subtitle for learnabout section of A SNAIL'S ODYSSEY
  Reproduction & development
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  Dispersal
 

Dispersal of littorines & relatives is by planktonic drifting if the species has a free-swimming veliger larva, or by crawling if such a stage is lacking. However, some species when small are able to disperse by use of a mucous thread that increases buoyancy.

This section on reproduction & development is divided into topics of dispersal, considered here, and MATE SELECTION & COPULATION, PATTERNS OF DEVELOPMENT, and GENETICS, considered elsewhere.

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Research study 1
 

photograph of lacunid snail Lacuna porrecta courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, WashingtonStudies on drift-dispersal of snails and other invertebrates at Bamfield Marine Sciences Centre, British Columbia show that despite lacking a pelagic larval phase in their development, many species of gastropods and mussels effectively disperse as juveniles using mucus-thread transport.  Use of off-bottom collectors demonstrate that at least 18 types of molluscs employ this type of dispersal, with snails Lacuna spp. and mussels Mytilus spp. representing by far the greatest components.  Mean sizes of organisms collected range from 1-2mm, but also present are a few littorinids Littorina sitkana at 2.3-3.2mm in size, and even some juvenile sea stars Leptasterias hexactis at 0.7mm diameter.  The authors analogise the strategy with the gossamer flight of young spiders, and suggest that it could constitute an effective alternative means of distribution for otherwise sedentary organisms, and for gastropods and other taxa that lack a pelagic larval phase, such as the sea-star Leptasterias just mentioned  It could also help to explain why many species with direct development are widely distributed. Martel & Chia 1991 J Exp Mar Biol Ecol 150: 131. Photograph courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

NOTE the collectors are nylon bags stuffed with red algae Gracilaria pacifica suspended between two supports in the intertidal region, about half a meter above the sand/gravel substratum

NOTE it is not clear whether these sea stars use a mucous thread, or whether they are simply lifted and carried bodily by currents

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Research study 2
 

photograph of lacunid snails Lacuna vincta feeding on the bulb of a bull kelp Nereocystis luetkeana, courtesy N. Elder and Linda Schroeder, PNWSCgraph comparing sinking rates of lacunid snails Lacuna variegata with and without threads used for buoyancyOther studies at the Bamfield Marine Sciences Centre, British Columbia show that Lacuna vincta and L. variegata can initiate mucus-thread drifting in currents.  An individual will do this by raising the foot surface until it is attached only at the front.  The mucus produced and accumulated at the sole of the foot is carried away with the water current and stretched 50-160 times the length of the shell.  The thread buoys up the snail and it drifts away. Note in the graph the degree of buoyancy conferred on the snail when a thread is present. The extended thread also enables the drifting snail to attach quickly and gain a foothold on substrata encountered.  Both juvenile and adult snails are capable of mucus-thread drifting.  The authors suggest that the behaviour may enable a snail to escape unfavourable conditions and to enable juveniles to relocate from their larval settling sites on seaweeds to their adult habitats.  Martel & Chia 1991 Mar Ecol Progr Ser 72: 247. Photograph courtesy N. Elder & Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

NOTE the authors include still photographs from videos for each species but, unfortunately, the mucous threads are too small to be seen

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Research study 3
 

schematic showing evolutionary consequences of having a pelagic larval stage in the life cycle of a marine invertebrateTo what extent do convergent ocean currents create a barrier to gene flow for an intertidal marine invertebrate?  This time-honoured and well-investigated question is addressed by a researcher at Friday Harbor Laboratories, Washington by use of simulation models of seasonal variation in ocean currents around Point Conception, California in relation to transport of  the pelagic larvae of the intertidal species Littorina scutulata and L. plena.  The model is complex and the results too detailed to present here (see schematic on Left; selected factors in the model are highlighted), but the most map of California showing location of Point Conception and associated summer water currentsimportant prediction from it seems to be that if a convergence of currents around a point of land presents a barrier to transport of larvae and if there is seasonal variation in these currents, then the longer the spawning season of a species, the better is the chance of overcoming the barrier by exploiting lapses in its effectiveness.  These could happen seasonally, or even on longer time scales.  The prediction may seem rather obvious, but the author remarks that past failures to detect a phylogeographic barrier at the Point may, in part, be explained by investigators not taking into account the extended spawning seasons, long pelagic life of larvae, and ability to extend time of settlement, of the species under study.  Any or all of these could make even the most impermeable barriers to gene flow “leaky”.  Hohenlohe 2004 Biol J Linn Soc 82: 169.

Location of Point Conception, California and
general pattern of summer water currents

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