title used in an account of west-coast marine invertebrates entitled A Snail's Odyssey
  Reproduction
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  Selection of substratum
 

Reproductive events include selection of substratum considered in this section, and
COPULATION & LARVAL DEVELOPMENT,
LARVAL BIOLOGY,
SETTLEMENT CUES
,
ATTACHMENT,
SETTLEMENT COUPLED WITH OCEANIC PROCESSES
, and
POST-RECRUITMENT EFFECTS ON COMMUNITY STRUCTURE, considered in other sections.

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Research study 1
 

histogram showing settlement of barnacle cyprids on plates set out at different intertidal levelsStudies at Friday Harbor Laboratories, Washington show that cyprid larvae of Balanus glandula and Semibalanus cariosus settle preferentially on experimental substrata bearing microflora characteristic of a lower intertidal level, rather than that of a higher intertidal level.  The experimental substrata consist of slate plates, 15 x 15cm in size and scored with grooves, set out in both high and low positions on the shore (vertical separation of 1m, high plates at 1.2m above MLLW) for 3mo over winter.  After this conditioning, involving settlement of bacteria, diatoms, and algal sporlings, and coincidental with the first appearance of cyprids in the plankton, half the plates at each location are reciprocally transferred to the other location.  After 4d they are removed and numbers of spat of each barnacle species counted. 

The data show that cyprids of both species settle preferentially on slate plates bearing microbiota characteristic of the lower intertidal position. These plates have a richer “set” of micro-algae than the high plates. The results are somewhat unexpected. Larvae of Balanus glandula settle widely throughout the intertidal zone, but rarely survive in the lower parts possibly owing to greater numbers of predators, so why would the cyprids be attracted to biota from the lower position?  In photograph of mixed assemblage of barnacles, including Semibalanus cariosus, Balanus glandula, and possible some Chthamalus dallicomparison, Semibalanus cariosus occupies a slightly lower level on the shore; hence, might be “expected” to be attracted as larvae to lower-zone microbiota. Furthermore, in comparison with the broader vertical settlement range of cyprids of Balanus glandula, settling larvae of Semibalanus cariosus actually avoid plates bearing high-intertidal microflora (see histogram above).  The cyprids of Semibalanus appear to use cues associated with the natural shore to avoid sites in the upper intertidal regions that are dry at low tide.  Thus, few larvae settle above the level where they will survive.  The authors suggest that mortality of larvae and/or early settlement stages from drying or high temperature may be the chief factor setting upper limits of distribution of the species.  Strathmann et al. 1981 Oecologia 48: 13; see also Strathmann 1979 p. 77 In, Reproductive ecology of marine invertebrates (Stancyk, ed) U South Carolina Press, Columbia, SC.

NOTE  the experiment is run over 3 consecutive seasons and the results averaged, both for year and for original conditioning zone.  Differences in counts relating to final “settling position” of the plates (i.e., whether a “low” plate is at a “high” or “low” larval-settling position) are not significant, and so the data are presented here as combined means

 

Mixed assemblage of barnacles in zone of overlap on a shore in Barkely
Sound, B.C. The assemblage comprises 4 Semibalanus cariosus, 13 Balanus
glandula
, and several B. glandula spat. The rock surface seems quite clean of
diatoms, possibly owing to the presence of 8 or so limpets Lottia digitalis 0.9X

 
Research study 2
 

histograms showing settlement of cyprid larvae of barnacles Balanus glandula onto differenst substrata at different tidal heightsStudies at Friday Harbor Laboratories, Washington show that some barnacle species preferentially settle in empty adult tests.  These unique microhabitats are common in some areas, occupying as much as 0.5% of the rock surface.  The walls of the old tests protect the early settlement stages from temperature extremes, desiccation, mechanical damage, and predators.  The histogram on the Right shows densities of settled B. glandula cyprids in tests of barnacles, on bare rock, and under brown algae Fucus.  Note the general preference for settling in tests of the small barnacle Chthamalus dalli.

Balanus settles broadly in all tidal zones but survives to reproduce only in the high intertidal zone. Other data presented by the authors show that cyprids of Semibalanus cariosus settle more in the low intertidal area and display no significant preferences for tests of other barnacle species over bare rock and under Fucus plants. Not only do cyprids of B. glandula gain a measure of protection from drying and wave photograph of a littorine Littorina scutulata in the empty test of a barnacle Semibalanus cariosusforces by preferentially settling in empty tests, they are exhibiting a bet-hedging strategy forsurvival because barnacles have previously lived on these spots. 

photograph of a barnacle Balanus glandula settled within the empty test of a thatched barnacle Semibalanus cariosusThe authors also note that littorine snails Littorina scutulata  and L. sitkana commonly inhabit empty barnacle tests, and tend to exclude settling barnacles by eating them, disturbing their settling behaviour with their shells, and passively occupying potential settling space. Qian & Liu 1990 J Exp Mar Biol Ecol 142: 63.


Balanus glandula settled within the
test plates of a thatched barnacle
Semibalanus cariosus
1.5X


By occupying an empty Semibalanus cariosus
test, this littorine snail Littorina scutula may prevent settlement by barnacle spat 1.2X

 
Research study 3
 

histogram showing growth of spat of barnacles Balanus glandula on different substrataphotograph of thatched barnacles settled onto the shell of a mussel Mytilus californianusIn Santa Barbara, California sea mussels Mytilus californianus are often found with growths of barnacles Balanus glandula and Chthamalus fissus,and limpets Lottia scabra, on their shells.  Given that it is risky for the larvae to settle on or around mussels because they could be sucked in and eaten, the question arises as which substratum, rock or mussel shell, represents the best substratum for growth and survival.  In other words, is there any selective advantage for a barnacle larva to settle preferentially on one substratum or the other. 

graph showing growth of barnacle Balanus glandula on different substrataFirst, size of barnacles does not differ significantly on the 2 substrata (see histogram upper Right). Second, growth of B. glandula does not differ significantly on the two substrata (see graph lower Right). However, survivorship of the barnacles is significantly better on mussels than on rock, indicating that a mussel may ultimately be the superior settlement substratum. 

graph showing survival of barnacles Balanus glandula on different substrataIs selection leading, then, to mussel-preferring barnacles in this region?  Not likely. The author notes that the availability of rock area and mussel area vary inversely, and since a larva cannot predict what the conditions will be where it lands, nor how much of a particular substratum will be available to it, it would be adaptive for a species to retain the ability to recruit to both rock and mussel rather than to specialise.  Lohse 1993 J Exp Mar Biol Ecol 173: 133.

NOTE  only results for barnacles are considered here; those for limpets Lottia scabra are considered elsewhere in the ODYSSEY: LEARN ABOUT LIMPETS: SETTLEMENT & RECRUITMENT The author also provides comparable data for barnacles Chthamalus fissus but, as they are similar to those shown for B. glandula, they are not included here

 
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