title for learn-about section of A SNAIL'S ODYSSEY
  Life cycle & reproduction
 


schematic showing life cycle of a colonial hydroidThe life cycle of a hydroid such as Obelia and related colonial species involves 2 phases: a feeding or polyp phase, and a reproductive or medusa, phase.  Reproduction begins in late spring or early summer with appearance of the reproductive polyps.  These bud off small medusae that feed and grow for several months.  Sexes are separate in hydrozoans, as in all cnidarians.  Later in the season the medusae reproduce sexually.   Eggs are released, fertilised by sperm from another medusa, and the resulting planula larva settles to the sea bottom to metamorphose into a juvenile polyp.  This grows, begins to divide asexually, and eventually takes on the familiar bushy form of the colony.  This textbook description applies only to a few types of colonial species; many solitary species vary in degree of separation of feeding and reproductive polyps, and in the free-living extent of the medusa stage.

NOTE  41 species of hydroids are identified from San Juan Island, Washington from their medusae, but little research work has been done on their reproduction.  Mills 1981 The Wasmann J Biol 39: 6.

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Let’s see if you have been paying attention.  Try to identify the single false statement among the following, then CLICK HERE for explanations:

Polyp and medusa forms reproduce, respectively, asexually and sexually; hence, have different genotypes. 

All polyps and medusae produced from a single colony are the same sex. 

All polyps, including feeding and reproductive, are genetically identical clones. 

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  Proboscidactyla flavicirrata
  Topics on reproduction of hydroids include a section on the comparatively well-researched species, Proboscidactyla flavicirrata, considered here, and a separate one on OTHER SPECIES, considered elsewhere.
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Research study 1
 

drawing of hydroid Proboscidactyla flavicirrata growing on the tube rim of a tubewormphotograph of gastrozooids of the hydroid Proboscidactyla flavicirrata inhabiting the tube rim of the sabellid tubeworm Pseudopotamilla ocellataResearchers at the Hopkins Marine Station, California discover an unusual symbiosis between a hydroid Proboscidactyla sp. and a sabellid tubeworm Pseudopotamilla ocellata (see drawing on Left). The hydroid colony inhabits the tube rim of the worm and consists of 2 types of individuals, gastrozooids and gonozooids, and possibly a third type, dactylozooids. The gastrozooids are about 0.8mm in height, possess 2 tentacles, and live at the tube rim (see photo upper Right).

drawing of gonozooid of hydroid Proboscidactyla flavicirrata bearing 4 medusa buds in different stages of developmentGonozooids are about 1mm tall and live on the sides of the worm tubes.  Medusa buds are borne about midway down the column of the gonozooid and are usually 4 in number (see drawing lower Right). 

The polyps appear to capture food particles carried in the current flow generated by the worm, that moves upwards and through the tentacular crown.  The gastrozooids can also reach over and pick food from the food grooves of individual filaments of the worm.  Hand & Hendrickson 1950 Biol Bull 99: 74.

NOTE  although listed here as an undescribed species of Proboscidactyla, apparently the medusa of P. flavicirrata had been described much earlier and, later, the medusa and polyp were united under this species name

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Research study 2
 

The 2-tentacled hydroid Proboscidactyla flavicirrata has an unusual life cycle commensurate with its mode of living in colonial form on tube-rims of different species of tubeworms.  Studies at the Friday Harbor Laboratories, Washington show that the free-swimming planulae are caught up in ciliary currents produced by the tentacle plume of the worm, and initially attach by nematocyst discharge onto the pinnules.  When the worm retracts into its tube, the settled larvae are scraped onto the tube rim.  Metamorphosis, which begins about 4-6h after settlement, requires the presence of the worm, but experiments show that the tube can be artificial or natural.  A tentacle rudiment appears by 4.5h and this is well developed by 7h. At about this time a protrusion or “foot” appears and the embryo uses this as a temporary means to glide about on.  Within a day or so a second tentacle has appeared, signifying the end of metamorphosis. Some of these developmental stages are shown below. Campbell 1968 Pac Sci 22: 336.

 

photograph of planula larva of hydroid Proboscidactyla flavicirrata in early metamorphosis
Planula 3h after contact with host worm. The endoderm is transforming (at Right)

photograph of planula larva of hydroid Probiscidactyla flavicirrata 4.5h after contact with host worm
Planula 4.5h after contact. The first of 2 tentacle buds is emerging
photograph of planula larva of hydroid Probiscidactyla flavicirrata 7h after contact with host worm
Planula 7h after contact. Tentacles have grown and the "foot" rudiment appears
photograph of planula larva of hydroid Probiscidactyla flavicirrata 4.5h after contact with host worm
Newly metamorphosed polyp. The "foot" protrusion is used to move about on
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Research study 3
 

photograph of sabellid tubeworm Eudistylia vancouveriThe author of a study on the 2-tentacled hydroid Proboscidactyla flavicirrata using specimens from Puget Sound, Washington depicts its relationship with several tubeworms as an obligate commensalism.  The relationship is obligate because only in the presence of a species of tubeworm does the hydroid maintain differentiated gastrozooids.  If a hydroid is isolated from contact with the worm, for example, by cutting off the tip of the tube without disturbing the hydroid, the gastrozooids dedifferentiate and are resorbed.  If contact with the worm is re-established, for example, by rejoining the excised bit of tube bearing the hydroid to the worm’s tube, the gastrozooids will redifferentiate.  Strickland 1971 Pac Sci 25: 88.

NOTE  tubeworms involved include Schizobranchia insignis, Pseudopotamilla occelata, and Eudistylia vancouveri

 


Cluster of sabellid tubeworms
Eudistylia vancouveri
0.25X

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Research study 4
 

photograph of planula of hydroid Proboscidactyla flavicirrata showing nematocysts used to attach to host wormgraph showing number of planulae of hydroid Proboscidactyla flavicirrata attaching to 2 different species of tubewormsA study at Friday Harbor Laboratories, Washington describes the preferential settlement of planula larvae of the hydroid Proboscidactyla flavicirrata onto the rims of tubes of the sabellid worms Schizobranchi insignis and Eudistylia vancouveri.  The planulae do not settle directly onto the tubes; rather, they begin their settlement by adhering with nematocysts to the pinnules of the tentacles. Later, they switch their attachment to the tube rim (again by nematocysts), likely when the worm withdraws into its tube.  However they do it, the process is remarkable, because in the time between initial attachment and transfer, found to be no less than 4min, the effective stimulus for nematocyst discharge shifts from pinnule to tube.  The planulae appear to be stimulated to metamorphose when they contact the rim of the tube.  By touching planulae sequentially to pinnulephotograph of tubeworm Schizobranchia insignisand either outside, inside, or middle layer of the worm tube, the author determines that it is the outside of the tube that is most effective in stimulating transfer of attachment. Interestingly, while the larvae settle and metamorphose on either of these similar-appearing sympatric worm species, they do so more on Schizobranchia insignis, and this is the only species that supports the growth of Proboscidactyla colonies (see graph). In some way, Eudistylia is deficient as a host and this is only manifested after metamorphosis. Donaldson 1974 Biol Bull 147: 573.

NOTE
  this occurs about 72h after fertilisation when nematocysts are abundant and mature

 

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