Metamorphosis and amphibian larvae

The word amphibian comes from ancient Greek words “amphi”, which means “both” and “bios”, which means “life”. Even if the word amphibious is an adjective used to describe animals that can live both on land and water, in the case of amphibians it also refers to both life stages through which these animals go through, as amphibians are born in an aquatic larval stage and become adults via a process of metamorphosis. In this new entry we’ll explain how metamorphosis works at a hormonal level, which anatomical changes occur during this period and the differences of this process among the different lissamphibian orders.

LISSAMPHIBIAN METAMORPHOSIS

Metamorphosis is present in the three lissamphibian orders. This process was already present in the first terrestrial tetrapods, which had to lay their eggs in water. Yet not all extant species present external metamorphosis, as some of them hatch as diminutive adults (as 20% of anuran species). In these species metamorphosis happens equally inside the egg before hatching, what’s called internal metamorphosis.

Red-eyed tree frog eggs (Agalychnis callydryas) just before hatching, by Geoff Gallice.

As a general rule, lissamphibians lay their eggs in water. In most species, aquatic larvae will hatch from gelatinous eggs, even if their morphology varies a lot between different species. Yet larvae of all lissamphibians present a set of common characteristics:

• External gills, thanks to which they can breathe underwater.
• Absence of eyelids and retinal pigments associated with sight outside of water.
• Presence of a lateral line (or equivalent), sensorial organ characteristic of fish which allow them to sense vibrations underwater.
• Thinner skin.
• Subaquatic anatomic adaptations.

Photo of a fire salamander (Salamandra salamandra) in which the external gills and the pisciform looks of the larva can be appreciated, by David López.

During metamorphosis, most structures useful during the larval stage are reabsorbed through apoptosis, a controlled cell death process. In many cases this process is highly conditioned by various environmental factors such as population density, food availability and the presence of certain chemical substances in water.

HORMONAL CHANGES

At the hormonal level, metamorphosis is characterized by the interaction between two kinds of hormones: thyroid hormones and prolactin. While the thyroid hormones as thyroxin (secreted by the thyroid gland) stimulate the metamorphosis process, prolactin (secreted by the pituitary gland or hypophysis) inhibits it. The concentration of these two hormones (regulated by the Hypothalamus→Hyphophysis→Thyroid) is what controls the different stages of metamorphosis.

Scheme by Mikael Häggström of the hypothalamus (green), hypophysis or pituitary (red), thyroid (blue) axis in human beings and the release of thyroid hormones.

PREMETAMORPHOSIS

This is the larval growth stage, and it lasts around the first 20 days of life (depending on the species). This stage is characterized by a low secretion of thyroidal hormones and by a high concentration of prolactin that inhibits the metamorphosis process. This is due to the fact that the hypothalamus→hypophysis system is still not mature.

PROMETAMORPHOSIS

It’s a period of reduced growth with slow morphological changes, due to the rise of thyroxin concentration in blood caused by the growth of the thyroid gland. Also, the hypothalamus→hypophysis axis starts developing, which will trigger even more the rise of the thyroxin concentration and will lower the prolactin, giving way to great morphological changes.

METAMORPHOSIS CLIMAX

It’s the point in which the hyothalamus→hypophysis→thyroid axis is at its maximum capacity and it is when great morphological changes happen in the larva, which will end up becoming a miniature adult. Finally, thyroxin levels will start to be restored by a negative feedback system of the thyroxin over the hypothalamus and the hypophysis.

Scheme from Brown & Cai 2007, about the general levels of thyroid hormones during the different metamorphosis’ stages.

MORPHOLOGICAL CHANGES

During the metamorphosis process, larvae will go through a set of anatomical changes that will allow them to acquire their adult form. Some changes common to most species are the acquisition of eyelids and new retinal pigments, the reabsorption of the gills and the loss of the lateral line. Other morphological changes vary among the different orders. For example in caecilians (order Apoda) larvae already look like miniature adults but with external gills. Also, most caecilians present internal metamorphosis and the hatchlings have no trace of gills.

Photo from Blog do Nurof-UFC of a caecilian egg, inside which we can see the larva with gills.

In urodeles (order Urodela), the external metamorphic changes aren’t that spectacular either. Larvae are pretty similar to adults, as their limbs develop quickly, although they present external filamentous gills, have no eyelids and present a largely-developed caudal fin. Even their carnivorous diet is similar to that of the adult’s. Yet the great diversity of salamanders and newts gives as a result a great variety of life cycles; from viviparous species that give live birth, to neotenic species that keep larval characteristics through their adult stage.

Photo by David Alvarez of the viviparous birth of a fire salamander (Salamandra salamandra), and photo by Faldrian of an axolotl (Ambystoma mexicanum) a neotenic species.

Frogs and toads (order Anura) are the group in which metamorphic changes are more dramatic. The anuran larva is so different that it’s called a tadpole, which differentiates from the adult both by its looks and its physiology and behaviour. Even if tadpoles are born with external gills, these are soon covered by skin folds that form a gill chamber. Also, tadpoles have a round, limbless body and a long, vertically-flattened tail, which allows them to swim swiftly in water.

Photo by J. J. Harrison of a southern brown tree frog tadpole (Litoria ewingii).

One of the main differences between adult and larval anurans is their diet. While adult frogs and toads are predators, tadpoles are herbivorous larvae, feeding by filtering suspended vegetal particles or by scraping off algae from rocks using a series of keratinous “teeth” present in some species. This is reflected in their spirally-shaped and extremely long digestive system in order to allow them to digest large quantities of vegetal matter. Tadpoles are tireless eating machines, with some filter-feeding species being able to filter eight times their body volume of water per minute.

Photo by Denise Stanley of a tadpole, in which we can see both the keratinous “teeth”, and the spiral-shaped intestine.

After metamorphosis, tadpoles will reabsorb their gills and tail, their digestive system will shorten, and will develop limbs and lungs, becoming small amphibians prepared for a life on land.

Recently metamorphosed spiny toad (Bufo spinosus) by David López.

As we have seen, the metamorphosis process varies greatly among the different species of each order. This process results in the fact that that most lissamphibians spend a part of their lives in water and the other on land, a representative fact of the transition of the first tetrapods from the aquatic to the terrestrial medium. Also, the great diversity of ecological niches occupied by both the adults and the larvae of the different species and the wide array of environmental factors that affect the metamorphosis process, make lissamphibians great bioindicators of an ecosystem’s health.

REFERENCES

The following sources have been consulted during the elaboration of this entry:

• Halliday & Adler (2007). La gran enciclopedia de los Anfibios y Reptiles. Editorial Libsa.
• Masó & Pijoan (2011). Anfibios y Reptiles de la Península Ibérica, Baleares y Canarias. Ediciones Omega.
• Brown & Cai (2007). Amphibian metamorphosis. Developmental Biology. Vol. 306. Pp: 20-33.
• Your Article Library. Process of Metamorphosis in Amphibians and it’s Hormonal Control.
• Metamorfosis de anfibios.
• Cover image by Brian Gratwicke.

Communication among plants: allelopathy

As always have been said, plants are unable to speak. But, even if they don’t speak, this does not mean they do not communicate with each other. Relatively few years ago, during the period from 1930 to 1940, it was discovered that plants also transmit certain stimuli to others. But, what kind of communication exist among them? What are their words and how are pronounced? And what involves this interaction?

INTRODUCTION

In 1937, Molisch introduced the term allelopathy referring to the two Latin words “Allelon” and “Pathos”, which mean “another” and “suffering”, respectively. But, the actual meaning of the word was determined by Rice in 1984. Allelopathy now means any effect that a plant transmits to another directly or indirectly through production of different metabolism compounds, causing either a positive or negative effect on the other organism. These compounds are called allelochemicals.

The allelochemicals are released on the environment by plants. But, they are not directly aimed to the action site, thus it is a passive mechanism. To be effective, allelopathic interaction needs that these substances are distributed along the ground or the air and that they reach the other plant. Once inside the recipient plant, this one may have defense and degradation mechanisms of the compounds while avoiding the effect, or conversely, it will suffer a pathological effect.

Allelopathy (Adapted image of OpenClips)

ROUTES OF RELEASE

The release of allelochemicals can be 4 main ways:

• Leaching: the aerial part of the plant lets go substances by rain effect. Then, they can fall on other plants or on the ground. Therefore, it can be direct or indirect effect, depending on whether they falls on another plant or not. Although, in principle, it is considered indirect.
• Decomposition: the plants drop their leftovers on the ground, where they decomposed under the microorganisms action, which help the release of the compounds. The plant leftovers range from leaves to branches or roots. The substances found there may be inactive until coming into contact with moisture or microorganisms, or can be active and then be inactivated by the microorganisms activity or by being retained on the ground. So, it is an indirect way. The decomposition is very important because the most of allelochemicals are released this way.
• Volatilization: the substances are released by the stomata (structures that allow the exchange of gas and transpiration). These are volatile and water-soluble, thus can be absorbed by other plant’s stomata or be dissolved in water. Commonly, plants using these pathways occur in temperate and warm climates. It is considered a direct route.
• Exudation: the plants can also release allelochemicals directly by live roots. The exudation system depends especially of roots state, of the kind of roots and of their growing level (if they are growing or not).

The 4 main pathways of allelochemical releasing: volatilization (V), leaching (L), descomposition (D) and root exudation (E). (Adapted image of OpenClips)

REGULATORY FACTORS

Factors influencing the release of allelochemicals are normally abiotic, such as high radiation, low humidity, unsuitable pH, ultraviolet light, temperature, nutrient deficiency, pollution or contamination (including pesticides ). The higher is the stress caused by this factors to the plant, highest is the allelochemicals amount released from secondary metabolic routes.

• This is important for research and pharmacy: for generating relevant oils many plants are grown under stressful conditions, as it is thanks to the production of these secondary metabolites that they can survive.

Furthermore, biotic factors also take part, such as insects, herbivores or competition with other plant species. These activate the plant defenses and then the organism is stimulated to secrete bitter substances, or substances that harden the tissues, that are toxic or give off unpleasant odors, etc.

Finally, each plant has its own genome and this makes synthesize those or other substances. But, they are also determined by the phenology (life stages) and the development (if the size of the plant is bigger, it can release more allelochemicals).

ACTION MODE

The allelochemicals are very diverse and, therefore, it’s difficult to establish a general action model; since it depends on the compound type, the receiving plants and how it acts.

When we talk about how the allelochemicals can act at internal level, there is a large number of physiological parameters that can be affected. They have action on the cellular membrane, disrupt the activity of different enzymes or structural proteins or alter hormonal balance. They can also inhibit or reduce cellular respiration and chlorophyll synthesis, leading to a reduction in vitality, growth and overall development of the plant. Furthermore, these substances can also reduce seed germination or seedling development, or affect cell division, pollen germination, etc.

On the other hand, at external level, the allelochemicals may be related to the release or limitation of nutrients that are found in the soil. Others act on microorganisms, leading to a perturbation on the symbiotic relationships they establish. In addition, these substances have great importance into the generations succession, as they determine certain competition tendencies and also act on the habitat ecology. Even so, it is a successive competition, as they do not directly compete to obtain the main resources.

EXAMPLES

One of the best known allelochemicals is the juglone, produced by the Eastern black walnut (Juglans nigra). Juglone, once released to soil, can inhibit the other plants growth around the tree. This allows the issuing organism to get more resources, avoiding competition.

Eastern black walnut  (Juglans nigra) (Photo taken by Hans Braxmeier)

A very curious case is that of the acacias (Acacia). These plants synthesize a toxic alkaloid that migrates to the leaves when the body is attacked by a herbivore. This substance’s toxicity is high, because it damages with the contact and ingestion, becoming deadly even for large herbivores.In addition, this alkaloid is volatile and transferred by air to other nearby acacias, acting as an alarm. When the other acacias receive this signal, this component is segregated to leaves, making them darker. Even so, the effect is temporary. This makes animals like giraffes have to constantly move to eat a few leaves of each acacia, and always against the wind, to avoid toxicity.

Acacias (Acacia) (Photo taken by Sarangib)

REFERENCES

• A. Aguilella & F. Puche. 2004. Diccionari de botànica. Col·leció Educació. Material. Universitat de València: pp. 500.

• A. Macías, D. Marín, A. Oliveros-Bastidas, R.M. Varela, A.M. Simonet, C. Carrera & J.M.G. Molinillo. 2003. Alelopathy as a new strategy for sustainable ecosystems development. Biological Sciences in Space 17 (1).
• J. Ferguson, B. Rathinasabapathi & C. A. Chase. 2013. Allelopathy: How plants suppresss other plants. University of Florida, IFAS Extension HS944
• Notes of Phanerogamae, Applied Plant Physiology and Analisi of vegetation, Degree of Environmental Biology, UAB