Arxiu d'etiquetes: sea urchins

Evolution for beginners 2: coevolution

After the success of Evolution for beginners, today we’ll continue  knowing the basics of biological evolution. Why  exist insects that seem orchids and vice versa? Why gazelles and cheetahs are almost equally fast? Why your dog understands you? In other words, what is coevolution?


We know that it is inevitable that living beings establish symbiotic relationships between them. Some depend on others to survive, and at the same time, on elements of their environtment as water, light or air. These mutual pressures between species make that evolve together, and as one evolve as a species, in turn it forces the other to evolve. Let’s see some examples:


The most known process of coevolution is pollination. It was actually the first co-evolutionary study (1859) by Darwin, although he didn’t use that term. The first to use the word coevolution were Ehrlich and Raven (1964).

Insects existed long before the appearance of flowering plants, but their success was due to the discovery that nectar is a good reserve of energy. In turn, the plants found in the insects another way more effectively to carry pollen to another flower. Pollination by the wind (anemophily) requires more production of pollen and a good dose of luck to at least fertilize some flowers of the same species. Many plants have developed flowers that trap insects until they are covered with pollen and then set them free. These insects have hairs in their body to enable this process. In turn some animals have developed long appendages (beaks of hummingbirds, butterflies’ proboscis…) to access the nectar.

Polilla de Darwin (Xantophan morganii praedicta). Foto de Minden Pictures/Superstock
Darwin’s moth (Xantophan morganii praedicta). Photo by Minden Pictures/Superstock

It is the famous case of the Darwin’s moth (Xanthopan morganii praedicta) of which we have already talked about. Charles Darwin, studying orchid Christmas (Angraecum sesquipedale) saw that the nectar was 29 cm inside the flower. He sensed that there should exist an animal with a proboscis of this size. Eleven years later, Alfred Russell Wallace reported him that the Morgan’s sphinxs had proboscis over 20 cm long, and a time later they were found in the same area where Darwin had studied that species of orchid (Madagascar). In honor of both it was added “praedicta” to the scientific name.

There are also bee orchids that mimic female insects to ensure their pollination. To learn more about these orchids and the Christmas one, do not miss this post by Adriel.

Anoura fistulata, murcielago, bat
The bat Anoura fistulata and its long tongue. Photo by Nathan Muchhala

But many plants not only depend on insects, also some birds (like humming birds) and mammals (such as bats) are essential to pollination. The record for the longest mammal tongue in the world is for a bat from Ecuador (Anoura fistulata); its tongue measures 8 cm (150% of the length of its body). It is the only who pollinates one plant called Centropogon nigricans, despite the existence of other species of bats in the same habitat of the plant. This raises the question of whether evolution is well defined, and occurs between pairs of species or it is diffuse due to the interaction of multiple species.


The cheetah (Acinonyx jubatus) is the fastest vertebrate on land (up to 115 km/h). Thomson’s gazelle (Eudorcas thomsonii), the second (up to 80 km/h). Cheetahs have to be fast enough to catch a gazelle (but not all, at risk of disappearing themselves) and gazelles fast enough to escape almost once and reproduce. The fastest gaelles survive, so nature selects in turn faster cheetahs, which are who eat to survive. The pressure from predators is an important factor that determines the survival of a population and what strategies should follow the population to survive. Also, the predators will find solutions to possible new ways of life of their prey to succeed.

Guepardo persiguiendo una gacela. Foto de Federico Veronesi
Cheetah hunting a Thomson’s gazelle in Kenya. Photo by Federico Veronesi

The same applies to other predator-prey relationships, parasite-host relationships, plants-herbivores, improving their speed or other survival strategies like poison, spikes…


Our relationship with dogs since prehistoric times, it is also a case of coevolution. This allows, for example, to create bonds with just looking at them. If you want more information, we invite you to read this post where we talk about the issue of the evolution of dogs and humans in depth.

Another example is the relationship we have established with the bacteria in our digestive system, essential for our survival. Or with pathogens: they have co-evolved with our antibiotics, so using them indiscriminately has favored these species of bacteria to develop resistance to antibiotics.


Coevolution is one of the main processes responsible for the great biodiversity of the Earth. According to Thompson, is responsible for the millions of species that exist instead of thousands.

The interactions that have been developed with coevolution are important for the conservation of species. In cases where evolution has been very close between two species, if one become extint will lead to the extinction of the other almost certainly. Humans constantly alter ecosystems and therefore biodiversity and evolution of species. Just declining one species, we are affecting many more. This is the case of the sea otter (Enhydra lutris), which feeds on sea urchins.

Nutria marina (Enhydra lutris) comiendo erizos. Foto de Vancouver Aquarium
Sea otter (Enhydra lutris) eating sea urchins. Photo by Vancouver Aquarium

Being hunted for their fur, urchins increased number, devastated entire populations of algae (consumer of CO2, one of the responsible of global warming), seals who found refuge in the algae nonexistent now were more hunted by killer whales… the sea otter is therefore a key species for the balance of this ecosystem and the planet, as it has evolved along with urchins and algae.

Coevolutive relations between flowers and animals depend on the pollination of thousands of species, including many of agricultural interest, so we must not lose sight of the seriousness of the issue of the disappearance of a large number of bees and other insects in recent years. A complex case of coevolution that directly affects us is the reproduction of fig.


As we have seen, coevolution is the evolutionary change through natural selection between two or more species that interact reciprocally.

It is needed:

  • Specificity: the evolution of each feature of a species is due  to selective pressures of the feature of the other species.
  • Reciprocity: features evolve together.
  • Simultaneity: features evolve simultaneously.




Summer is the perfect season of the year to go and enjoy the sea and you probably are one of these people who practise snorkel or who dive. In this case, I want to explain which are the main features of starfishes, with which animals can be confused and some examples of the Mediterranean sea. 


Starfishes (Asteroidea) are included in the phylum of Echinoderms, together with sea urchins (Echinoidea), sea cucumbers (Holothuroidea), crinoids (Crinoidea) and brittle stars or ophiuroids (Ophiuroidea).

Equinoderms: (A) Asteroïdeus, (B) Ofiuroïdeus, (C) Holoturoïdeus, (D) Crinoïdeus i (E) Equinoïdeus. (Fotos de Fresno.pntic)
Echinoderms: (A) Asteroidea, (B) Ophiuroidea, (C) Holothuroidea, (D) Crinoidea y (E) Echinoidea. (Pictures: Fresno.pntic)

Echinoderms are all marine animals, which present the following main features:

  • Body with pentamerous radial symmetry in the adult phase, but with bilateral symmetry in the larval phase (there is just one symmetry axis).
  • They are all mobile, except some sessile species (affixed on the sea-floor) of crinoids.
  • Endoskeleton consisting of ossicles.
  • Water vascular system: system projected from the body wall with some expansions similar to tentacles called podium, which can be spread out due to de liquid pressure. Normally, they present an opening called madreporite.

It is important to remember that they are marine animals and, if you want to observe them, you must not extract them from the water because they begin to die in just 10 seconds. 


There is about 1,500 species of starfishes, which are all included in the Asteroidea class. Starfishes live in sandy, muddy, rocky and coral reefs seafloors, depending on the species. They can measure from some centimetres to one metre.

Externally, starfishes have a central disc from which the arms are originated. From the mouth, which is placed in the lower part (or oral part), and throughout the arms there is the ambulacral ridge, from which the podiums are originated. The upper part (or aboral part) is usually coarse and with spines. In the base of this spines there is structures called pedicellarie, which function is to remove the particles that remove debris from the body surface and in some cases are used to capture small fishes. Gas exchange takes place through papulae, which are thin-walled bulges on the aboral surface of the disc and arms. Anus and madreporite are placed in the aboral surface.

External anatomy of a starfish (Picture: JKL Bahweting Middle School).
External anatomy of a starfish (Picture: JKL Bahweting Middle School).

An important feature of echinoderms is the water vascular system. In the case of starfishes, it plays an important role in locomotion, in food capture, in excretion and in breathing.

Many starfishes are carnivorous and feed on molluscs, crustaceans, worms, echinoderms and other invertebrates, sometime on small fishes too. Some starfishes can feed on small particles of plankton or other organic particles.

WATCH OUT! Starfishes can be confused with ophiuroids, but ophiuroids have thinner and more mobile arms than starfishes, in addition to the absence of anus and the fact that they do not use podium to get around, so they move the arms. 



This group includes 6 Mediterranean species, which live in sandy and muddy seafloors and with 5 arms covered by scales and spikes. The most common is the red comb-star (Astropecten aranciacus), which present two lines of big and sharp spines, and with a red – orange colouration.

Estrella de sorra (Astropecten aranciacus) (Foto: Natura Mediterráneo).
Red comb-star (Astropecten aranciacus) (Picture: NaturaMediterráneo).

BLUE SPINY STARFISH (Coscinasterias tenuispina)

This starfish measures between 7 and 10 cm and usually present between 7 and 9 arms of different sizes, which are covered by small spines. It is bluish white and brown. The most common reproduction system of this species is fission, and for this reason a large section of the coast can be occupied by the same genetic individual. The blue spiny starfish usually lives under the stones.

Estrella blavosa (Coscinasterias tenuispina) (Foto: Animales Marinos).
Blue spiny starfish (Coscinasterias tenuispina) (Picture: Animales Marinos).

SPINY-STARFISH (Marthasterias glacialis)

The spiny-starfish, which can measure 80 cm of diameter, always presents 5 arms, which are covered by hard spikes. Its colouration is greenish or brownish, with light spots when they live deeper. It can live in rocky or sandy seafloors, until 180 metres deep.

Marthasterias (Foto: Tato Grasso, Creative Commons).
Spiny-starfish (Marthasterias glacialis) (Picture: Tato Grasso, Creative Commons).

RED STARFISH (Echinaster sepositus)

Red starfish, which can measure 30 cm, has a red to orange body, with 5 long and cylindrical arms, covered by small spines. It can be found utill 1000 metres deep, always in rocky bottoms.

Estrella vermella (Echinaster sepositus) (Foto: , Creative Commons).
Red starfish (Echinaster sepositus) (Picture: Tato Grasso, Creative Commons).

HACELIA (Hacelia attenuata)

This starfish, with also 5 cylindrical arms, has a red to orange body. It can be confused with the red starfish, but this has the papulae in longitudinal lines, while in Echinaster their distribution is irregular.

Hacelia (Hacelia attenuata) (Foto: European Marine Life).
Hacelia (Hacelia attenuata) (Picture: European Marine Life).

STARLET (Asterina gibbosa)

The arms of this little star (from 2 to 4 cm) are not many differentiated from the central disc. It can present different colours, from greenish grey to red. It can be observed on rocks, sand or behind sea-grass.

Estrelleta (Asterina gibbosa) (Foto: Glaucus).
Starlet (Asterina gibbosa) (Picture: Glaucus).