title of learn-about section on goose barnacles of A SNAIL'S ODYSSEY

photograph of hummocks of goose barnacles on the west coast of Vancouver IslandGoose barnacles Pollicipes polymerus are similar to other barnacles in that they are hermaphroditic, and therefore must be within penis-length of a neighbour to be reproductively viable.  Eggs are brooded for several weeks until they hatch into nauplii larvae. 

NOTE  self-fertilisation is known for some species of acorn barnacles, but has not been described for goose barnacles



Hummocks of goose barnacles Pollicipes polymerus on
the west coast of Vancouver Island, British Columbia.

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Egg release & brooding

  Topics on reproduction include egg release & brooding, considered here, and LARVAL BIOLOGY and SETTLEMENT considered in other sections.
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Research study 1

drawing of dissected goose barnacle Pollicipes polymerus to show digestive and reproductive systemsGoose barnacles Pollicipes polymerus in Monterey Bay, California are sexually mature at 17mm rostrum-carina length, but full reproductive potential is probably not realised until they reach a size of 27mm.  Reproduction occurs throughout the warmer months of the year, around 12-17oC, and may extend over 8mo or more.  Eggs incubated in vitro at 13oC take 30d to develop from fertilised egg to free-swimming larva.  The eggs are released from paired oviducts into the mantle cavity in batches of 100,000-240,000.  On their release the eggs are coated with a sticky material and, after falling to the floor of the mantle cavity, they adhere to one another and form a pair of flattened egg masses called ovigerous lamellae.  Eggs vary in size from 50-100µm.  The testes surround the gut and extend within the body mass towards the bases of the feeding appendages.  Paired vas deferens collect the spermatozoa from smaller feeder ducts and conduct them to a pair of large thin-walled seminal vesicles which, in turn, become fused and lead via a single duct to the penis. The author estimates that adults may release 3-4 broods per year, with up to 7 broods being produced by the largest individuals in a population.  Individuals separated by a distance greater than 20cm are sterile, indicating that self-fertilisation does not occur.  Hilgard 1960 Biol Bull 119: 169.

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Research study 1.1

photograph of goose barnacles Pollicipes polymerus in a cave environmentA researcher at Hopkins Marine Station, Pacific Grove, California describes 6 types of carotenoid pigments in the hemolymph of goose barnacles Pollicipes polymerus.  That these pigments are involved in some way in reproduction is suggested by the presence of 5 of them in eggs removed from the ovigerous lamellae on the floor of the mantle cavity.  During  reproduction, moreover, the stalk fluid that bathes the ovaries becomes coloured bright orange, the same colour as the ovigerous lamellae.  As the eggs develop, their colour changes to yellow and then to brown.  In the stalk fluid and ovigerous lamellae the astaxanthin component is bound to a highly unstable protein and appears to be transported about in this form.  However, the exact role that it plays is unknown.  Holter 1969 Comp Biochem Physiol 28: 675.

NOTE  beta-carotene, dihydroxy carotenoid, isozeaxanthin, lutein, zeaxanthin, and astaxanthin, of which the ones in bold comprise about 90% of the total 

A UV-light-protecting function is commonly suggested for carotenoid pigments
in animals, but the normal black stalk and shell plates of goose barnacles
Pollicipes polymerus
would make this unlikely. But this particular cluster
is in a dark cave environment in Barkley Sound, British Columbia...hmmm!

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Research study 2

photographs of constriction rings passing over the egg of a goose barnacle Pollicipes polymerusFertilised eggs of goose barnacles Pollicipes polymerus undergo peristaltic contractions.  The contractions appear as constriction rings about an hour after fertilisation and continue for another 4-5h. The rings, usually 3 in number, move from the animal to vegetal pole and are likely caused by contractions of myofilaments.  At 14oC a single ring takes about 10min to transit the egg.  At the time of the publication, such contractions were described for only a few marine invertebrates, such as shrimps, and their function remains unknown.  Lewis et al. 1973 Experientia 29: 1533.

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Research study 3

map showing study sites for collections of goose barnacles Pollicipes polymerus in southern CaliforniaStudies on brooding activity of goose barnacles Pollicipes polymerus in southern California indicate the presence of 2 physiological races, one that broods maximally in colder seawater temperatures, 14oC or less, and one that broods maximally at warmer temperatures, 20oC.  The disjunction latitude appears to be at Point Conception or slightly south (see map).  Brooding at the Pismo Beach site occurs in summer, but at temperatures of 15oC or less.  Brooding at Latigo Point is greatest during summer and increases significantly as seawater temperature rises to 20oC.  Finally, brooding at Santa Catalina Island is highest during winter at temperatures around 13oC, and decreases as temperatures increase.  The author proposes that these last individuals are likely transported south from populations to the north of Point Conception, thus forming a “cold-water” physiological race, with the Latigo Point individuals forming a “warm-water” physiological race.  At the 2 southerly sites, brooding is greater in the lower intertidal part of the distribution of Pollicipes than in the higher part.  The possibility that food availability, which correlates with water temperature, may be the proximate factor that triggers seasonal reproduction is rejected by the author based on results of laboratory experiments using different combinations of temperature and nauplii of brine shrimp Artemia salina as food.  The author suggests that ample bodily energy reserves in Pollicipes make temporal changes in planktonic food availability less important than they might be for smaller barnacle species living at similar intertidal levels.  Low-temperature brooding of P. polymerus at Pismo Beach, just north of Point Conception, is comparable to that exhibited by populations further north, such as at Friday Harbor, WA: 9-11oC or Bodega Bay, CA: 10-12oC.  The presence of 2 types of brooders in southern California suggests a genetic difference.  Cimberg 1981 Biol Bull 160: 31.

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Research study 4

graph showing proportion of goose barnacles Pollicipes polymerus bearing egg masses at different times of the yearGoose barnacles Pollicipes polymerus live at low- to mid-intertidal levels, and percentage of individuals carrying egg masses during the year varies with intertidal height occupied. Studies at Goleta Point near San Diego, California show that while 22-70% of individuals bear egg masses at any given time of year at 0.15m tidal levels, no more than 15% bear egg masses at 1m levels.  Note in the goose barnacles Pollicipes polymerus intermixed with sea musselsgraph the seasonal variability in the data, from highs in late winter/spring to lows in late summer/autumn.  The author suggests that the differences in brooding percentages are likely related to differences in time available for feeding at the different heights.  Page 1984 J Exp Mar Biol Ecol 74: 259.

NOTE   this idea is supported by laboratory data on feeding on different rations in an earlier paper by the same author.  Of course, there are likely to be many other features of life at high intertidal levels that will affect brooding in Pollicipes.  Page 1983 J Exp Mar Biol Ecol 69: 189.

Pollicipes polymerus growing intermixed with
sea mussels Mytilus californianus 1X

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Research study 5

With respect to the suggestion in Research Study 3 above that genetic differences may explain temperature differences in brooding, a later investigation on allelic variations of 8 allozymes in 5 populations of Pollicipes polymerus along the same part of the California coast reveals no significant genetic differences.  Is it possible, then, that the reproductive differences between the 2 "physiological races" described in the earlier study may simply result from phenotypic plasticity in brooding activity.  The present author speculates that the larvae may sense water temperature at a critical stage in early development and that this later determines the temperature at which the adults brood.  The author proposes an experiment to test this hypothesis, namely, to collect brooding adults from northern and southern populations, reciprocally rear the brooding embryos in warm (20oC) and cold (14oC) water and, following metamorphosis, place all juveniles in a common environment and rear them to sexual maturity.  The expectation would be that individuals would brood at the same temperature they experienced as larvae.  Miner 2002 Invert Biol 121: 158.

NOTE it is not known whether this experiment has ever been done, but it might be worth doing so

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Research study 6

photograph of goose barnacle Pollicipes polymerus showing penisIn a recent publication, researchers at the Bamfield Marine Sciences Centre, Brtish Columbia describe sperm dissemination in goose barnacles Pollicipes polymerus (see photograph on Left).  This behaviour contrasts with all previous descriptions for reproduction in the species, including transfer of sperm via a penis to a neighbouring individual and, for isolated individuals bearing fertilised eggs, self-fertilisation.  The researchers test which method has occurred by analysing eggs  from both isolated individuals and isolated pairs (where at least one individual is carrying egg masses) for genetic relationships between egg masses and potential parents.  Results show, surprisingly, that all isolated individuals with eggs have actually have captured free-swimming allosperm from the seawater and have not self-fertilised, and that 24% of egg-bearing individuals with a penis-accessible partner have also captured sperm.  With regard to the last, the authors note that P. photograph of a goose barnacle Pollicipes polymerus with one individual leaking spermpolymerus is unusual amongst barnacles in having a rather short and poorly extensible penis.  In comparison with acorn barnacles Balanus glandula, for example, whose penises can extend almost 3 body lengths, those of Pollicipes are extensible to only 0.7 body lengths. The results are remarkable not just in giving us a new look at how goose barnacles reproduce and in casting doubt on the possibility of self-fertilisation, but in uniquely demonstrating broadcast fertilisation in an aquatic arthropod. Barazandeh et al. 2013 Proc RSoc B 280: 20122919. Photos courtesy the authors.

NOTE  the authors refer to this as spermcast “mating” although, of course, there is no physical contact between the individual releasing sperm and the one receiving it

NOTE  use of 16 single-nucleotide polymorphism-markers

Goose barnacle Pollicipes polymerus with one individual
"leaking" sperm. The authors do not know whether free sperm
in the water originates in this way, through wastage during
transfer to another individual, or by free release into the water

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