Caught in the Act! This pair of mating amphipods was found in the Ulva area. The larger male often clasps the smaller female for days at a time, hoping to mate with her when she molts. |
The majority of the
fauna observed in False Bay consist
of polychaete worms, bivalves, snails, and crustaceans. Reproductive strategies
can be divided into two categories: those species in which development
takes place away from False Bay in the plankton (free-spawning and epitoky),
and those in which development occurs benthically in the bay (oviposition
and brooding). This page will focus on a sampling of species exhibiting
these habits.
Free-Spawning
The majority of the
fauna observed in False Bay spend part or all of the larval stage in the
plankton. The sand dollar Dendraster
excentricus spawns both eggs and sperm (separate individuals) in
late spring/summer. Although cues for spawning are unknown, it has been
suggested that following low tide, spawning is induced by the influx of
cold water on the incoming tide (Strathmann 1987). This strategy would
ensure that Dendraster embryos are carried safely out into the plankton
on the next ebb tide and also that embryos are not exposed to the harsh,
hot conditions of low tide at False Bay. Sand
dollar larvae spend a variable period of time in the plankton, and
often prefer to settle near pre-existing populations of adults (Strathmann
1987).
Epitoky
is an unusual mode of reproduction unique to polychaetes in which the worm
undergoes a partial or entire transition into a pelagic, sexually reproductive
morph known as an epitoke. Polychaete epitokes can often be seen swarming
around submerged lights during summer nights in the San Juan Islands. In
many cases, epitoky involves loss or degeneration of digestive structures
and enhancement of swimming, sensory, and gonadal structures (Strathmann
1987). In the case of False Bay, worms such as nereids and glycerids that
engage in epitoky leave the sandflats and enter the plankton
in order to reproduce. Populations are replenished when larvae come in
on the incoming tide and settle out of the plankton to take up the benthic
lifestyles of adult polychaetes.
Benthic Oviposition
Many snails and some
worms of False Bay lay their eggs in gelatinous masses anchored to sand,
algae, or bits of debris. Bubble snails such as Melanochlamys
and Haminoea develop in embryo masses to at least the veliger
stage, at which time they become planktonic for a period before settlement.
Embryo masses are exposed to extreme temperature
fluctuations and other hazards of low tide, as well as predation
at both high and low tide.
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Brooding
Many False Bay animals,
particularly arthropods, exhibit brooding behavior. Upogebia
pugettensis, the mud shrimp, carries eggs cemented to its abdominal
appendages throughout the winter. Larvae are released in spring and
spend 2 or 3 weeks, as a stage called a zoea, in the plankton before returning
to the mud flats on nocturnal spring flood tides (Dumbauld et al 1996).
Brooded embryos and larvae are protected from predation and from the worst
extremes of tidal fluctuations. However, brooding incurs a significantly
higher cost of reproduction to the female parent than reproductive strategies
in which parental responsibility ends with egg release.
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Taxon | Reproductive Mode | Taxon | Reproductive Mode | |
Annelida: Polychaeta | Mollusca: Bivalvia | |||
Maldanidae | brood to 7-setiger or later | Macoma inquinata | Free-spawn | |
Terebellidae | unknown | Macoma nasuta | Free-spawn | |
Arenicolidae | brood or free-spawn | Clinocardium nuttalii | Free-spawn | |
Glyceridae | epitokous | Orbitella rugifera | Brood to veliger | |
Lumbrineridae | benthic oviposition | Tresus capax | Free-spawn | |
Nereidae | epitokous | Transennella tantilla | brood to juvenile | |
Onuphidae | brood to 6-setiger stage | |||
Orbiniidae | benthic oviposition? | Mollusca: Gastropoda | ||
Phyllodocidae | epitokous | Lacuna variegata | benthic oviposition | |
Chaetopteridae | free-spawn | Haminoea vescicula | benthic oviposition | |
Nephytidae | free-spawn | Melanochlamys diomedia | benthic oviposition | |
Hesionidae | free-spawn? | |||
Arthropoda | ||||
Echinodermata | Grandifoxus grandus | Copulatory; brooding | ||
Leptosynapta | internal brooding | Other amphipods | Copulatory; brooding | |
Dendraster excentricus | free spawn | Upogebia | Copulatory; brooding | |
Neotrypea | Copulatory;? | |||
Nemertea | Pinnixa tubicola | Copulatory;? | ||
Paranemertes peregrina | free-spawn | Pagurus spp. | Copulatory; brooding | |
Amphiparous | free-spawn | Leptochelia (Tanaid) | Copulatory; brooding | |
Tubulanus | free-spawn |