Cuticle and muscle variation underlying phenotypic plasticity in barnacle feeding leg and penis form

Abstract

Abstract. Many aspects of barnacle body form are known to be developmentally plastic. Perhaps the most striking examples of such plasticity occur in their feeding legs and unusually long penises, the sizes and shapes of which can change dramatically and adaptively with changes in conspecific density and local water flow conditions. However, whether variation in overall appendage form is mirrored by structural responses in cuticle and muscle is not known. In order to determine how structural variation underlies phenotypic plasticity in barnacle appendages, we examined barnacles occurring at low and high population densities from one wave-protected and one wave-exposed site. We used histological sectioning and fluorescence microscopy of feeding legs and penises to compare cuticle thickness, muscle thickness, and muscle organization, and artificial penis inflation to compare penis extensibility. We observed striking differences in cuticle thickness, muscle thickness, and muscle organization between sites that differed in water velocity, but we found no clear differences associated with variation in conspecific density. Penis extensibility also did not differ consistently between sites. These results are consistent with an adaptive explanation for much of the remarkable and complex variation in barnacle feeding leg and penis morphology among sites that differ in water velocity.Additional key words: Balanus glandula, cirri, wave exposure Rocky shores are among the most extreme and variable hydrodynamic environments on earth. On wave-exposed shores, the maximum velocity of breaking waves can exceed 25 m s À1, while in nearby protected bays, water velocity may be two orders of magnitude slower, never exceeding 10 cm s À1 (Denny 1988;Denny et al. 2003). Adding to this extreme spatial variability, water velocity at any one place can change dramatically as waves break and recede, and it may also vary significantly throughout the year due to the frequency and severity of storm events (Denny 1988). Surprisingly, despite this widespread variation in water velocity over space and time, a diverse assemblage of plants and animals thrives under these extreme conditions.Intertidal barnacles are one particularly successful group that attains large population sizes on rocky shores worldwide. These small sessile crustaceans are filter feeders that use six pairs of modified biramous thoracic legs, called cirri, to capture particles from the surrounding water. In most acorn barnacles, the last three pairs of cirri align to form a fan-like structure that is extended into the water column, while the first three pairs of cirri are used to filter smaller particles and to clean larger particles from the extended cirri (Anderson 1994). Barnacles extend these feeding legs by increasing hydrostatic pressure generated within a closed circulatory system, and retract them with a bundle of striated muscle fibers that runs from the base to the tip of each ramus (Anderson 1994). However, filter feeding poses a challenge to many intertidal barnacles...

Effects of tidal height and wave exposure on cirrus and penis morphology of the acorn barnacle Tetraclita stalactifera

“…). Penis thickness did not exhibit the strong variation observed in other species (Hoch , , ; Neufeld & Palmer ; Neufeld & Rankine ; López et al. ), although it did vary with tidal height.…”

“…; Marchinko ; Marchinko & Palmer ; Chan & Hung ; López et al. , , ; Hoch , , ; Neufeld ; Neufeld & Palmer ; Neufeld & Rankine ; Table ). However, we did note variation in cirrus and penis morphology with respect to tidal height.…”

“…; Marchinko ; Marchinko & Palmer ; Chan & Hung ; López et al. , , ; Hoch ; Neufeld ; Neufeld & Rankine ), which show dramatic morphological differences between environments and which had a large proportion of the variance in morphology explained by environmental variables (Table ). The barnacles from this study may lack such strong responses because they use a different feeding strategy and thus respond to the risks of crashing waves differently.…”

“…; Hoch ), with those in wave‐exposed areas growing shorter, stouter cirri to reduce drag and increase strength (Arsenault et al. ; Neufeld & Rankine ). This response is phenotypically plastic, and barnacles transplanted between wave‐exposed and wave‐protected areas finely tune their morphology to local conditions (Marchinko ; Neufeld ).…”

“…Other barnacles respond to hydrodynamic conditions by varying the strategies used to draw water across their cirral fan, including active beating or passive extension (Crisp & Southward 1961;Marchinko 2007). Some barnacles have a morphological response to wave conditions (Arsenault et al 2001;Marchinko 2003;Marchinko & Palmer 2003;Chan & Hung 2005;L opez et al 2010;Hoch 2011), with those in wave-exposed areas growing shorter, stouter cirri to reduce drag and increase strength (Arsenault et al 2001;Neufeld & Rankine 2012). This response is phenotypically plastic, and barnacles transplanted between wave-exposed and wave-protected areas finely tune their morphology to local conditions (Marchinko 2003;Neufeld 2012).…”