-1, a velocity commonly exceeded within the typical habitat of  B. glandula.  At higher velocities, the large drag forces might not allow the legs to act as effective filters, and buckling could occur.  Li & Denny 2004 Biol Bull 206: 121.

NOTE  water velocities are measured daily at each site and averaged for 10-30d prior to collection and measurement

 
Research study 6
 

Plasticity of feeding limbs in acorn barnacles Balanus glandula is well documented, with individuals from wave-protected shores having longer and thinner limbs than conspecifics from wave-exposed shores. But no-one has actually observed the functional significance of this difference, that is, watched what happens to the limbs of both morphological forms in a range of current velocities in an experimental flow tank.  This is done at the graph showing effect of increasing wave velocity on feeding behaviour in barnacles Balanus glandulaBamfield Marine Sciences Centre, British Columbia using 2 populations of B. glandula, one from a protected shore and one from a wave-exposed shore, subjected to 5 different water velocities. 

Differences are highly significant, with individuals from the wave-exposed population being able to feed with their cirral nets fully extended at all velocities tested up to 50cm . sec-1 (= graph showing effect of water velocity on cirral beat frequency in barnacles Balanus glandula0.5m . sec-1), while individuals from a wave-protected population cease feeding with cirral nets fully extended at between 7-21cm . sec-1 (see graph on Left).

Individuals from the long-limbed population also decrease the frequency of their cirral beating in the 2 highest test velocities and curl in their limbs to a more protected location near the operculum (see graph on Right).  In contrast, individuals from the short-limbed population actually increase their cirral beating as current velocity increases.  The results confirm an earlier hypothesis that differences in cirral lengths between wave-protected and wave-exposed shores are, in fact, adaptive.  Marchinko 2007 Biol Bull 213: 12.

NOTE  individuals from the wave-protected population have mean cirral lengths of 4.1mm (in mean maximum tidal current velocity of 1cm . sec-1), while those from the wave-exposed population have lengths of 2.7mm (in mean maximum tidal current velocity of over 400cm . sec-1)

NOTE  the author’s water-velocity measurements (some expressed to hundredths of a cm . sec-1 in the paper) are rounded off here for visual clarity

 
Research study 7
 

An interesting study by researchers at the Oregon Institute of Marine Biology concerns scaling of cirral-net “leakiness” in relation to body size in barnacles Balanus glandula.  Three size-classes of barnacles are chosen: “small” (0.4mm aperture diameter), “medium” (3.4mm), and “large” (6.5mm).  Feeding behaviour differs among the 3 size-classes, with small individuals feeding mostly passively, and the other size-classes feeding by active cirral-sweeping.  Large individuals have significantly fewer setae per cirrus length than medium or small individuals, and leakiness, as would be expected from predicted scaling relationships, is relatively greater in these large (4.6% leakiness) and medium (4.1%) individuals in comparison with small (0.3%) ones.  Small barnacles also have a higher cirral beat-rate than large- and medium-sized individuals.  photograph of Cirrus VI of barnacle Balanus glandula showing measuring points to determine "leakiness"Another difference noted by the authors is that because of changing relative viscosity of seawater with size of barnacle, cirral fans of small, newly settled barnacles tend to behave as paddles, while those of medium and large barnacles tend to behave as rakes.  Geierman & Emlet 2009 J Exp Mar Biol Ecol 379: 68.

NOTE  3 larger pairs of biramous appendages covered with setae (cirri IV-VI) form the main feeding sructure, known as a cirral fan, while 3 smaller pairs (cirri I-III) act to clean the fan and transfer food to the mouthparts.  As noted, each cirrus is biramous (2 branches), so 12 rami make up the large cirral fan and 12 smaller rami are used for collecting and transferring food.  Most often the cirri beat actively at low current speeds (e.g., 4cm . sec-1) and are held passively in the water stream at high current speeds.  At current velocities in excess of 4m . sec-1 a barnacle usually ceases to feed.  The researchers take measurements from videotapes of the barnacles feeding in a stream of milk

NOTE  calculated as the volume of fluid that does not get filtered as the cirral fan sweeps past.  For a single cirrus it is equivalent to approximately one-half of the volume of an elliptical cone as measured between the rami at locations “b” and “c” in the photograph

 
Research study 8
 

An idea based upon the physical proximity of the cirral appendages and penis in barnacles is that they may “compete” for nutrients required for growth and, thus, in environmental circumstances where phenotypic plasticity leads to extra-long cirrus length, penis length may, perforce, decrease, and vice versa in other conditions.  This seems a strange point of view because all other organs in a barnacle’s body could be construed as being in competition for nutrients, so extra growth of one would be expected to affect growth of all others.  However, the idea of coupling of traits is not new, and has been examined in other animals including vertebrates and insects.  Here, it is investigated in barnacles Balanus glandula at the Bamfield Marine Sciences Centre, British Columbia in comparisons of cirrus and penis lengths as a function of wave-exposure and population density.  Interestingly, while the results indicate that cirrus length and penis length show coupled responses to wave exposure, both decreasing as wave exposure increases, responses to density are uncoupled.  Thus, as density of conspecifics increases, cirrus lengths increase by about 6%, while penis lengths decrease by about 18%.  How a barnacle is able to sense  density of conspecifics is unclear (the author suggests degree of perception of some chemical emanation or other), but shorter penises would be an adaptive energy-savings strategy because closer inter-individual distances means that the penises don’t have to reach so far.  Neufeld 2011 J Exp Zool (Mol Dev Evol) 316: 254.

NOTE  for some reason, the author considers wave exposure and density to be conflicting environmental cues and, in fact, this forms the basis for the hypotheses to be tested, but it is never explained by the author

 
Research study 9
 

photomicrograph of cross-section of barnacle penis Balanus glandulaIn seeking an explanation for the remarkable phenotypic plasticity in cirrus and penis form in acorn barnacles Balanus glandula, researchers at the Bamfield Marine Sciences Centre, British Columbia compare cuticle thickness and extensibility, and muscle form between sites that differ greatly in wave exposure.  Results for analyses of Cirrus VI show a 25% thicker cuticle in wave-exposed individuals than in ones from quiet-water habitats, an adaptation for resisting compression and buckling under wave stress.  The cuticle in barnacles in both habitats is significantly thicker on the convex, posterior side of all cirri than on the concave, anterior side.  This is presumably an energy-savings strategy, for while the posterior surfaces must resist strong posteriorward buckling forces when the cirral fan is fully extended into the current, the anterior surfaces will be stretched and under less buckling tension.  The cirri are extended by hydrostatic pressure and retracted by contraction of striated muscle fibers that run from the cirrus base to its tip.  These retractor muscles attach to the inner surfaces of the cuticle at the bases of each articulation.  As predicted from previous work, penises are significantly longer in wave-protected individuals than in wave-exposed ones and about 20% longer in populations with lower density than in ones with higher density.  Unlike a cirrus, which has thicker cuticle on the posterior surface and bends only in one direction, a penis has to resist buckling forces in all directions; hence, is thickened more evenly around its entire circumference.  The relatively thicker cuticle on the penis also constrains its circumferential expansion, thus allowing it to extend further at a given pressure and to reach more mates.  The penis extends through muscle contractions that force hemolymph into it and its cuticle has annulations all round that allow unfolding.  In a fully extended penis the annulations are completely unfolded.  During extension, which may be up to 8 times body length, layers of longitudinal muscles beneath the cuticle contract differentially to  direct the penis tip towards and into the mantle area of a copulating partner (see accompanying photograph).  Based on their observations of penis operation, the authors propose that penises of wave-exposesd barnacles operate at higher internal pressures than penises  in wave-protected ones.  Neufeld & Rankine 2012 Invert Biol 131 (2): 96.

NOTE although penises are included here and in Research Study 8 along with cirri, this is done so for convenience. More on barnacle penises can be found at LEARNABOUT/BARNACLE/COPULATION & LARVAL DEVELOPMENT/BALANUS GLANDULA

 
Research study 10
 

graphs showing cirrus lengths of barnacles Balanus glandula after reciprocal translocations to different wave-protected and wave-exposed sitesHow quickly do these morphological adaptations occur in the appendages of barnacles?  This is examined for Balanus glandula at the Bamfield Marine Sciences Centre, British Columbia in a series of translocation experiments.  Barnacles growing on mussels shells are collected from 2 source populations, one on a wave-exposed shore; the other, on a wave-protected shore.  Individuals from each population are isolated, each on its own mussel-shell base, and glued in groups to plastic plates.  The plates are then mounted in the mid-zone of barnacle distribution at 4 new sites that vary in degree of wave exposure, from protected to full exposure, and left in place for a period of 11wk (start date: 12 Sept).  Every 2wk samples are taken for morphometric measurements of cirrus and penis lengths.  Results show that response rates in growth of cirri depend on translocation direction, with individuals moved from the protected site to a wave-exposed site taking twice as long to change than ones moved in the reverse direction (8wk vs. 4wk; see graphs).  Growth of barnacles moved from the wave-exposed source location to a protected location is effected through a combination of adding more segments and growing longer segments.  Specifically, barnacles moved from the protected-source site to a wave-exposed site produce after 8wk feeding cirri with 15% fewer segments and 25% shorter segments than ones moved to a protected site (top graph).   The asymmetrical directional responses noted in this study are contrary to those previously predicted for barnacles in an earlier study (see Research Study 4 above) and in studies on other taxa, a difference that the author discusses at length.  Neufeld 2012 J Exp Mar Biol Ecol 429: 20.

NOTE  included in the study, but not presented here, are seasonal monitoring of cirrus and penis lengths over a 2yr period

 
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