When we hear of vertebrate larvae, we think primarily of amphibian tadpoles. You can read in textbooks for high school students and on educational platforms that fish larvae are also found in nature. However, we are not familiar with such examples from everyday life. Educational materials only mention leptothecal as a “child” of the European eel. Not a word about salmon parr and smolts or symmetrical flatfish fry, although these are species we know from the plate. And the phenomenon of di- and even age polymorphism, i.e. differences in the appearance of individuals of a species at different developmental stages, is not all that uncommon in fish [1]. So let’s take a look at the distinct juvenile forms in their different species!

Fish larvae – a tadpole is not just a frog

The tadpole stage most of us associate primarily with amphibians. However, according to Professor Dzik [2], it arose in the course of evolution much earlier and was common in primitive fish. To this day, it is preserved in several species, the so-called. “living fossils” – three Dipnoi dipnoid fishes (the African great whale Protopterus annectens, the American great whale Lepidosiren paradoxa and the Australian barramundi Neoceratodus forsteri) and the Polypterus polypterus. The tadpole stage probably occurred in three-finned fishes, although it disappeared in the individual development of their only modern representative, the latimeria Latimeria chalumnae. This is because it gives birth to fully formed cubs, which are essentially miniatures of their parents [2].

However, it is not uncommon for fish “fathers” and their “children” to differ so much that the organisms were once classified as separate families or even genera. You can mention such oddities as:

  • casidoron in deep-sea Gibberichthys;
  • querimana in Mugilidae;
  • vexillifer in Carapidae;
  • Acronurus in Acanthuridae fish ;
  • ptax in butterfly fish (gempyli) Gempylidae;
  • exterilium in the slippers Ophididae;
  • Kanazawaichthys in the histrionids Antennariidae;
  • Stylophthalmus in a variety of not at all related fish from the orders Stomiiformes and Myctophiformes [3].

Examples of such age polymorphism can also be found in closer native fish species, such as lampreys (and their blennies) and eels (and their leptocephals).

Eel worms and blanks

From the eggs of Petromyzontid lampreys hatch creatures not very similar to their parents: worm-like, toothless, with hard-to-see, short fins, filtering water like a lancefish and, like it, equipped with an endostyle on the bottom of the throat. Due to their elongated shape, they have been called worms or eels since time immemorial. The tiny eyes, hidden beneath impermeable skin, made them what we usually refer to today as blinkers.

They have been known in Poland for a very long time. He mentions them as early as the 18th century. Gabriel Rzączyński, author of the first natural history monographs of the Polish-Lithuanian lands. For a long time, however, they were considered an undefined “worm,” possibly a juvenile stage of the European eel. No one for centuries thought to connect them with lampreys, also elongated, but endowed with good eyesight, a throat armed with an impressive number of teeth and hunting other fish [4].

The most important occupation of the larvae of most animals remains foraging. In some species, this is the only period of life in which they take food at all. This is no different for many lampreys, including those found in Poland – the Ukrainian Eudontomyzon mariae and the stream Lampetra planeri. The adult forms of both species do not take food, so they pose no threat to other fish. Since the 1970s. of the last century, the view has become widespread that lampreys “non-eaters,” foraging only at the worm stage, arose in the course of evolution through shortened migrations and accelerated aging of adult specimens, and even atrophy of the esophagus. Each river basin is believed to have its own species of stream lamprey, derived from the local river lamprey Lampetra fluviatilis, which has lost the ability to parasitize fish [5].

Smoltification of noble salmon

As a result of the spawning of the Salmo salar salmon, the streams of Europe were filled with lots of small fish, completely unlike salmon spawners. In England and Scotland, the smaller ones were called parries and the larger ones smolts. For a long time they were considered a dangerous fish weed, eating the eggs and fry of more valuable species. They were therefore caught on an industrial scale, fattened for pigs and poultry, or spread over fields as fertilizer. What was the astonishment when the studies of ichthyologists showed that these despised parr and smolts are the offspring of the increasingly valuable Atlantic salmon!

The life cycle of Salmo salar, in a nutshell, is that spawners return from the seas and estuaries up rivers, where they lay eggs in nests. Most of them fall right after breeding, but a few, called kelts, try to breed a second or even third time. The eggs hatch into fry, or alevins, which grow and transform into parr. These, in turn, when they grow up, undergo another metamorphosis, becoming smolts. Smoltification involves not only a change in appearance and environment (from freshwater to marine), but also a comprehensive remodeling of the entire osmoregulation at the level of molecules (primarily Na+/K+ – ATPase in the gills), cells, tissues and systems.

The rate of transformation is determined by both external factors (mainly photoperiod) and internal factors (the process is accelerated by growth hormone, insulin-like growth factor, cortisol and thyroid hormones, while inhibited by prolactin). Some contaminants strongly interfere with this process, for example, molecules of the herbicide atrazine are recognized by salmon cell receptors as their own hormones. The next, and for most individuals the last, developmental leap is sexual maturation, i.e. transformation into spawners [3, 6, 7].

Research has shown some modifications to this cycle. It turned out that some males become sexually mature as early as the parr stage, remaining in freshwater. They will then, as snikers, secretly fertilize the eggs guarded by typical males, pretending to be sexually immature youngsters. Thus, they implement alternative reproductive tactics [1, 6-8] That’s another story, however!

Asymmetrization of flatfish

The larvae of Pleuronectiformes flatfish (sidereal rays) do not differ in structure from typical, non-sided ray-finned fish. They lead a pelagic lifestyle and view the world with their eyes located on the sides of their heads. During adolescence, the left or right side of the body develops faster. While the odd fins do not change their position, the eye undertakes a wildly spectacular wandering. In some flatfish (especially the evolutionarily oldest scramblers Psettodidae) it will stop at the top of the head, in others it will move entirely to the other side.

The side with the eyes will therefore take over the function and coloration of the back, and the side on which the flounder in question lies becomes the underside. It will therefore turn white, and its gill slit will cease to function. Eyes on the left side are prevalent in the turbot-like Bothidae and the blowfish Cynoglossidae, and on the right side in the sole-like Soleidae and the flounder-like Pleuronectidae [2, 3].

Larvae of our eels and their oceanic relatives

The life cycle of the eel includes the stages of egg, leptocephalus, glass eel, elver, yellow eel, silver eel and finally spawner.

Leptocephalus is unique among vertebrates in that it takes food not through the epithelium of the digestive tract, but with the entire surface of the body (through the skin). The presence of leptocephalus-type fish larvae became the reason for distinguishing a separate superorder of fish: the tarpon-like Elopomorpha, combining such dissimilar species as herring-like tarpon and elopses with snake-like moray eels and congers [2, 3].

From the life of ichthyoplankton

Humans have fished the sea for thousands of years, but it wasn’t until the 19th century. discovered their microscopic eggs and larvae, or ichthyoplankton, floating in the pelagial. Fish larvae face the same problems as crustacean zooplankton, being, literally and figuratively, suspended between starvation and devouring. Successfully hiding from a predator easily leads to death by starvation. Plankton, both fish and crustaceans, feeding efficiently in full light, expose themselves to predator pressure.

What’s more, there is a constant arms race between the fish larvae and their food (algae and invertebrates) – analogous to the plant-herbivore. Fish continually synchronize their life cycle with the mass occurrence of phyto- or zooplankton to optimize their foraging, while their prey “strive” to diverge in time and space from their fishy pursuers [3].

The huge loss of eggs and larvae leads one to ask why typical marine fish need such stages in their life cycle at all. Couldn’t they give birth to live young like sharks? Or take care of offspring like needlewomen? As Helfman and co-authors [3] point out, several hypotheses have been formulated to answer this question. One speaks of a strategy to free oneself from parasites (breaking their development cycles by manipulating one’s own life cycle), another to increase the chances of colonizing new territories, and another to avoid competition between parents and offspring in this way.

In summary, fish are as diverse in the larval stage as they are as adults. The variety of shapes, habitats and life strategies of “fish babies” and the evolutionary pathways of their formation seem endless. Instead, the lifespan of certain fish, especially those famous for their spectacular transformations, such as lampreys and salmon, is sometimes finite. In the world of fish, this seems to be an exception, since most species are characterized by unlimited growth and potential immortality.

In the article, I used, among other things. From the works:

[1] Shine R. (1989). Ecological causes for the evolution of sexual dimorphism: a review of the evidence. The Quarterly Review of Biology, 64(4), 419-461.

[2] Boar J. (2018). Zoology. Diversity and affinities of animals. Published. UW, Warsaw.

[3] Helfman G., Collette B., Facey D., Bowen B. (2009). The Diversity of Fishes: Biology, Evolution, and Ecology. Wiley & Blackwell, West Sussex (UK) – New York (US).

[4] Samek A. (1992). Historya naturalna obejmująca zwierząt opisanie z dykcjonarzów i dzieł dawnych polskich naturalistów zebrane albo jako Polakowi ciekawemu wiadomości z histori naturalnej podawano. KAW, Warsaw.

[5] Mateus C.S., Almeida P.R., Quintella B.R. & Alves M.J. (2011). MtDNA markers reveal the existence of allopatric evolutionary lineages in the threatened lampreys Lampetra fluviatilis (L.) and Lampetra planeri (Bloch) in the Iberian glacial refugium. Conservation Genetics, 12, 1061-1074.

[6] Nichols K., Edo A., Wheeler P., Thorgaard G. (2008). The Genetic Basis of Smoltification-Related Traits in Oncorhynchus mykiss. Genetics. 179 (3): 1559-1575.

[7] Prunet P., Boeuf G., Bolton J.P. & Young G. (1989). Smoltification and seawater adaptation in Atlantic salmon (Salmo salar): plasma prolactin, growth hormone, and thyroid hormones. General and Comparative Endocrinology, 74(3), 355-364.

[8] Saunders R.L., Henderson E.B. & Glebe B.D. (1982). Precocious sexual maturation and smoltification in male Atlantic salmon (Salmo salar). Aquaculture, 28(1-2), 211-229.

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