Living on the Edge: Settlement Patterns by the Symbiotic Barnacle <i>Xenobalanus globicipitis</i> on Small Cetaceans

<div><p>The highly specialized coronulid barnacle <i>Xenobalanus globicipitis</i> attaches exclusively on cetaceans worldwide, but little is known about the factors that drive the microhabitat patterns on its hosts. We investigate this issue based on data on occurrence, abundance, distribution, orientation, and size of <i>X</i>. <i>globicipitis</i> collected from 242 striped dolphins (<i>Stenella coeruleoalba</i>) that were stranded along the Mediterranean coast of Spain. Barnacles exclusively infested the fins, particularly along the trailing edge. Occurrence, abundance, and density of <i>X</i>. <i>globicipitis</i> were significantly higher, and barnacles were significantly larger, on the caudal fin than on the flippers and dorsal fin. Barnacles were found more frequently and in greater numbers on the dorsal rather than ventral side of the caudal fin and on the central third of dorsal and ventral fluke surfaces. Nearly all examined individuals attached with their cirral fan oriented opposite to the fluke edge. We suggest that <i>X</i>. <i>globicipitis</i> may chemically recognize dolphins as a substratum, but fins, particularly the flukes, are passively selected because of creation of vortices that increase contact of cyprids with skin and early survival of these larvae at the corresponding sites. Cyprids could actively select the trailing edge and orient with the cirri facing the main direction of flow. Attachment on the dorsal side of the flukes is likely associated with asymmetrical oscillation of the caudal fin, and the main presence on the central segment of the flukes could be related to suitable water flow conditions generated by fluke performance for both settlement and nutrient filtration.</p></div

Percent occurrence of <i>Xenobalanus globicipitis</i> on flukes, flippers, and dorsal fin of 96 striped dolphins (<i>Stenella coeruleoalba</i>).

<p>Triangles indicate actual figures, and circles indicate expected figures in a total of 96 striped dolphins, <i>Stenella coeruleoalba</i>, assuming that the probability of colonization depends on space provided on each fin (see text for details). Bars represent the 95% confidence interval.</p

Average values of median shell diameter (S.D.) of <i>Xenobalanus globicipitis</i> on the dorsal and ventral sides of 3 sections of the flukes (see Fig 3) of the striped dolphin, <i>Stenella coeruleoalba</i>, from the western Mediterranean Sea.

<p>‘n’: sample size of dolphins.</p><p>Average values of median shell diameter (S.D.) of <i>Xenobalanus globicipitis</i> on the dorsal and ventral sides of 3 sections of the flukes (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127367#pone.0127367.g003" target="_blank">Fig 3</a>) of the striped dolphin, <i>Stenella coeruleoalba</i>, from the western Mediterranean Sea.</p

Parameters of predictors in a mixed model that accounts for the median shell diameter of <i>Xenobalanus globicipitis</i> on the flukes, flippers and dorsal fin of the striped dolphin, <i>Stenella coeruleoalba</i>, from the western Mediterranean Sea.

<p>Parameters for ‘flippers’ and ‘dorsal fin’ were obtained by setting that of ‘flukes’ to zero.</p><p>Parameters of predictors in a mixed model that accounts for the median shell diameter of <i>Xenobalanus globicipitis</i> on the flukes, flippers and dorsal fin of the striped dolphin, <i>Stenella coeruleoalba</i>, from the western Mediterranean Sea.</p

Area colonized by <i>Xenobalanus globicipitis</i>.

<p>Diagram of dolphin flukes (a), flipper (b), and dorsal fin (c), highlighting the area where individuals of <i>X</i>. <i>globicipitis</i> were found on the striped dolphin, <i>Stenella coeruleoalba</i>.</p

Microhabitats.

<p>Microhabitats defined for the study of the habitat selection for <i>Xenobalanus globicipitis</i> on the flukes of the striped dolphin, <i>Stenella coeruleoalba</i> (see the text for details).</p

Basal shells of <i>Xenobalanus globicipitis</i>.

<p>Shells attached on the dorsal side of the flukes, close to the notch, of the striped dolphin, <i>Stenella coeruleoalba</i>. Scale bar: 2 cm.</p

Orientation criterion for the basal plate.

<p>Diagram of a basal plate of <i>Xenobalanus globicipitis</i> and criterion used to orient it with respect to the edge of the fluke of the striped dolphin, <i>Stenella coeruleoalba</i> (see the text for details).</p

Average abundance of <i>Xenobalanus globicipitis</i> on caudal fin.

<p>Dorsal side (squares) and ventral side (diamonds) of three sections of the caudal fin of striped dolphins (see also <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0127367#pone.0127367.g003" target="_blank">Fig 3</a>). Bars represent the 95% confidence interval.</p