Odd Couples by Fairbairn Daphne J

Author:Fairbairn, Daphne J. [Fairbairn, Daphne J.]
Language: eng
Format: mobi, epub
Publisher: Princeton University Press
Published: 2013-08-04T21:00:00+00:00

FIGURE 10.1. Top panel: A mature female burrowing barnacle, Trypetesa lampas, removed from her burrow. Only the exterior surface of her mantle is visible, and the opening to her mantle cavity can be seen at the top of the photograph. The arrow points to one of four dwarf males attached to her mantle. The scale bar indicates 0.5 mm. Bottom panel: A close-up of the male indicated by the arrow with his penis (not inflated) extending to the right. The scale bar indicates 0.1 mm. Photo reprinted with permission from Gotelli and Spivey (1992), figure 1.

In the photograph we can see only the exterior surface of the mantle that would normally be pressed against the burrow wall, so you will have to be content with imagining the curled, shrimplike female within. Mature female Trypetesa lampas are large by burrowing barnacle standards, with mantle sacs averaging 5–11 mm in diameter. The large, round disk that makes up most of the visible mantle sac in the photograph is called the ovarian disk because the paired ovaries lie immediately beneath it, along the curved back of the female’s body. It is flattened or disk-shaped to conform to the inner surface of the narrow burrow, which in turn must fit within the thickness of the host shell wall (usually about 2 mm). Between the ovarian disk and the mantle opening you can just discern a small, raised circular knob of tissue. This is the horny stalk by which the female has cemented herself to the burrow wall. The white arrow to the right of this points to one of four males attached to the outer surface of the female’s mantle, just below the attachment stalk, and the lower photograph shows this male in detail. His anatomy is straightforward. His long penis extends to the right and even though it is not inflated, it dwarfs his rounded, lobular body. To fertilize the female’s eggs, the penis expands and stretches well into the mantle cavity so that the sperm can be deposited near the openings of the female’s oviducts. Other than the penis the main constituents of the male’s body are a single, large testis, a seminal vesicle for sperm storage and lipid globules stored for fuel. With no mouth, gut or digestive organs of any sort, it is clear that the males subsist only on their stored resources. Like Osedax males, their lifespan and sperm production must depend on the amount of fuel they have stored prior to maturation into adults.

The sexual dimorphism of adult T. lampas is truly extraordinary in every way. Females average ten times the length and 500 times the weight of their males,10 and, once mature, the two sexes share virtually no morphological characteristics. If the adult males were not found living on the females and fertilizing their eggs, one would never identify them as members of the same species. Surprisingly, this remarkable sexual dimorphism develops only after the larval barnacles settle down and metamorphose to their juvenile forms. Both sexes

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