title used in an account of west-coast marine invertebrates entitled A Snail's Odyssey
  Food, feeding, & growth
  black dot
  Growth & energy budgeting
 

photograph of a moult of a giant barnacle Balanus nubilisLike all arthropods, barnacles moult their exoskeletons as part of growth.  This is done several times a year during the early stages and less often as a breeding adult.  A barnacle moult is quite distinctive once it is recognised for what it is.

NOTE  this simply means “outside skeleton” and could refer either to the cuticle or calcarous plates; however, the outside plates are usually referred to collectively as the “test”.  A test is a non-living skeleton located outside of the body, as found in barnacles and bryozoans.  Sea urchins also have a test but, while non-living, it is actually covered by epidermis and is part of the sea urchin’s endoskeleton.  In fact, it is phylogenetically homologous with our own endoskeleton of bones

NOTE  more on moulting of arthropods can be found in the section LEARN ABOUT CRABS & RELATIVES: MOULTING & GROWTH


Cast-off moult of a giant
barnacle Balanus nubilis 2X

  This section deals with growth & energy budgeting, while FOODS and CIRRAL-NET FEEDING are dealt with in other sections.
  Research study 1
 

schematic drawing of plates and cement glands of a barnacle Balanus glandulaOn sight and feel a barnacle Balanus glandula seems solidly built, in shape like a tiny teacup.  So, how does a teacup expand from the inside as the barnacle grows in size?  The answer is that the test is not solid like a teacup.  In fact, it is made up of several calcified plates that are separated from one another and the substratum by thin cement or adhesive lines.  As calcium carbonate is secreted at the edges of the plates by underlying tisues during growth, fresh adhesive is produced by the cement glands photograph of several thatched barnacles Semibalanus cariosusand ducted down a ramified system of channels.  The adhesive fills gaps between the test plates and between the base plate and the substratum surface, and excludes water at the same time.  The adhesive cures within a few hours to a tough, rigid mass. Balanus glandula has a solid basal plate that is cemented to the substratum (see schematic drawing on Right), and grows at the circumference edge. Other species, such as Semibalanus cariosus, have membraneous bases and it is the cement lines on the bottom edges of the test plates that fastens the barnacle to the substratum. Information and drawings of Balanus sp. cement glands taken from several sources: Lacombe 1970 Biol Bull 139: 164; Wiegemann 2005 Aquat Sci 67: 166; Khandeparker & Anil 2007 Intern J Adhesion Adhesives 27: 165.

NOTE  the adhesive cement is comprised of >90% protein


Thatched barnacles Semibalanus cariosus, one
dead one showing the lack of a basal plate 1.5X

 
Research study 2
 

graph showing moulting frequency of barnacles Balanus glandula in southern British Columbia over 1 yearTable showing energy budget of barnacles Balanus glandula over the first year of lifeAn energy budget calculated for the first year of life of laboratory-maintained Balanus glandula in the West Vancouver Laboratory, British Columbia provides insight into growth in this species.  First, newly settled barnacles moult 4-5 times per month during the first few months of life. Moulting ceases over winter, then picks up again in the warmer conditions of springtime.  Loss of energy in moulted exuviae is thought to represent 2% of the total annual energy budget.  The physical components of the shell, represented by chitin and protein, are thought to comprise only about 7% of the total annual budget.  This cost does not include energy needed for production of the shell, but this is thought by the authors to be insignificant and would, in any case, be included as part of the overall cost of respiration.  Determination of an energy budget for an intertidal barnacle is made more difficult by having 2 components for respiration: aerial and aquatic – the former, especially, having an added influence of fluctuating air temperatures, both daily and through the course of the year.  It is not clear from the results of the study how these effects are accounted for.  Wu & Levings 1978 Mar Biol 45: 225.

NOTE  such budgets are notoriously difficult to balance, mostly owing to assumptions made in translating respiratory energy costs obtained in laboratory respirometers to normal activity in the field.  This may be less of a problem for barnacles, which perhaps behave more “normally” in the confines of a respirometer flask than do most motile animals.  The budget balances here because the researchers simply sum all measured components to yield 100%. However, when the authors actually measure rates of consumption in 3 experimental groups of barnacles, the data yield budgetary imbalances ranging over 24 percentage points

NOTE the authors do not comment on the apparent zero moulting frequency in the last month of the study

 
Research study 3
 

graph comparing growth rates of barnacles at Santa Cruz, CaliforniaA study of acorn barnacles at Santa Cruz, California reveals that growth of Chthamalus fissus is slowest, followed by Balanus glandula and then the speedily growing Tetraclita squamosa.  Annual allocation of energy to production of eggs is 11 times body mass for Chtamalus, 9 times body mass for Balanus, and 4 times body mass for Tetraclita.  Survival follows the same pattern, with Chthamalus being the shortest-lived at an average of 3yr, Balanus the next at 8yr, and Tetraclita the longest-lived at 15yr.  Hines 1979 p. 213 In, Reproductive ecology of marine invertebrates (Stancyk, ed.) U South Carolina Press, Columbia.

 
Research study 4
 

graph showing effects of current speeds on growth in barnacles Balanus glandulagraph showing effect of current speeds on growth in barnacles Balanus crenatusWhat is the effect of current speed on growth of barnacles?  This is tested in Friday Harbor Laboratories, Washington using 3 acorn-barnacle species Balanus glandula, B. crenatus, Semibalanus cariosus, and a goose-barnacle species Pollicipes polymerus (only results for the first 2 species are presented here).  Barnacles are collected either by allowing cyprids to settle onto plastic strips attached in the low intertidal zone (B. glandula), or by removing small individuals from rocks in the field and gluing them onto plastic strips (B. crenatus, S. cariosus, and P. polymerus).  The barnacle-bearing strips are then mounted in plastic pipes through which field seawater is pumped at different speeds1.  Results show that growth rates of species that live in high-energy environments (B. glandula, S. cariosus, and P. polymerus) are relatively insensitive to flow speed. Note that growth rates for B. glandula, for example, show a non-significant effect of current speed on growth over a 54d study period (see graph on Left).  However, species2 that typically experience weak current flows in their natural habitats (B. crenatus) appear to be significantly affected by current speed (see graph on Right).  Note in the graph3 that growth rate of B. crenatus increases from low speeds to a maximum at intermediate speeds of about 8-10cm sec-1, then decreases at higher speeds.  Interestingly, at the maximum current speeds used in the growth experiment and at rates up to about 50% greater, cirral nets of the acorn-barnacle species tend to hold their integrity, that is, they tend not to buckle (an exception is B. crenatus whose cirri are deflected at rates >25cm sec-1). The authors speculate that the effects of current flow may ultimately relate to how effectively a species can handle and process food particles impacting the feeding apparatus.
Eckman & Duggins 1993 Biol Bull 185: 28.

NOTE1  each pipe is 3m long x 2-5cm diameter and contains several sets of barnacles mounted on plastic strips (species are intermixed on the strips).  Field seawater is pumped through at a single, continuous velocity for 54- to 69-d study periods after which growth is measured.  There are 5 speeds of flow, ranging from 2-15cm sec-1.  Each speed treatment has 2 replicates. Growth is measured as increase in basal area

NOTE2  this generalisation for B. crenatus is supported by data showing similar patterns, not presented here, obtained for a bryozoan and tubeworm, both from similar low-energy environments

NOTE3  the authors have used a 2nd-degree polynomial regression line to fit their data.  Visual observation of the data (which show high variability) suggests that a linear regression might also be applied, but a 2nd-order polynomial gives better statistical fit and is also statistically significant. It also supports the authors’ hypothesis that feeding and, thus, growth will reach a maximum at a certain current velocity and become less at higher velocities

 
Research study 5
 

drawing of settling plates used in study of growth of barnacles Balanus glandula and Chthamalus dalli in OregonGrowth of barnacles is obviously governed by availability of phytoplankton food, but how do near-shore factors such as wave exposure and upwelling affect growth.  This is tested with 2 species Balanus glandula and Chthamalus dalli at 2 sites in Oregon differing in phytoplankton productivity.  Spat are allowed to settle onto graph showing growth of barnacles Balanus glandula at 2 sites in Oregon differing in phytoplankton productivityplastic plates in April and photographically monitored for the next 14mo.  Some plates are translocated from areas of high settlement to areas of low settlement (the latter is typical for C. dalli especially in wave-exposed areas). 

Results show that mortality of both species is high (50%) over the first 25d, likely owing to physical stresses rather than predation because whelks and other predators are not seen to visit the plates. Balanus glandula grows faster in wave-exposed habitats than wave-protected ones, presumably because higher flows deliver more food.  Both species grow more at the site with higher productivity (Strawberry Hill; see graph on Right, data only for Balanus glandula) and grow less during a persistent upwelling event.  Unexpectedly, growth of both species appears to vary out of synchrony with phytoplankton blooms, suggesting involvement of other factors.  One possibility, suggested by presence of abundant zooplankton in stomach-content analyses in other studies, is that zooplankton may be an important alternative food for these barnacle species.  Higher water temperatures during non-upwelling periods may also be involved in faster growth at these times.  Thus, a combination of warmer temperatures, more phytoplankton, and more zooplankton may interact to produce faster growth in the 2 species.  Sanford & Menge 2001 Mar Ecol Progr Ser 209: 143.

NOTE  barnacles are allowed to settle onto pitted, plastic plates attaced to level areas of intertidal rock substratum.  The pits are favoured for settlement, and this produces an array of solitary spat whose subsequent growth is unaffected by potential effects of crowding

NOTE  only one set of (condensed) results is shown here, out of a paper rich in data

 
Research study 6
 

map of California showing location of Salton Seagraphs of growth of barnacles Balanus amphitrite in the Salton Sea, CaliforniaA study on growth of barnacles Balanus amphitrite in the Salton Sea, California by researchers at the University of California, Riverside focuses on whether enhancement of numbers of these “living filters” could lead to greater water clarity and possibly significant reduction of N and P levels in the lake.  Apparently the high nutrient content in the water leads to excessive phytoplankton blooms and to sometimes low levels of dissolved oxygen.  Hard substratum for recruitment of barnacles is in short supply, so an “artificial” substratum of vertically hanging burlap sheets is used for growth experiments by the researchers.  Settlement of cyprid larvae onto the sheets following their installation is virtually instantaneous (colonisation rate of up to 6000 cyprids m-2 after 15min of deploying the sheets).  Results from a 44d experiment in springtime show good growth and with lots of calcium carbonate being precipitated, but not enough incorporation of P and N to justify the expense of maintaining the required amount of surface area of burlap (e.g., estimated at 140 million m2 just to remove the annual phosphorus load), and the time and effort involved.  Geraci et al. 2008 Hydrobiologia 604: 77.

NOTE  more on this interesting species and its even more interesting habitat can be found at HABITAT & COMMUNITY ECOLOGY: PHYSIOLOGICAL ECOLOGY

NOTE  the idea is annually to grow masses of barnacle on the burlap sheets, and then dispose of the sheets and barnacles, to effect a significant reduction in nutrient load in the lake

 
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