My fair larvae
Without holding too biased an opinion, I regard the transformation of a small bag of muscles and intestines, called a tadpole, into a handsome little frog, a matter for awe and wonder. The frequency with which I watch it happen does nothing to dim the drama of the event.
The larva while it is yet in growth is a soft egg.
WHAT DO BABY BIRDS, SNAKES AND HUMAN BEINGS HAVE IN COMMON? Not much, you might say. Well, maybe so; but they do share one feature — they all look basically similar to their parents. As a general rule, upon either hatching from their egg or being born, young mammals, reptiles and birds look quite similar to what they will look like when mature. Fur or feathers may be lacking, and bodily proportions may deviate from the adult form, but they have all their members and organs, and nobody would have much trouble predicting the ultimate outcome for the youngster.
Nature's rules, as we all know, invariably are broken by her mavericks. Some frogs lay eggs large enough to contain sufficient nutrient for the young to pass through the tadpole stage while egg-bound, hatching as a tiny but fully-formed frog. As a general rule, however, amphibian hatchlings sport a considerably different overcoat from mum and dad. What is more, not only do they look different, but they live quite independently from their folks, pursuing a different lifestyle for which their organs are designed. Carnivorous frogs or salamanders start out lives as scum-eating tadpoles and their ilk.
Few people realize that hatchling fish, too, generally differ quite markedly from their elders. Though the difference is rarely as significant as that between polliwog and toad, it is nevertheless great enough to make scientists refer to the young as "larvae", the same term they use for the young of amphibians. The term simply refers to pre-adult forms which are distinctly different from the sexually mature adult in form, and are capable of fending for themselves.
Neither fish nor amphibian holds a candle to many animals without backbones, known as "invertebrates", for dissimilarity between juvenile and adult. Of land lubbers, the main large group that characteristically goes through a larval phase is the insects. Grubs turn into beetles, maggots into flies, and caterpillars into butterflies. (Not all insect orders experience such a major transformation between babe and teenager. In some cases the newly-hatched have a general similarity to the final form, becoming more and more like the adult through a series of successive molts. The true "bugs" fit into this group, as do grasshoppers, cockroaches, dragonflies and others. In this case, the young are generally termed "nymphs".) Some other groups, such as flatworm flukes, go through five larval stages before attaining maturity.
Dramatic differences between larva and adult occur most commonly among animals that, for two reasons, we never see: they live in the sea, and the larval stages generally are microscopic in size and transparent. A good example of the difference between larva and adult is found among many echinoderms — starfishes, sea cucumbers, brittle stars, and the like. The microphotographs below by Wim van Egmond beautifully illustrate the point.
Professor Higgins may have performed a mighty work in transforming the low-class flower seller into a sophisticated lady, but the transformation involved cannot be compared with nature's many miraculous makeovers. You may well be astonished to find out just how thoroughly the phenomenon of transformation permeates the world of living things.
We will say little more about insect larvae or amphibians in this article. Our attention will be fixed on fish, flukes, and marine invertebrates. Almost all major groups of marine organisms, and about 80% (90,000 or more species) of all intertidal marine invertebrates pass through a larval stage before achieving maturity (Carefoot 1977, p. 57), as do many free-swimming and deep-sea invertebrates, the figures for which are less certain. Starfish go through a larval stage, as do most worms, snails, slugs, sea urchins, sponges, barnacles, arrow worms, comb jellies and more — at least fifteen phyla of marine organisms follow this pattern with its endless variations. Believe it or not, larvae vary so greatly in form that they have spawned a vocabulary of their own. Experts use names such as actinotroch, actinula, bipinnaria, brachiolaria, cercaria, cyphonautes, cypris, Desor's, doliolaria, glochidium, etc, to identify various kinds.
The two paths
God's mind knows no limits, and can think infinite thoughts all at once. His ingenuity, likewise, knows no bounds; the creation demonstrates these noble truths in its seemingly infinite variations on every theme of creation.
Demonstrating yet again the boundless ingenuity of the Creator, some phyla of invertebrates, for example peanut worms, exhibit both forms of development among different members of the group — some reach maturity directly while others take the circuitous, metamorphic path.
So the term "larva" can be misleading in its own right, covering a broad range of biological phenomena. In many instances the larva bears virtually no resemblance to the adult form, while in others you see hints of the glory to come; the larvae of ragworms (family Nereidae of the phylum Annelida), for instance, bear bristles grouped in three pairs, representing three bristle-bearing legs (parapodia) of the worm to come. In some cases a larva undergoes dramatic and rapid transformation to attain maturity, while in others a larva is merely an early phase in the development, or differentiation of an organism; it is called a larva because it looks so different, and functions differently, from the product that finally graduates from finishing school.
Both the gradual transformation and metamorphic paths required staggering design and planning on the Creator's part, not to mention unbelievably precise and intricate engineering. When the metamorphic path is taken, critical timing issues are involved, too. The creature has to survive during the switchover period when the old organs are being radically transformed like boys' transformer toys or are literally dissolving away and new ones being built to take their place. How do tadpoles survive when they are shutting down their gills and turning to breathing with lungs, for instance? The details of how these miracles occur are beyond the scope of this article.
The first path: hobbyist transformers
Time to explain these differences with some examples of both kinds. One of the best places to look for gradual transformationists is in the fish kingdom. Though some fish, such as sharks, are "born" or hatch from the egg as miniature versions of the parents, many others go through a larval stage. Compared with insects and many marine invertebrates, such fish are lightweights in the transformation stakes, inasmuch as their transformation from larva to adult generally follows the path we have defined as developmental rather than involving metamorphosis. But it would be a mistake to view the watery ocean through the metaphor of the womb, and thus to interpret the change from fry to fish as nothing more than a quirky marine version of uterine development. Fish larvae have what it takes to survive independently, and they generally experience changes in organ structure or function during the process.
A good model of the changes that take place in body form in many fish is provided by the northern clingfish, with the charming Latin name Gobiesox maeandricus. Found in the rocky intertidal and subtidal zones along the Pacific coast of North America, this little chap grows from about 6 mm long as a hatchling to about 12 mm as a young adult (Allen 1983, p. 553). Though it does not show any details of internal systemic change, the illustration here clearly shows the degree of external change involved. A cursory examination will show that, even though the hatchling is obviously a fish, details of its final form would be difficult to predict by extrapolation.
Sometimes the larvae of fishes may be so different from the adult, both in appearance and habits, that they are unrecognizable as the young of the adults they are destined to become. In a number of cases these dissimilar larvae were thought to be a completely different species. Such was the case, for example, with an eel larva which until fairly recently was thought to be a fish in its own right, and was called a "Leptocephalus". The discovery that it was the larva of an eel was made by two Italian biologists who had caught a leaf-shaped Leptocephalus in a plankton net. When placed in an aquarium for observation, it slowly transformed into a young glass eel before their eyes.
Another remarkable example of a fish that experiences some strange changes as it matures is the ocean sunfish. The hatchling is so different from the mature fish that it can properly be deemed a larva. Soon after hatching, the minuscule (one-eighth inch) larva sunfish develops a suit of amour which resembles the business end of a medieval mace. In one species five spines grow into long horny spikes. Another transformation leaves the one inch fish deeper than it is long, with its spines shortened and with a single new tail fin. When full grown, an ocean sunfish may weigh as much as a ton, and measure over three meters in length.
Many marine invertebrates also follow this path of gradual development via a larval stage. Polyclad flatworms that have a larval stage, known as a Muller's larva, "gradually become more complex. as they grow more massive" (Russell-Hunter 1979, p. 577). Many others, by contrast, follow the metamorphic path. No pattern can be elucidated to predict which creatures will undergo gradual developmental change and which will experience a classical metamorphosis. Indeed, as we will see shortly, the line between the two methods proves so blurry that the distinction must be considered somewhat arbitrary.
The argument that parasitic flatworms known as flukes demonstrate the infinite inventive genius of God would endear few people to them. The fact is, parasites rarely harm their host! Quite the contrary; they generally benefit the host; if not the individual host, they benefit the species as a whole by weeding out the infirm and diseased. All God's works are good, and those who love Him will find benefit in considering them all. Thousands of different species of flukes exist; about any species of animal you care to name has its own species of custom-designed fluke to parasitize it. The startling thing about flukes is that many of them undergo five distinct larval stages before attaining to maturity! The animal in which a parasite comes to maturity (ability to reproduce) is called the final host. (In technical literature it is called the definitive host.) Other animals that are parasitized by various larval stages are called intermediate hosts for obvious reasons. The first intermediate host is invariably an aquatic snail or clam, the second intermediate host often a fish, but the final host can be just about anything — frog, dog, fish, kingfisher, fox, snake, tortoise — you name it.
The general life cycle goes something like this. Eggs are voided in the feces or urine of the final host into the water where they hatch into a ciliated larva called the miracidium (plural, miracidia). (Cilia are tiny muscle-power whips found on many larvae and other microscopic creatures as well as in your lungs.) Although details of structure vary considerably among species, most miracidia look roughly the same. The vast majority of miracidia perish because they fail to find a specimen of the creature they victimize. A miracidium feels very pleased with itself if it chances upon a suitable intermediate host, generally a mollusk, where it wastes no time losing its cilia and boring into its victim's juicy flesh — no mean feat in its own right.
Though many exceptions to the general rule can be found, flukes usually pass through the second and third larval stages — the sporocyst stage and the redia stage — in the first intermediate host. In reality, the sporocyst stage exists for the sole purpose of producing redia. Rediae form from "germ balls" within the sporocyst, then burst out and start growing themselves. All this is taking place inside the helpless mollusk.
One may legitimately ask what function the redia performs. It exists to produce the next larval stage called the cercaria. And just as rediae form inside the sporocyst, cercariae form inside rediae. Confused? God could have written out all of these phases; numerous flukes don't go through such a rigmarole. But doesn't such a bewildering array of incredible activities going on under our very noses, each one painstakingly worked out in detail, illustrate the endless ingenuity of the One who devised them?
The cercaria larva that has been produced by the redia now has to vacate the mollusk and move on to greener pastures to keep the cycle going. The cercaria swims around until it is able to locate the final host or its next intermediate host, such as a dragonfly nymph or a fish. The cercaria usually heads for the abdomen of the host, and somehow penetrates the skin. Before entering, it casts off its tail. Once inside it transforms into the fifth larval stage called the metacercaria, a kind of infective cyst. In order for the cycle to be completed, this creature must fall prey to the final host. In many cases, the intermediate host is eaten by the wrong thing, and the metacercaria perishes. When eaten by the correct creature, the metacercaria unencysts and miraculously migrates to the organ it plans to live in, such as the liver, where it develops or transforms into the adult fluke. How does it know where to go, and how, exactly, does it get there? These questions have too many answers to deal with here.
And there you have it. What a story!
Just what is a larva? What is metamorphosis?
Flukes illustrate perfectly the difficulty, mentioned earlier, of hammering down a definition of both larva and metamorphosis. Each larval stage in the fluke life cycle bears some similarity to the previous stage, yet it also betrays some differences. You are confronted with the question of whether they are more similar to each other or more different from each other. In addition, in a couple of the larval stages, the new phase doesn't develop by the recruitment to a new use, or the destruction and rebuilding, of organs of its progenitor, but, as we have seen, it begins its existence as a "germ ball" within its predecessor. The earlier larval stage contains clumps of cells which develop into the new stage by differentiation. The final product of all this monkey business is a flatworm, which looks and functions so differently from the miracidium larva that you could probably say that metamorphosis is involved. But when? The final product is reached through a number of stages, each of which is considerably different from the one before.
For such reasons, defining metamorphosis and, by extension, a larva, presents a challenge. It's a bit like defining a beard; how many hairs does it take on a man's face to constitute a beard? How much change and what kind of change must occur, and in what sequence, for a transformation to amount to metamorphosis? Nobody would object to interpreting the caterpillar-to-butterfly transformation as a kind of metamorphosis; but miracidium to fluke via a series of less dramatic changes? Maybe that's quite different. As one author puts it, “… there is no definition of the amount of change that constitutes metamorphosis” (Williamson 1992, p. 9). We will take as our definition of larva that of Russell-Hunter, who says that larvae are “immature stages basically dissimilar to the adults which they will become” (p. 577).
Trying to define metamorphosis is akin to attempting to knit with soot. Ballinsky speaks of it as a reactivation of developmental processes "after development has almost reached a standstill" (1970, p. 609). So he brings postponed development under the same umbrella as a total reorganization of body parts; the development of flukes through a number of larval stages therefore counts as metamorphosis. To illustrate further the difficulty of definition, we note that Barnes calls the change of some larvae to adults as "slightly metamorphic" (p. 606), while Gilbert and Raunio (1997, p. 160) speak of “subtle metamorphosis” — an oxymoron if ever there was one. To speak of partial metamorphosis is like talking about a man who almost has a beard. We don't say these things to mock such experts but to illustrate the almost infinite spectrum of developmental strategies the Intelligent Designer has used. The richness of the Divine Mind makes it impossible to always speak with precision in talking about the natural world. To do so would require the invention of many thousands of new words. Scientists may wish to do that so that they can speak more intelligently with other experts, but then they would find it virtually impossible to make sense to us laymen. What a dilemma.
Another fact underscores the difficulty of hammering down a definition of metamorphosis. Most texts speak only of the metamorphosis of juveniles into adults. Truth is, some fully mature organisms transform to such a degree that nobody would argue against calling the event a metamorphosis. One author says of certain marine worms,
In a number of… cases, whole adult worms metamorphose to sexual swarming forms which… temporarily enter the plankton (Russell-Hunter, p. 579).
At an even more basic level, how is one to interpret the phenomenon of larvae and metamorphosis? What, really, is a larva? Aristotle interpreted the larva as a "soft egg". As late as the latter half of the twentieth century, scientists thought that larvae and metamorphosis are "to be explained by the immaturity of the insect at the time of hatching from the egg" (Wigglesworth 1959, p. 2); somehow some creatures escape the nursery before they are really ready. Presumably the same interpretation applied not just to insects but to all creatures that appeared to metamorphose. Wigglesworth feels such an interpretation is deficient, and suggests a different view:
Instead of regarding the larva, the pupa and the adult as three stages in the progressive unfolding of a complex organism, it is possible to think of them as alternative forms which an already complex organism assumes at different periods in its development (p. 2).
Speaking primarily about the phenomenon in insects, Gould, similarly, sees the two stages of larva and adult as having the purpose of providing an efficient division of labor between "larvae as machines for feeding and imagoes [adults] as devices for reproduction" (1986, p. 16). The quality approach is to recognize the stamp of the Divine Mind in such intricate phenomena, all of which redound to the glory of the supreme intellect of the One who planned the endless variations in detail.
Babes in larval clothing
The fun continues. Not only does the distinction between metamorphosis and straightforward development appear blurred at times, so too does the distinction between embryo and larva. When egg and sperm of any animal come together in the act of fertilization, an embryo springs into being — that difficult-to-define stage between fertilized ovum and larva or adult. Convention decrees that this growing ball is called an embryo until the moment of hatching from its membrane- or shell- bound egg or until being born. While still in its egg, a fish-to-be is an embryo, and doesn't become a larva until it hatches. Well, it isn't that simple when it comes to many marine invertebrates. Many female gametes ("egg" cells) and male sperm are released directly into the water where fertilization occurs; the resulting "embryo" is not contained within any kind of protective membrane. The question arises, when can the embryo be said to turn into a larva? Received wisdom seems to plump for the onset of feeding or of independent locomotion through the development of cilia as announcing a larva's debut.
The sheer speed with which some buoyant embryos attain such a free-swimming stage highlights the difficulty of definition. The microscopic, ciliated, larval stage of some polychaete worms is reached within one day of fertilization (Barnes 1974, p. 277) while still, obviously, in a very early developmental state. Lancelets (Branchiostoma), confuse the issue even further.
The mind of God is bound by no constraining limits, His works time and again defying attempts to be hammered down into watertight classification schemes.
We conclude that the terms larva and metamorphosis amount to an attempt to systematize that which possibly cannot be systematized. We must make such attempts; indeed, we could not communicate intelligently without engaging in such analyses and interpretations of natural phenomena. But let us recognize that our interpretive systems often amount to little more than a fallible human imposition upon the realities of creation, a creation whose endless intricacies and variations on a theme defy simple categorization. We would be foolish to get into arguments about whether or not a particular creature undergoes metamorphosis during its life; to do so would be to indulge in that least profitable polemical pastime — arguing about words. Rather, let us probe the creation for its endless insights into the glory of God's infinite mind.
When we use the terms larva and metamorphosis in this article we are simply expressing the idea of considerable change of form from post-embryonic young to adult.
1 The course of development from the fertilization of the egg to the production of a new generation of reproductive germ cells. In many animal species (such as butterflies) the cycle includes one or more intermediate larval phases in addition to the sexually-reproducing adult phase. Some plants, such as ferns, also have two distinct, independent stages in their life cycles.
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