Reference: Biol. Bull. 206: 121-124. (June 2004) 2004 Marine Biological Laboratory Limits to Phenotypic Plasticity: Flow Effects on Barnacle Feeding Appendages NATASHA K. LI* AND MARK W. DENNY Hopkins Marine Station, Stanford University; Pacific Grove. California 93950 Phenotypic plasticity, the capacity of a given genotype to produce differing morphologies in response to the environ-ment, is widespread among marine organisms (II For example, acorn barnacles feed by extending specialised appendages {the cirral legs) into flow, and the length of the cirri is plastic: the higher the velocity, the shorter the feeding legs (2, 3). However, this effect has been explored onl\ for flows less than 4.6 m/s. slow compared to typical flows measured at sites on wave-exposed shores. What happens at faster speeds? Leg lengths o/Balanus glandula Danvin, 1854. an acorn barnacle, were measured at 15 sites in Monterey; California, across flows ranging from 0.5 to 14.0 m/s. Similar to previous findings, a plastic response in leg length was noted for the four sites with water veloc-ities less than 3 m/s. However, no plastic response was present at the 11 sites exposed to faster velocities, despite a 4-fold variation in speed. We conclude that the velocity at which the plastic response occurs has an upper limit of 2-4 m/s. a velocity commonly exceeded within the tvpical Inih-itat of this species. Acorn barnacles provide an excellent opportunity for examining plastic response because they are sessile (and therefore cannot move in response to the environment), molt their exoskeleton (providing periodic opportunity for morphological change), and occur across a wide range of flow conditions. Helmuth and Denny (4) measured maximal wave-induced water velocities at 222 sites along the rocky intertidal shore at Hopkins Marine Station (HMS) in Pacific Grove, California (3636'N, 12153'W), and the variation Received 5 February 2004; accepted 31 March 2004. * To whom correspondence should be addressed. Present address: American Institute of Mathematics. 360 Portage Ave, Palo Alto. CA 94305. E-mail:
[email protected] Abbreviations: ADM, average daily maximum; ANCOVA. analysis of covariance; HMS. Hopkins Marine Station; MLLW. mean lower low water; OM. overall maximum. in velocity at each site was expressed as a function of offshore significant wave height (the average height of the highest one-third of waves). These measurements allowed us to select sites exposed to a range of wave-induced water velocities. Eleven sites, each 1.5 m above the mean lower low water (MLLW), were selected for collection of B. glandula. Because of the exposure of this shore, all HMS sites except one are subjected to water velocities greater than those encountered in previous studies on B. glandula (2, 3). Therefore, four additional sites were selected at the Monterey Wharf in Monterey, California (~2 km from HMS), where barnacles are subjected to a range of slower flows. At each site. 10 barnacles were collected, and the length of each cirrus was measured. Offshore significant wave height was measured four times per day for 30 days prior to the barnacle collections, and the largest significant wave height occurring when the tidal height was greater than 1 .5 m above MLLW was noted for each day. These data, in conjunction with the relation-ships measured by Helmuth and Denny (4), allowed us to estimate the daily maximal water velocities imposed at each collection site. Marchinko (3) found that transplanted spec-imens of B. glandula begin modifying their cirral length in response to their new environment somewhere between 7 and 18 days after first exposure. There was no evidence of alteration at day 7 and significant alteration by day 18, continuing through day 30. To incorporate this lag in re-sponse time, we used the water velocities recorded 10-30 days prior to sampling as an index of the flows to which the barnacles could have responded. Previous studies have ex-amined only the relationship between cirrus length and average daily maximum velocity, although the barnacles could be responding to maximum velocity, average veloc-ity, or some other aspect of flow. We employ both the average daily maximum (ADM) velocity and overall max-imum (OM) velocity.
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