Barnacles 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

The 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

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

LARVAL BIOLOGY,
SETTLEMENT CUES,
SELECTION OF SUBSTRATUM,
ATTACHMENT,
SETTLEMENT COUPLED WITH OCEANIC PROCESSES, and
POST-RECRUITMENT EFFECTS ON COMMUNITY STRUCTURE, considered in other sections.

Balanus crenatus

Studies 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 17-21^OC.  Herz 1933 Biol Bull 64: 432. Photograph courtesy Dave Cowles, Walla Walla University, Washington wallawalla.edu.

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

Balanus glandula

As 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

In Ladysmith Harbour, British Columbia reproduction in Balanus glandula follows this schedule at
6-8^OC:

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 15-18^OC. 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

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 the 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 times 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 5^OC above ambient); the other, nearby shore populations living at ambient temperatures.  Only T. squamosa shows significant differences in reproductive cycling in the 2 locations

Three 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

A 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

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.

Barnacles 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 breaking 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

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 

Balanus nubilis

Naupliar development in the giant barnacle Balanus nubilis is similar to that described for other balanoid species, save for 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 tows

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 0.5X

Chthamalus spp.

Can 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
glandula at the extreme upper limits of their
distributions. A solitary C. dalli is highlighted 0.4X

Megabalanus californicus

Semibalanus cariosus

Release 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

A 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

Tetraclita rubescens

Larvae 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