title used in an account of west-coast marine invertebrates entitled A Snail's Odyssey
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photograph of penis of barnacle Balanus glandula courtesy Chris Neufeld & Rich Palmer, University of Alberta, EdmontonBarnacles are hermaphroditic and one individual usually cross-fertilises with another individual.  The penis in a barnacle is extensible, highly maneuvreable, and can extend several body diameters. If one individual is out of penis-reach of another then it may be reproductively sterile, as self-ertilisation is found in only a few species. Photograph courtesy Chris Neufeld & Rich Palmer, University of Alberta, Edmonton.

NOTE if separated by more than about 5cm, both Chthamalus dalli and C. fissus are apparently capable of self-fertilisation

The penis in Balanus glandula can
extend up to 8 body diameters


diagram of life cycle of a barnacleThe life cycle of a barnacle involves 3 unique phases: 1) a pelagic, suspension-feeding nauplius larva, 2) a pelagic, non-feeding cypris larva, which is the settlement stage, and 3) a bottom-dwelling, suspension-feeding adult. Pechenik et al. 1998 Bioscience 48: 901.

NOTE (lit. “a kind of mollusc” G.).  The name reflects an early belief that barnacles are related to snails and bivalves, presumably based on the nature of their calcareous shell-plates.  This idea persisted until the mid-1800s, when scientists recognised distinctive arthropodan features in the barnacle larvae

NOTE the term settlement describes the behaviour of a larva of a marine invertebrate dropping out of the plankton, selecting a place to live, and adopting a juvenile way of life usually after passing through metamorphosis.  Recruitment refers to the addition of this individual to the population



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  Copulation & larval development

Reproductive events include copulation & larval development considered in this section, and

This section is organised by genus and species.

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Balanus crenatus

Research study 1

diagram of life cycle of the barnacle Balanus crenatusphotograph of a barnacle Balanus crenatus courtesy Dave Cowles, Walla Walla University, WashingtonStudies on development of Balanus crenatus at Hopkins Marine Station, Pacific Grove, California provide information on naupliar and cyprid morphology.  The author divides development into 8 naupliar stages instead of the usual 6. The cypris is 0.55mm in length. Development through metamorphosis to a spat takes 2-3wk in summer laboratory seawater temperatures of 17-21oCHerz 1933 Biol Bull 64: 432. Photograph courtesy Dave Cowles, Walla Walla University, Washington wallawalla.edu.

Barnacle Balanus crenatus
living on a Dungeness crab Metacarcinus magister 7X

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Balanus glandula

Research study 1

photograph of barnacles Balanus glandula courtesy Dave Cowles, Walla Walla University, Washingtondrawings of developmental stages of barnacle Balanus glandulaAs in other acorn barnacles, fertilised eggs in Balanus glandula are retained in the mantle cavity until they hatch to nauplii larvae.  After their release from the parent the nauplii swim and feed on phytoplankton using 3 pairs of appendages. After a few weeks, during which time the nauplius moults 5 times, it transforms into a settling stage known as a cypris. The cypris is non-feeding, but is loaded with fat droplets for buoyancy and energy.  The cypris soon drops to the sea bottom and begins searching for place to settle.  Special sensory endings on each antennule enable the cypris to assess the minute physical and chemical features of the sea bottom.  When a suitable place is found, the larva attaches via sticky secretions from the antennae and begins to metamorphose. Brown & Roughgarden 1985 J Crust Biol 5: 574. Photograph of Balanus glandula courtesy Dave Cowles, Walla Walla University, Washington wallawalla.edu/inverts

Research study 2

graph showing relationship of number of eggs/embryos versus animal volume in barnacles Balanus glandulaIn Ladysmith Harbour, British Columbia reproduction in Balanus glandula follows this schedule at

fertilisation (eggs 0.15mm dia): Dec-Jan
eyed nauplii in mantle cavity: end of Feb
nauplii released: Mar
all nauplii gone: mid-April

There may be an additional, smaller, spawning in autumn at 15-18oC. The graph shows number of embryos in egg masses of gravid Balanus glandula at Ladysmith Harbour, British Columbia, plotted against estimated volume of the adult (length in mm3).  Maximum brood size is about 12,000.

Comparative data on the same species in La Jolla, California:

fertilisation: Oct
eyed nauplii in mantle cavity: end of Dec
nauplii released: Jan
all nauplii gone: mid-Feb

There may be a smaller brood produced in springtime.  Barnes & Barnes 1956 Pac Sci 10: 415.

NOTE  the incubatory volume in the mantle cavity of a barnacle scales roughly as the cube of length.  Numbers of eggs/embryos represent an estimate of volume; hence, the relationship is predicted to be linear. The estimate is crude and further research on this relationship in different species would be interesting

Research study 3

The previous Research Study suggests that Balanus glandula in British Columbia has only a single brood during the year, but a comparison of 3 species of barnacles, including B. glandula, in central California indicates several broods in this species, as well as wide variability in reproductive cycles among schematic showing breeding cycles in 3 species of Californian acorn barnaclesthe species.  Thus, Balanus glandula produces 3-6 relatively large broods from Dec-May, while Tetraclita squamosa produces 3 intermediate-sized broods from Jun-Sep, and Chthamalus fissus produces up to 16 small broods from Mar-Oct.  The broods are produced in rapid succession, with occasional overlap between them.  In the laboratory, brood frequencies in all species appear to be regulated primarily by food availability, and not by temperature (except possibly for T. squamosa) or photoperiod.  The accompanying schematic shows the patterns of nutrient storage in the ovaries of the 3 species, expressed as number of brood equivalents (in mass) in the ovary. In C. fissus a brood is deposited as soon as enough nutrients are accumulated.  This is slow in late autumn and winter, but speeds up as more food is available during spring and summer.  In contrast, B. glandula stores nutrients for at least 3 broods during summer then broods up to 6 photograph of barnacle Tetraclita squamosatimes from Dec-Apr, storing nutrients during springtime feeding.  Finally, T. squamosa accumulates yolk for only one brood at a time, for a total of 3 during the summer.  The author groups barnacle species into 5 categories based on patterns of reproductive timing and brood production, and the present paper illustrates 3 of these patterns.  Hines 1978 Biol Bull 154: 262.

NOTE  the author studies barnacle reproduction in 2 areas, one, a warm-water discharge canal of a large power plant at Morrow Bay, California (discharge temperatures are 5oC above ambient); the other, nearby shore populations living at ambient temperatures.  Only T. squamosa shows significant differences in reproductive cycling in the 2 locations

Research study 4

schematic showing shore heights occupied by 3 species of west-coast barnaclesgraphs showing reproductive cycles in 3 species of west-coast barnacles, Chthamalus fissus, Balanus glandula, and Tetraclita squamosaThree species of acorn barnacles are common on intertidal shores around Santa Cruz, California. Chthamalus fissus at 8mm maximum diameter lives in the highest regions and extending into the splash zone to about 2.2m above MLLW (see illustration on Left). Balanus glandula at 2cm diameter is most abundant in the mid-high zone from 1-1.5m above MLLW, and Tetraclita squamosa at 6cm diameter occupies the low intertidal area from 0-1m above MLLW.  Brooding in C. fissus occurs Mar-Oct, in B. glandula Dec-May with up to 6 broods per season, and in T. squamosa Jun-Sept with up to 3 broods per season (see graph on Right). Hines 1979 p.213 In, Reproductive ecology of marine invertebrates (Stancyk, ed.) U South Carolina Press, Columbia.

NOTE  the 3 species are considered together here for convenience

Research study 5

graph showing fertilisation success of barnacles Balanus glandula at different separation distancesA common assumption is that aggregation in barnacles principally serves to increase fertilisation success between neighbouring mature adults through increased penis access.  However, studies at the West Vancouver Laboratory, British Columbia on Balanus glandula show that there is no significant effect in this regard at nearest-neighbour distances less than 1.75cm (see graph). However, at 5cm distance the adults are out of effective penis-reach distance.  Wu 1981 Can J Zool 59: 890.

NOTE  although the author concludes that distance, represented by maximum penis length of a mature individual, has no effect on fertilisation success in B. glandula, the data leave a sizeable and possibly critical gap between the 1.75 and 5cm distances, and perhaps this should be re-examined

Research study 6

An adult Balanus glandula on the central California coast is estimated to produce 2-6 broods per year and the larvae spend approximately 3-4wk in the plankton.  All but the last few days or so are spent feeding by the 6 naupliar stages.  Only the terminal, cypris stage is non-feeding.  Gaines et al. 1985 Oecologia 67: 267.

Research study 7

graph showing penis length in barnacles Balanus glandula with increasing wave exposure in the habitatphotograph of penis of a barnacle Balanus glandula courtesy Neufeld & Palmer 2008 Proc Roy Soc B 275: 1081Barnacles cross-fertilise with a penis that may be 8 or more times longer than their body diameter.  Evolution in barnacles has been a trade-off between an ever-increasing penis length to service more mates and a need to control these longer penises in turbulent waves and currents.  It is not so all-or-none, however, for a study at the Bamfield Marine Sciences Centre, British Columbia shows that at least one species, Balanus glandula, has phenotypic plasticity in penis morphology, permitting individuals in wave-exposed areas to have shorter, stouter, and more massive penises than ones in wave-protected areas. Thus, variation in penis size and shape correlates with maximum velocity of diagram showing 25% greater length of penis in barnacles Balanus glandula in quiet-water habitats than in wave-exposed onesbreaking waves (see graph1 on Right). On average, quiet-water individuals have penises2 that are 25% longer than rough-water individuals. 

Note in the diagram on the Left that a 25% increase in penis length in quiet-water habitats translates into a 90% increase in reachable3 area.  Penis size also scales allometrically with body size, leading to disproportionately stouter penises in larger animals. 

For final confirmation, the authors translocate barnacles to different habitats and show that, after 20wk, individuals moved to wave-exposed shores produce shorter (by 25%) and wider (by 20%) penises than ones moved to a protected harbour, confirming that the size variation in penises owes to phenotypic plasticity. The phenotypic flexibility in penis size enables B. glandula to inhabit a greater range of habitat conditions than would otherwise be possible.  Neufeld & Palmer 2008 Proc Roy Soc B 275: 1081.

NOTE1  the graph presented here shows the relationship of penis length and wave exposure, but the authors provide data on penis basal width and mass relative to both body size and penis length that also correlate significantly with wave exposure. The data in the graph are expressed for "standard"-width barnacles of 8mm

NOTE2   how do you measure the size of a barnacle penis?  It requires use of a dissecting microscope with ocular ruler. Inflate the penis, confirm that the size of a relaxed (i.e., dead) penis is a valid indicator of  extended penis length, and measure its dimensions

NOTE3  as copulatory partners are accessible around the entire circumference of the barnacle, the searchable area expands as the square of penis length

Research study 8

Researchers at University of California, Santa Cruz and University of Hawaii construct a model for naupliar development of Balanus glandula based on published data on effects of temperature and food concentration on rates of growth.  The model uses total chlorophyll to represent food concentrations in the field which, because of variability in nutritional content of different phytoplankton species, may lead to larval developmental rates being over-estimated.  Still, durations of larval life predicted from the model compare favourably, perhaps not surprisingly, with previous observations of larval duration of the species.  The authors suggest that their model will be a useful tool in simulations of larval dispersal under varying conditions of current flow, upwelling, and so on.  Pfeiffer-Hoyt & McManus 2005 J Plankton Res 27 (12): 1211.

NOTE  the cyprid stage is non-feeding; hence, is not included in the model 

Research study 9

schematic showing arrangement of treatments in each experimental blockAs simultaneous hermaphrodites, barnacles have both sexes contained within the same body and both sexes mature at the same time.  Given that self-fertilisation does not occur, or occurs only rarely, what is the relative investment1 into male and female function in an individual (i.e., equivalent to sex ratio) in a species?  Previous theoretical models developed specifically for acorn barnacles predict that relative allocation to male function will increase as number of competitors for fertilisations increase.  This is tested in a field study at Friday Harbor Laboratories, Washington for Balanus glandula2 under different levels of crowding.  Experiments are done on mid-intertidal-level populations manipulated by selective removal of individuals to form 4 experimental treatments containing low or high numbers3 of individuals in dense or sparse aggregations (see diagram).  Each treatment occupies 10x10cm area of rock surface and is separated from each other treatment by a 5cm distance to prevent mating outside of the treatment group.  Six replicate sites are used.  After 4wk, investment is measured in selected individuals from each treatment at each site.  Results are complex, but show that while allocation to male function does not change as predicted with number of competitors, it is affected by body size (greater relative allocation to female function with increasing size) and site, and does increase with increased crowding.  How the barnacles perceive the density of crowding in an aggregation is not known, but common ideas are that it could relate to perception of chemical emanations, to a sense of non-self test proximity, or perhaps even to intensity of penis probings.  The topic is a challenging one, and is
certain to generate further research.  Hoch & Levinton 2012 Evolution 66 (5): 1332.

NOTE1 investment is defined for females as total mass of eggs, and for males as total mass of reproductive parts including testes, ducts, and penis.  The authors note that their definition of “investment” is just one method that could be used.  Other measures of investment that have been used for similar studies in other hermaphroditic taxa as, for example, sea hares Aplysia, are frequency and duration of the copulatory act, and these parameters might have been useful for this study as well 

NOTE1  the researchers  do a parallel study on an Atlantic species Semibalanus balanoides, results for which are not considered here

NOTE1  numbers of individuals are 3-4 or 10-25, and are touching one another in dense aggregations, but physically separated in sparse aggregations

Research study 10

photographs of nauplius larvae of barnacle Balanus glandula within egg capsule and free-swimmingIn Balanus glandula, nauplii larvae that are competent to swim can be held by the parent within their egg capsules for some time until conditions are suitable for their release (see photograph). The time spent in this immotile state varies depending upon season, with winter-brooded larvae spending longer as the adults wait for the advent of springtime hatching stimuli, including warmer temperature, more light, and phytoplankton blooms. photograph of cancroid crab Glebocarcinus oregonensisSummer-brooded larvae, in contrast, are released into more regularly available food conditions after shorter periods of brooding than the winter ones (means of 16 and 45d, respectively, in the present study). This plasticity in time of hatching and release has obvious survival benefits. Other environmental factors that may come into play include feeding activities of predators, and it is about how the different behaviours of certain predators differently affect survival of the nauplii that researchers have investigatd at Friday Harbor Laboratories, Washington. For example, when crabs such as Glebocarcinus oregonensis attack a brooding barnacle they tend to crush the shell plates, allowing many of the nauplii to hatch from the brood lamellae and swim safely away. In comparison, when whelks such as Nucella ostrina attack a brooding barnacle photograph of whelk Nucella ostrinaby boring through or between the plates, the shell plates including the opercular ones most often remain intact, trapping larvae within the broodlamellae and/or swimming within the mantle cavity, and causing their deaths. By dissecting gravid barnacles and tearing apart lamellae in the laboratory, the authors confirm that physical damage such as that caused by crabs will cause larvae to be released. Branscomb et al. 2014 Invert Biol 133 (2): 158.

NOTE the proximal stimulus for hatching is thought to be a pheromone released by the brooding parent, but hatching can also be stimulated by experimentally separating the encapsulated nauplii from the egg lamellae, as well as by other disturbances

NOTE formerly Carcinus oregonensis

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Balanus nubilis

Research study 1

drawings of naupliar stages of the barnacle Balanus nubilisNaupliar development in the giant barnacle Balanus nubilis is similar to that described for other balanoid species, but of a larger size.  Studies on B. nubilis larvae in laboratory-culture at Friday Harbor Laboratories, Washington provide information on general morphology and carapace widths. Barnes & Barnes 1959 Can J Zool 37: 15.

NOTE the authors use stage I-III nauplii from lab culture and stage IV-VI nauplii from plankton towsphotograph of barnacles Balanus nubilis covered in corallimorpharians Corynactis californica

NOTE carapace widths are measured as the distance between the fronto-lateral horns.  The authors note that stage I nauplii are not usually free-living, but are recognisable by their posteriorly directed horns

Two Balanus nubilis growing among corallimorpharians
Corynactis californica


Chthamalus spp.

Research study 1

photograph of mixed populations of barnacles Balanus glandula and Chthamalus dalliCan barnacles self-fertilise?  Such a strategy would be of survival value particularly to Chthamalus spp., where isolated individuals commonly occur high up the shore.  The answer is “yes” for both C. fissus and C. dalli on the west coast.  This is confirmed for the former species in Santa Monica and Malibou Beach, California, and for the latter species in Coos Bay, Oregon, and Anacortes and San Juan Islands, Washington. The authors examine contiguous and separated (>5cm) populations for the presence of viable embryos and larvae.  In all instances, the separated individuals lag behind their contiguous conspecifics in terms of stage of development, suggesting that they wait for cross-fertilisation, and only self-fertilise as a last-ditch strategy.  Barnes & Barnes 1958 Ecology 39: 550.

NOTE  the authors confirm self-fertilisation in other European barnacle species, but it is not known whether this occurs in west-coast forms other than Chthamalus


Mixed populations ofChthamalus dalli and Balanus
at the extreme upper limits of their
distributions. A solitary C. dalli is highlighted 0.4X

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Megabalanus californicus

Research study 1
  drawings of developmental stages of barnacle Megabalanus californicusLarvae of Megabalanus californicus from California are described for the first time. Note the large sizes of the various stages (the cypris is about 1cm in length).  Miller & Roughgarden 1994 J Crust Biol 14: 579.
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Semibalanus cariosus

Research study 1

photograph of barnacles Semibalanus cariosusRelease of nauplii by Semibalanus spp., and perhaps other species, is synchronised with the spring plankton bloom.  It makes sense that an invertebrate species with phytoplankton-eating larvae would spawn or release their larvae synchronously with the seasonal phytoplankton bloom but, unlike sea urchins and mussels that spawn their gametes in response to a chemical produced by the phytoplankton, barnacles appear to release their larvae in response to physical contact with the phytoplankton cells.  Starr et al. 1975 Can J Zool 53: 582.






Semibalanus cariosus 1.5X

Research study 2

drawings of larval stages of a barnacle Semibalanus cariosusA study at Friday Harbor Laboratories, Washington provides details on naupliar development of 3 barnacle species Semibalanus cariosus, Balanus crenatus, and B. glandula.  In contrast to the last 2 species which breed throughout the year, S. cariosus produces only one brood per year in early spring.  These outline views of the 6 naupliar stages of each species may help somewhat in identifying them in the plankton but, for precise differentiation, the antennule structures have to be compared (not presented here). Branscomb & Vedder 1982 Crustaceana 42: 83.

NOTE  as features of the larvae of the other 2 species are shown elsewhere in this section, only those of S. cariosus are presented here

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Tetraclita rubescens

Research study 1

Drawings of larval stages of barnacle Tetraclita rubescensphotograph of barnacles Tetraclita squamosaLarvae of Tetraclita rubescens from California are described for the first time. Miller & Roughgarden 1994 J Crust Biol 14: 579.

NOTE  this species may be the same asTetraclita squamosa

Barnacles Tetraclita.
from Pukhet, Thailand,
possibly T. squamosa 0.5X

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