title for learn-about section of A SNAIL'S ODYSSEY

Sexes are separate in mussels and gametes are spawned freely into the ocean.  Fertilisation leads to a veliger larva that feeds for several weeks on phytoplankton, and then settles to the sea bottom to metamorphose. 

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  Spawning & larval life

There are many ideas about how the 2 sexes of broadcast-spawning invertebrates, such as mussels, sea urchins, clams, corals, worms, and others, coordinate their behaviour and physiology to spawn synchronously.  Obviously, this must happen or the eggs would mostly go unfertilised.  Any number of factors may be involved, but a few that have been proposed are:

photograph of mussels Mytilus spawning courtesy 1. state of tide
2. lunar phase
3. temperature change
4. mechanical stimulation by water flow
5. water-borne chemicals (pheromones)
6. phytoplankton
7. rainfall

Based on the generality of this list it is likely that when an individual is ripe with gametes, almost any physical or chemical disturbance may induce spawning.  This may even include the physical bumping by phytoplankton in the water, representing the food of the soon-to-be larvae.  We know little about inter-individual chemical communication involving pheromones, and this has been suggested for several taxa of spawners, including sea urchins, corals, nereid worms, and others. Photograph courtesy OSFImages, London.

Mussels, probably Mytilus edulis, spawning. It seems by
the presence of sperm (milky white) and particulate
matter (possibly eggs?) that both sexes may be spawning 1X

  Topics on spawning and larval life are considered here, while those on SETTLEMENT & METAMORPHOSIS are found elsewhere.
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Research study 1

An early study on reproduction in sea mussels Mytilus californianus around San Francisco, California reveals that spawning occurs throughout the year, but with possible maxima in early October, January-February, and May-June.  One point raised by the author is that there appears to be no effect of “mussel poison” on seasonal spawning activity.  This is an interesting idea, one that appears not to have looked at by later investigators, and one that may justify further research.  Whedon 1936 U Cal Publ Zool 41: 35.

NOTE  although not specified, this presumably refers to Paralytic Shellfish Poisoning that seasonally may be present in mussels and other shellfish

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Research study 2

graph showing spawning cycles of mussels Mytilus californianus at La Jolla, CaliforniaSeveral later studies on the reproductive cycle of mussels Mytilus californianus in southern California reveal that spawning is fairly consistent from year-to-year, occurring mainly Oct-Mar/Apr, but with some spawning occurring at other times of the year by some individuals (see graph). Settlement of larvae follows spawning by 1-3mo.  Experiments at the Scripps Institution of Oceanography, La Jolla involving stimulation of spawning in M. californianus indicate that laboratory-held animals spawn more-or-less at the same time as do field animals. Next to presence of gametes in the surrounding seawater, the author notes that mechanical shock such as scraping and byssus pulling has proven the most effective stimulus for spawning in M. californianus. Young 1946 Ecology 27: 354; Young 1942 Ecology 23: 490; Young 1945 Ecology 26: 58.  In these several papers the author reviews some of the earlier literature on spawning in mussels.

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Research study 3

Monthly gonadal smears of mussels Mytilus galloprovincialis from waters around Long Beach, California reveal that mature sperm are present throughout the year in some individuals.  In comparison, mature ova are present only during Nov-May.  Moore & Reish 1969 Veliger 11: 250.

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Research study 4

Larvae of mussels, like most other bivalves, spend several weeks floating in the ocean feeding on phytoplankton.  At this time, and moreso during actual settlement, they are vulnerable to predation by many different types of filter-feeding animals.  Among these are other bivalves, including mussels. Studies in Wales and elsewhere reveal that if an adult mussel Mytilus edulis does suck in the larvae, there are 2 possible outcomes.  The first, more common in clean non-silted water conditions, is that they will be filtered out like other food particles on the gills, moved in mucus strands to the mouth, and eaten.  The second, more common in silty water, is that they will be rolled up in mucus by the labial palps, and rejected along with the silt as pseudofeces.  Interestingly, in the first outcome they may actually emerge alive in the feces and be able to swim off; in the second outcome they generally become too tangled in mucus and die.  Bayne 1964 J Anim Ecol 33: 513; Mileikovsky 1974 Mar Biol 26: 303.

NOTE  feeding in bivalves is considered elsewhere in the ODYSSEY: LEARN ABOUT CLAMS: FOODS & FEEDING

Research study 5

photograph showing mussel Mytilus californianus zone in Barkley Sound, British ColumbiaSpawning is generally in late winter/early spring for bay mussels Mytilus trossulus and M. galloprovincialis, and throughout the year for California mussels M. californianus, although summer peaks are not uncommon depending upon area. There are only 2 reports of synchronous spawning by both sexes in M. californianus in the field, both occurring in Barkley Sound, British Columbia. The most notable of these involved some 35% of the mussels on a 20-40m length of intertidal shore, representing an estimated 54,000 individuals.  Eggs collected shortly afterwards in nearby surge channels showed approximately 80% fertilisation.  The authors estimate the total number of fertilised eggs produced from this relatively small group of spawning individuals to be 4.3 x 1010 (43 billion).  This tells us something of the level of energy input into coastal plankton from sea mussels, and the large potential for recruitment, not just by mussels, but by many other broadcast-spawning shore invertebrates, most notably, sea stars and sea urchins. Gosselin 2004 J Shellf Res 23: 529; Curiel-Ramírez & Cáceres-Martínez 2004 J Shellf Res 23: 515.





Zone of sea mussels Mytilus californianus
in Barkley Sound, British Columbia



Research study 6

During their life in the plankton a mussel larva may encounter widely varying conditions of food and other factors, and little is known of natural variability in larval condition at the time of settlement that may result from this.  For example, early in its life a veliger may feed well, grow large, and have ample lipid reserves.  It might be expected that these better-condition larvae would settle in higher numbers, but do they?  A study in southern California investigates the variability in size and condition, measured by lipid content, of settling larvae of Mytilus spp.1 from different settling cohorts2 throughout the summer settlement period. Interestingly, most of the variation in size (82%) and lipid content3 (58%) occurs within cohorts, rather than among cohorts.  Also, coefficients of variation within a cohort for larval size is much smaller (3-8%) than those for lipid content (44-93%), suggesting that lipid storage is a much more plastic attribute than size for mussel larvae.  Unexpectedly, settlement success and larval condition are decoupled, such that no significant relationship is found between condition and number of larvae settling.  The study highlights the need for further study to determine what factors influence settlement success in the field.  The author cautions that although size and lipid content are used graph showing sizes and lipid contents of veliger larvae of Mytilus spp. at different dates in southern Californiahere as measures of larval “condition”, the precise physiological mechanisms by which differences in larval history operate to affect metamorphic performance are not yet known.  Phillips 2006 Integr Comp Biol 46: 598.

NOTE1  the author cannot distinguish between the larvae of different mussel species, but suggests that they are likely M. californianus.  Also considered in the study are condition indices for barnacles Pollicipes polymerus and Chthamalus dalli but, as the results are similar to those for mussels, these other species will not be considered further

NOTE2  larvae are collected on tuffies (plastic-mesh scrub pads), set out in the mid-intertidal region of rocky shores on 6 dates between May-September.  Each collection is considered a cohort, defined in this case as a number of larvae collected at the same time in the same place

NOTE3  this is actually represented as a lipid index, calculated from a 2-dimensional microscopic image as lipid area/total larval area x 100%

Research study 7

A related study by the same author at the University of California, Santa Barbara monitors maternal investments, measured as egg volume and energy content, and larval size, in sea mussels Mytilus californianus at 2 sites north of Point Conception, California and 2 sites south.  Gonads are sampled twice (June and September) and the expectation, given the markedly different  environmental conditions that exist north and south of the Point, is that these differences will be reflected in extent of maternal investment.  Results, however, show no such effects, with the only significant differences being related to size of the adult females.  Thus, depending upon female size and independent of site, egg volume may differ by up to 57% and egg energy content by up to 116%.  Phillips 2007 Biol Bull 212: 12.

NOTE  in a later paper the author provides similar data on total reproductive outputs of M. californianus at sites north and south of Point Conception.  Phillips 2007 Mar Biol 151: 1543.

NOTE  mussels north of the Point have lower body masses and growth rates than ones south of the Point

Research study 8

map showing study locations for gonad colour in sea mussels Mytilus californaianusphotograph of male sea mussel Mytilus californianus with orange gonadsAsk a marine biologist what colour are the gonads of sea mussels Mytilus californianus and the answer will invariably be orange for females and white for males.  Some biologists, though, may not be familiar with the work of researchers in Oregon who have systematically sampled mussels from different locations and intertidal heights and concluded that colour is actually not a reliable indicator of sex.  This is especially evident in the high reaches of the intertidal zone where male gonads are more likely to be orange (or orangeish) in colour and indistinguishable from the females, as compared with gonads of males in the lower intertidal region (see photograph on Right).  Overall, the data show that mussels at the high edge of the mussel bed are inaccurately sexed based on colour more frequently than mussels at the low edge (62 vs. 36%).  Food appears not to be involved, as the 4 sites sampled are found from analysis of water samples to vary greatly in phytoplankton content (see map).  Instead, the authors propose that it is environmental stress, most apparent in the higher intertidal zone, that for whatever reason (the authors suggest antioxidant properties) leads to greater deposition of carotenoid pigments in the gonad. Petes et al. colour chart used to assess colour of gonads of sea mussels Mytilus californianus2008 Aquatic Biol 3: 63. Photograph of orange gonads above courtesy Jane Lubchenko, Oregon State University.

NOTE  the researchers use a standardised scoring system for the gonad coloration in the field (see figure on Right), then check the accuracy of the scoring later from prepared histological sections of each gonad sample.  An abundance of statistical information is provided on site, intertidal height, and sex differences, and the results are highly convincing