Precise tuning of barnacle leg length to coastal wave action

Abstract

Both spatial and temporal variation in environmental conditions can favour intraspecific plasticity in animal form. But how precise is such environmental modulation? Individual Balanus glandula Darwin, a common northeastern Pacific barnacle, produce longer feeding legs in still water than in moving water. We report here that, on the west coast of Vancouver Island, Canada, the magnitude and the precision of this phenotypic variation is impressive. First, the feeding legs of barnacles from protected bays were nearly twice as long (for the same body mass) as those from open ocean shores. Second, leg length varied surprisingly precisely with wave exposure: the average maximum velocities of breaking waves recorded in situ explained 95.6-99.5% of the variation in average leg length observed over a threefold range of wave exposure. The decline in leg length with increasing wave action was less than predicted due to simple scaling, perhaps due to changes in leg shape or material properties. Nonetheless, the precision of this relationship reveals a remarkably close coupling between growth environment and adult form, and suggests that between-population differences in barnacle leg length may be used for estimating differences in average wave exposure easily and accurately in studies of coastal ecology.

Precisely proportioned: intertidal barnacles alter penis form to suit coastal wave action

“…Six sites in the order of decreasing wave exposure (Seppings Island, SI, Bordelais Island, Wizard Islets, Kelp Bay, Self Point and Ross Islets, RI) are described in Arsenault et al (2001) and two additional low-velocity sites (Bamfield Inlet, BI and Grappler Inlet) are described in Marchinko & Palmer (2003). Barnacles were collected between 21 and 24 February 2006, when the majority of individuals in this area are reproductively active (Strathmann 1987) and have fully developed penises (Barnes 1992).…”

“…To avoid potential density-induced differences in penis form, 25 solitary barnacles-barnacles with plates not touching another barnacle but with an approximately equal number of neighbours within 1-2 cm-were collected in the middle of the Balanus glandula zone at each site on 14 December 2007. After freezing for 24 hours at K88C, barnacles were thawed in seawater and the soma was removed, photographed under a dissecting microscope at 6 -8!, blotted dry and weighed to the nearest 0.1 mg following Arsenault et al (2001). The soma was then cut between the first and second pair of thoracic legs, inserted onto the tapered end of a seawater-filled plastic capillary tube (1.09 mm in outside diameter, 0.38 mm in inside diameter and approx.…”

“…In addition, we used empirically determined summer velocities at these sites ( (c) Field transplant experiment To differentiate between genetic control (differential settlement and/or selective mortality) and environmental control (phenotypic plasticity) of penis form, we transplanted barnacles from two source populations to each of two destination sites. On 25 September 2006, adult B. glandula were collected on mussel shells (Mytilus californianus and Mytilus trossulus) from two source populations chosen for a substantial difference in wave force between sites (Arsenault et al 2001) and for a sizeable supply of adult barnacles growing on mussels: a protected shore (RI; Arsenault et al 2001) and an exposed shore (SI; Arsenault et al 2001) in Barkley Sound, BC, Canada. Mussel shells were cut using a variable speed rotary tool so that one barnacle occupied each mussel shell fragment.…”

“…Owing to errors during dissection, sample sizes used to determine site means differed slightly (from nZ17 to 20) among sites and analyses. Owing to the difficulty of measuring winter water velocity using standard techniques, we used the known relationship between leg length and maximum velocity of breaking waves (Arsenault et al 2001) to estimate the maximum water velocity at each site. In addition, we used empirically determined summer velocities at these sites ( (c) Field transplant experiment To differentiate between genetic control (differential settlement and/or selective mortality) and environmental control (phenotypic plasticity) of penis form, we transplanted barnacles from two source populations to each of two destination sites.…”

“…Consequently, individuals with penises well suited for mating in quiet waters may be poorly suited for copulating on wave-exposed shores. Indeed, other structures that must extend into flow, like the long feathery feeding legs of barnacles, differ dramatically in form between protected harbours and nearby wave-exposed sites (Arsenault et al 2001). Furthermore, in one species, these differences in feeding leg form arise primarily due to phenotypic plasticity (Marchinko 2003).…”