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Larvae

lpetrich

lpetrich

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Most multicelled organisms have a single-cell phase, whether a spore or fused gametes, and they grow from there to where they can produce spores or gametes. The immature forms usually have some resemblance to the adult, reproducing forms, even if an imperfect resemblance.

But some immature forms have a lot of difference, and those forms are called larvae (Latin: ghost, demon, mask) because some early-modern biologists thought that they were not the true form of the organism.

Larval forms have evolved several times, and they may make possible very different lifestyles over an organism's life.  Larva has a sizable list of larval forms, and I'll go over some of them.

Tadpoles are larval frogs, and they are a legacy of tetrapods' aquatic ancestors. "Amphibian" (Greek "both life") is from living in the water, then living on land.

Some species of frogs hatch as miniature adults, however, going through a tadpole-like phase before they hatch. Amniotes also go through a tadpole-like phase as embryos.

Some fish have distinct larval forms:
  • Lamprey - ammocoete (wormlike, burrows in mud)
  • Eel - leptocephalus ("glass eel")

Tunicates have larval forms that look much like tadpoles and primitive chordates like the amphioxus and the ammocoete (lamprey larva). Their adult forms are a reshaping of their larval forms.
 
Insect wormlike larvae

Some insects have a larval phase with wormlike larvae, variously called grubs and maggots and caterpillars. The next phase is the pupa (Latin, "doll"), a phase where the insect rests and rebuilds itself for adulthood. Thus a total of four stages: egg, larva, pupa, and adult (or imago).

Some of these larvae have legs that become the adult legs, some don't.

Caterpillars have not only adult-leg predecessors, but also extra legs called prolegs, legs that they lose in their adult phase as butterflies and moths. The adult legs are thin while the prolegs are fat and stubby.

The origin of worm larvae is still not very well-understood, though a common hypothesis is that they represent a late embryonic phase that persists outside the egg.

In insects with more direct development, where the immature ones look much like adult ones, the immature ones are called nymphs. Thus having three-stage development: egg, nymph, adult.

I must note that whether four-stage or three-stage, most winged insects have wings only in their adult phases, after their final molts.

Four-stage development is sometimes called "complete metamorphosis" and three-stage development "incomplete metamorphosis". Four-stage development developed only once, as far as we can tell, and four-stage insects are grouped within a taxon called Endopterygota ("inside wings", from the wing buds being inside for the larval phases) or Holometabola.

Crustacean larvae

 Crustacean larva - most marine crustaceans and some freshwater ones hatch as a nauplius, a form with only three segments and pairs of swimming legs on each. But as they grow, they add segments at their rear ends, segments that also have pairs of limbs on them, except maybe some of the later segments. The first two pairs of limbs become antennae, the next three pairs mouthparts, and those after that become walking or swimming legs. In some crustaceans, it's a zoea before it reaches its adult form.

Some freshwater crustaceans go through a nauplius-like phase in the egg, hatching with more segments and limbs.
 
Other marine-invertebrate larvae

Cnidarians have a planula. Some mollusks and annelids have a trochophore. In some mollusks, the trochophore grows into a veliger. Bryozoans have a cyphonautes. Some hemichordates have a tornaria, and most echinoderms have a pluteus. Starfish have a bipinnaria that grows into a brachiolaria. All of them are planktonic, with the adults mostly being benthic, living on the ocean floor.

These larval forms are not universal in these groups, because some members go through similar forms in the egg.

Some of these larvae grow to adults from the whole larva. In Cnidaria, a planula settles down and becomes a polyp, either becoming an adult polyp or else making a medusa phase - jellyfish. In Annelida, the trochophore extends its digestive tract rearwards by making rearward segments, giving the adult worm with the trochophore as its head. This in itself is not very remarkable, because it's like how tadpoles become frogs, insect worm larvae become adult insects, etc. Also, addition of rear-end segments is how a nauplius grows into an adult crustacean.

But sea urchins grow to adulthood in a remarkable way. The adult grows out of a small "adult rudiment" or "imaginal rudiment" in the larva.

The origin of these larval phases is a big conundrum in evolutionary biology, connected to the early evolution of animals (Metazoa). Is their origin larva-first or adult-first? As an illustration, here are some larval forms whose origins are clearer. For tunicates, comparison for other chordates suggests larva-first, with the adult phase being added on to the ancestral state, which then became the larva. For four-stage insects, comparison to other insects suggests adult-first, with the larval phase being a heavily-modified immature phase, likely a persistent late-embryonic phase.

If larva-first, then the ancestor of most animals was some little blob, much like a planula or a placozoan, while if adult-first, then the larval forms were adaptations for dispersal.
 
Let's see where the larval stages are, phylogenetically.

Trautwein_et_al_-_annurev-ento-120710-100538-with-cover-page-v2.pdf - insect phylogeny

Insects branch off within crustaceans, making them land shrimp.
  • Wingless insects: springtails, silverfish, ...
  • Winged insects (Pterygota)
    • Straight-wing insects (Palaeoptera: "old wings"): dragonflies
    • Folded-wing insects (Neoptera: "new wings")
      • Three-stage ones: Orthoptera (grasshoppers, crickets), stick insects, mantises, cockroaches & termites, lice, hemipteran bugs, ... (overall relationships unclear, and are likely not monophyletic relative to four-stage ones)
      • Four-stage ones (Endopterygota: "inside wings", Holometabola) - worm larvae

The four-stage ones have phylogeny
  • Hymenoptera: wasps, bees, ants (bees are hairy wasps, ants are ground wasps)
  • -
    • -
      • Coleoptera: beetles (front wings modified into wing covers)
      • Neuroptera: lacewings
    • -
      • -
        • Trichoptera: caddisflies
        • Lepidoptera: moths, butterflies
      • -
        • Diptera: flies, mosquitoes
        • Siphonaptera: fleas

    The Phylogeny and Evolutionary History of Arthropods - ScienceDirect
 
Arthropods as a whole are
  • Chelicerata
    • Pycnogonida: sea spiders
    • Arachnida (spiders, scorpions, ticks & mites, ...), Xiphosura: horseshoe crabs
  • Mandibulata
    • Myriapoda: centipedes, millipedes
    • Pancrustacea: crustaceans, insects

Their closest relatives (Panarthropoda) are:
  • Onychophora (velvet worms: Peripatus, ...)
  • Tardigrada ("water bears")
  • Euarthropoda
    • Radiodonta: Anomalocaris, ...
    • Extant arthropods, extinct ones like trilobites

The Compact Body Plan of Tardigrades Evolved by the Loss of a Large Body Region - PubMed
With some exceptions, segmented animals (arthropods, annelids, vertebrates) grow their segments at their rear ends. The main exceptions are some insects, which lay down some or all of their segments all at once (medium or long germ-band ones vs. short germ-band ones). Among the long g-b insects are flies (Diptera), including a favorite model system, the fruit fly Drosophila melanogaster. Which indicates a risk of using model systems: atypical features.

Tardigrades have five segments: one head segment and four limbed segments. Arthropod heads typically have 6 or 7 segments, so a tardigrade is homologous to an arthropod head. Sort of like some nauplius that stopped growing and became sexually mature in that state.

Going further, their closest relatives (Ecdysozoa) include nematodes (roundworms) and some very obscure seafloor worms. They are all direct developing -- none of them have very distinct larval stages.
 
Another big bilaterian group is Lophotrochozoa. This is a motley group containing a variety of marine invertebrates, though some molluscs and annelids live in freshwater and on land (land snails, earthworms). I've looked at their phylogeny, and there isn't much consensus on that.
They have a variety of kinds of larvae, and many molluscs, annelids, and some others share a trochophore larva.


The remaining big bilaterian group is Deuterostomia. Overall phylogeny:
  • Ambulacraria
    • Echinodermata
    • Hemichordata - acorn worms, ...
  • Chordata
    • Cephalochordata: amphioxus
    • -
      • Urochordata: tunicates
      • Vertebrata

Some hemichordates have a larva called a tornaria. Echinoderm larvae:
  • Crinoidea: sea lilies - doliolaria
  • Eleutherozoa (free animals)
    • Asteroidea: starfish or sea stars -- bipinnaria then brachiolaria
    • Ophiuroidea: brittle stars -- ophiopluteus
    • -
      • Holothuroidea: sea cucumbers -- auricularia
      • Echinoidea: sea urchins, sand dollars -- echinopluteus
So the ancestral ambulacrarian likely had a well-defined larval form.

Some of the echinoderm larvae look like jugs with spikes coming out of one end at an angle. More generally, echinoderm larvae are bilaterally symmetric, unlike the adults, which are radially symmetric.
 
Going further, we consider the overall phylogeny of bilaterian animals:
  • Protostomia
    • Ecdysozoa -- (semi)direct
    • Lophotrochozoa -- (semi)direct, indirect
  • Deuterostomia -- (Chordata) direct, (Ambulacraria) indirect

Even further, all the animals:
  • Porifera: sea sponges -- parenchymella, amphiblastula
  • Ctenophora: comb jellies
  • Placozoa (small blobs)
  • Planulozoa
    • Cnidaria: sea anemones, jellyfish, ... -- planula
    • Bilateria
I'm leaving the root-level branching unresolved, because there is a long-running controversy over whether the first brancher is sponges or ctenophores. Placozoa may be closest to Cnidaria or else to both Cnidaria and Bilateria (Planulozoa).

 
Yet,
Larval body patterning and apical organs are conserved in animal evolution - PMC
Planktonic ciliated larvae are characteristic for the life cycle of marine invertebrates. Their most prominent feature is the apical organ harboring sensory cells and neurons of largely undetermined function.
Click to expand...
Then describing work on the marine annelid Platynereis dumerilii, a common model system for animal development. It has a trochophore larva, shaped like two cones joined at the base, with two strips of cilia and a mouth around the equator, an apical tuft at one pole, and an anus at the other pole. It grows into the adult animal by adding segments at the anus end, lengthening the gut as it extends outward.
We propose that a simple apical organ - comprising an apical tuft and a basal plexus innervated by sensory-neurosecretory apical plate cells - was present in the last common ancestors of cnidarians and bilaterians. One of its ancient functions would have been the control of metamorphosis. Various types of apical plate cells would then have subsequently been added to the apical organ in the divergent bilaterian lineages. Our findings support an ancient and common origin of primary ciliated larvae.
Click to expand...
But it's hard to say whether this supports adult-first or larva-first development of early animals.

Nervous systems may have originated twice.
Convergent evolution of neural systems in ctenophores | Journal of Experimental Biology | The Company of Biologists
Ctenophores, or comb jellies, represent an example of extensive parallel evolution in neural systems. First, recent genome analyses place ctenophores as a sister group to other animals. Second, ctenophores have a smaller complement of pan-animal genes controlling canonical neurogenic, synaptic, muscle and immune systems, and developmental pathways than most other metazoans. However, comb jellies are carnivorous marine animals with a complex neuromuscular organization and sophisticated patterns of behavior. To sustain these functions, they have evolved a number of unique molecular innovations supporting the hypothesis of massive homoplasies in the organization of integrative and locomotory systems. Third, many bilaterian/cnidarian neuron-specific genes and ‘classical’ neurotransmitter pathways are either absent or, if present, not expressed in ctenophore neurons (e.g. the bilaterian/cnidarian neurotransmitter, γ-amino butyric acid or GABA, is localized in muscles and presumed bilaterian neuron-specific RNA-binding protein Elav is found in non-neuronal cells). Finally, metabolomic and pharmacological data failed to detect either the presence or any physiological action of serotonin, dopamine, noradrenaline, adrenaline, octopamine, acetylcholine or histamine – consistent with the hypothesis that ctenophore neural systems evolved independently from those in other animals. Glutamate and a diverse range of secretory peptides are first candidates for ctenophore neurotransmitters.
Click to expand...
This adds to the mystery of early animal evolution, since the common ancestor of cnidarians-bilaterians and ctenophores would not have had much of a nervous system, if any.

ctenophore | marine invertebrate | Britannica
In Pleurobrachia and in other Cydippida, the larva closely resembles the adult, so that there is little change with maturation. Most ctenophores, however, have a so-called cydippid larva, which is ovoid or spherical with two retractable tentacles. The metamorphosis of the globular cydippid larva into an adult is direct in ovoid-shaped adults and rather more prolonged in the members of flattened groups. Only the parasitic Gastrodes has a free-swimming planula larva comparable to that of the cnidarians.
Click to expand...
Which suggests a small-blob ancestor for ctenophores, much like the larva-first scenario for cnidarians and bilaterians.
 
 Marine larval ecology
  • Direct developing - look much like an adult
  • Lecithotrophic - live off of their egg yolk, making this phase an extended embryonic phase
  • Planktotrophic - eat plankton and grow
Direct development is common among small animals.
 
Ciliated larvae -- larvae with cilia for locomotion (moving themselves) (bolded if yes):
  • Porifera
  • Ctenophora - adults also ciliated (the "combs")
  • ParaHoxozoa
    • Placozoa - no distinct larval form
    • Cnidaria - has placozoan-like planula larvae
    • Bilateria:
      • Deuterostomia:
        • Chordata
        • Ambulacraria - in Hemichordata, Echinodermata
      • Protostomia:
        • Ecdysozoa
        • Lophotrochozoa - in Mollusca + Annelida (trochophore), Brachiopoda (three types), Bryozoa (cyphonautes), Phoronida (actinotroch), Nemertea (pilidium), Platyhelminthes (Müller's), ...
Early animal evolution continues to be unresolved, and I find papers like

Evolutionary transcriptomics of metazoan biphasic life cycle supports a single intercalation origin of metazoan larvae | Nature Ecology & Evolution
Our findings support an adult-first evolutionary scenario with a single metazoan larval intercalation, and suggest that the first appearance of proto-larva probably occurred after the divergence of direct-developing Ctenophora from a metazoan ancestor.
Click to expand...

This scenario is hard to distinguish from larva-first evolution with the adult emerging only once in the ancestor of the Cnidaria and Bilateria.
 
In one paper, I found reference to "head larvae", so I investigated further. That's a larva becoming the head of an animal as it grows.

Multiple origins of feeding head larvae by the Early Cambrian - "In many animals the head develops early, most of the body axis later." - preprint(?) - cjz-2019-0284.pdf

The crustacean nauplius is a head larva, because it becomes the forward part of the animal's head as it grows. Life cycle of the Western King Prawn (Source: Kailola et al. 1993). | Download Scientific Diagram

The annelid trochophore is also a head larva, because it also becomes a head. It first becomes a nectochaete, with three segments with limbs rearward of the mouth, before growing further by adding segments at its rear end. For marine annelid Platynereis dumerilii, often used as a model system for development, LAB MODEL | Stem cells, development and evolution and The Platynereis life cycle The fertilized eggs develop via spiral... | Download Scientific Diagram

As mentioned earlier, tardigrades grow only a few segments - segments homologous to arthropods' and onychophorans' head segments. So it's like a head larva but it matures in that state.


The amphioxus (Cephalochordata) also has a ciliated larva, a neurula, named after a similar vertebrate embryo phase: [PDF] Study of the evolutionary role of nitric oxide (NO) in the cephalochordate amphioxus, Branchiostoma lanceolatum | Semantic Scholar - Figure 1.11

But this larval phase doesn't seem much like a head larva.

Can't find out whether tunicate larvae are ciliated, and their larvae don't seem much like head larvae.
 
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