Leonardo’s Mountain of Clams and the Diet of Worms (46 page)

But when the virgin externa pokes through the abdomen and lies flush against the crab’s underside, all “fight” has evaporated
from the host. The crab still possesses an active cleaning response, but makes no attempt to remove the externa. Why not? What has happened to the crab? In a remarkable paper published in 1981 in the
Journal of Crustacean Biology
, authors Larry E. Ritchie and Jens T. Høeg answer these questions in studying the root-head species
Lernaeodiscus porcellanae:

Since the externa could presumably be reached and removed,
the second and more interesting alternative probably applies. In other words, the parasite has somehow evolved to turn off the host’s defenses, presumably by disarming the crab’s immune response with some chemical trickery that fools the host into accepting the parasite as part of itself. The authors continue:

The phrase “absolute service” may sound extreme, but a compendium of parasitic devices for usurpation, takeover, and domination of the host can only elicit an eerie feeling of almost macabre respect
for the unparalleled thoroughness (and cleverness) of parasitic management!

First of all, the adult parasite castrates the host, not by directly eating the gonadal tissue (as in most cases of “parasitic castration,” a common phenomenon in this grisly world), but by some unknown mechanism probably involving penetration of the interna’s roots around and into the crab’s nervous system. In
Sacculina
(but not in most other rhizocephalans), the parasite also cuts off the host’s molting cycle, and the crab never again sheds its outer shell (an obvious benefit to the externa, which can easily be dislodged by molting).

Lernaeodiscus porcellanae
turns control of the host into a fine art. After castration by the parasite, male crabs develop female characteristics in both anatomy and behavior, while
females become even more feminized. The emerging externa then takes the same form and position as the crab’s own egg mass (in normally developing uncastrated females)—attached to the underside of the abdomen. The crabs—both male and female (for both sexes are feminized by the parasite)—then treat the externa as their own brood. In other words, the parasite usurps all the complex care normally
invested in the crab’s own progeny. Crabs ventilate the externa by waving their abdomens; they actively (and carefully) groom the externa with their cleaning limbs. Moreover, Ritchie and Høeg proved that this behavior may be indispensable for the externa’s survival—for when they removed the cleaning limbs from a parasitized crab, “the externa soon became fouled and necrotic.” Finally, the “simple”
root-head even fools the crab into treating the release of parasite larvae from the externa as the discharge of her own fertilized eggs! Ritchie and Høeg write:

In short, rhizocephalans are the cuckoos of the marine invertebrate world—laying their eggs in another species’s “nest,” mimicking the host’s own eggs (similarity of the externa to the crab’s egg mass), and then eliciting parental care from the host. But rhizocephalans are
even more thorough, for they always castrate their host, while only some cuckoos kill the legitimate nestlings of their foster parents.

In short, the root-head turns the crab into a Darwinian cipher, a feeding machine working entirely in the parasite’s service. The castrated crab can make no contribution to its own evolutionary history; its “Darwinian fitness” has become flat zero. All feeding
and growth now work in the evolutionary interest of the root-head, which continues to reproduce at a prodigious rate, entirely at the crab’s expense—as the interna’s roots drain the crab’s nutrition. But ever so carefully, for the parasite must maintain the crab in constant and perfect servitude—not draining the host enough to kill this golden goose, but not letting the crab do anything for its
own Darwinian benefit, either.

Root-heads can maintain this delicate balance for a long time. Ritchie and Høeg kept infested crabs for two years in the laboratory—with the parasites showing no ill effects and reproducing all the time. Moreover, the root-head can produce prodigious numbers of larvae—all supported by the crab’s feeding. In a 1984 article on
Sacculina carcini
, Jørgen Lützen found
that a single externa, during a breeding season lasting from mid-July to October, can produce up to six batches of eggs, with an average of 200,000 per clutch—for a total of more than a million eggs per season. Complex indeed—and devilishly effective. If I were a conscious rhizocephalan, I would adopt this motto: Don’t call me a simple sac with roots.

3. W
HAT ABOUT MALES? (OR, FURTHER COMPLEXITY
IN THE ROOT-HEAD LIFE CYCLE
). These first two categories of complexity only consider the female root-head. Delage and all early students of rhizocephalans regarded the externa as a hermaphrodite, with both male and female organs. But the externa forms part of the adult female only. Male rhizocephalans were not well documented until the 1960s, when a group of Japanese zoologists finally worked
out the full sexual system of rhizocephalans, and recognized the true nature of males.

The male life cycle differs in a striking way from the development of females—another testimony to rhizocephalan complexity when we consider full biology rather than adult anatomy alone. The beginning stages of nauplius and cyprid differ little between the sexes. But whereas the female cyprid settles on a crab
to begin the stage of internal penetration, the male cyprid alights instead on the female externa. In
Sacculina
and close relatives, the virgin externa contains no opening. But this initial externa soon molts to a second stage containing an orifice known as the “mantle aperture.” This opening leads into two passageways known as “cell receptacles.”

Successful male cyprids settle on the externa’s
aperture. A unique male stage, called the
trichogon
, then forms within the cyprid. The trichogon, clearly the homolog of the female kentrogon, but much simpler in form, has no muscles, appendages, nervous tissue, or sense organs. The trichogon looks like a small mass of undifferentiated cells surrounded by a cuticle covered with small spines (the name means “hairy larva”). The trichogon passes
through the antennule of the cyprid, into the aperture of the externa, and down the passageway of the cell receptacle. (Two trichogons may successfully enter the externa, one in each receptacle.) The trichogon then sloughs the spiny cuticle and lodges as a small group of cells at the end of the passageway. (Other rhizocephalans form no trichogon; the male cell mass must then be injected through the
cyprid antennule right into the body of the externa.) These tiny male cell masses become sources for the production of sperm.

These facts may not warm the hearts of superannuated macho blusterers among humans, but male root-heads end up as tiny dwarfs, injected into the body of a vastly larger female, and finally coming to rest as a small mass of loosely connected cells deep inside the externa.
Biologists refer to such males as hyperparasites—for they are parasitic upon a parasite. The female parasitizes a crab, but the tiny male depends entirely upon the female for nourishment. The male cells, permanently enclosed deep within the body of a relatively enormous and protective female (an odd kind of Freudian fantasy even for human males, I suppose), then spend their days producing sperm
in synchrony with egg production by the externa.

In summary, the intricate and different life cycles of both male and female root-heads, and the great behavioral sophistication shown by the female in reconfiguring a host crab as a support system, all underscore the myopia of our conventional wisdom in regarding rhizocephalans as degenerate parasites because the adult anatomy of internal roots
and external sac seems so simple.

This reassessment of root-heads forces me to revise my initial take on the lessons of parasites for correcting the bias of equating evolution with progress—for I can no longer hold that parasitic degeneration argues for a slight preponderance of simplification over complexification among evolutionary trends. But this correction leads to an even better argument
against predictable progress—one that also takes us back to the roots of our intellectual heritage in Darwin’s ideas.

In his famous 1880 essay
Degeneration: A Chapter in Darwinism
, E. Ray Lankester correctly identified belief in progress as the principal inference falsely drawn from Darwin’s theory of natural selection. Lankester wrote:

Lankester then cited supposed cases of degeneration, including root-heads as a primary example, to prove that natural selection does not guarantee such progress. “Degeneration,” he wrote, “may be defined as a gradual change of structure in which the organism becomes adapted to less varied and less complex conditions
oflife.” Lankester, in other words, remains true to Darwin’s deeper principle that natural selection leads only to local adaptation, not to global progress. He correctly states that simplified conditions of life might lead, by natural selection, to less complex anatomies—and that these simplified descendants would be just as well adapted to their habitats as more complex ancestors to previously
more elaborate modes of life. But Lankester erred in regarding root-heads as degenerate, forms properly responding to simplified conditions. An enlarged view of the entire root-head life cycle reveals great complexity and corresponding adaptation through several intricate phases of growth. The simple adult sac of the female externa tells only a tiny part of a fully elaborate tale.

Rhizocephalans,
instead, provide a superb example of Darwin’s genuine principle—the production of appropriate local adaptation by natural selection. Rhizocephalans are phenomenally well suited to their complex conditions of life. But in evolving their unique specializations, rhizocephalans did not become better (or worse) than any close relative. Are root-heads better than barnacles because they live in crabs
rather than on rocks? Are they worse than barnacles because the adult female looks like a bag, rather than a set of gills enclosed in a complex shell? Is a crab better than a barnacle? Do we prefer seahorses over marlins, bats over aardvarks? Such questions are foolish and diversionary.

Natural selection can only adapt each creature to its own local conditions—and such a mechanism therefore cannot
serve as a rationale for our oldest and most pernicious prejudice of progress. Rhizocephalans derail the bias of progress not because they are degenerate, but because they are so well adapted and uniquely specialized to their own intricate series of lifetime environments—and how can we possibly rank all the disparate uniquenesses of the animal kingdom as cosmically better or worse? May this
aid provided to
our
poor benighted intellects—not only
their
undoubted success in commandeering crabs for Darwinian advantage—represent the triumph of the root-heads.

CAN WE TRULY KNOW SLOTH AND RAPACITY?

T
HE CLASSIC GENERALIZED STATEMENT OF A COMMON PROBLEM IN INTELLECTUAL
and practical life may be found in Tennyson’s lament that his dearest (and deceased) friend Arthur Hallam seemed so close in loving memory, yet so unreachable in actuality:

The classic particularized statement of the same problem describes Coleridge’s
Ancient Mariner, adrift in an utterly unusable but completely enveloping bounty:

The common experience of being so close that you can almost touch, yet so utterly distant by any available way of knowing, provides a decidedly mixed blessing—as both a primary frustration of daily life and a major prod to scientific advance. My favorite example
has a largely happy ending still vigorously in progress; consider how much of medicine’s sorry history of so little advance over so many centuries (until very recently) arose primarily from a diagnostic problem involving only an inch or two. The trouble requiring visualization lies just below the opaque covering of our skin. Untold millions (probably billions) of premature, and often painful,
deaths have occurred because no one could see a developing tumor or an internal source of infection. The old surgeons could do little more than cut it off or (on occasion) take it out—where “it” refers to something quite large (a limb, for example), removed
in toto
because a small and local lesion could not be pinpointed. Imagine, then, the triumphant benevolence of a host of inventions from X
rays to CT scans to MRIs. The ability to see an inch or two inside has revolutionized our lives and greatly improved our prospects.