Crinoids: Characteristics, Morphology, Reproduction, Species

The crinoids  or sea lilies are a class of animals that belongs to the phylum of the echinoderms, which are characterized by presenting a very similar appearance to that of a plant. Because of this, they are commonly known as sea lilies.

These animals first appeared on Earth during the Paleozoic era, specifically in the Ordovician period. The fossil record of these animals is abundant, allowing an adequate study of their characteristics or their evolutionary development.

Sea lily specimen. Source: Syahrul Harijo [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

At present, approximately 600 species have survived, which are located in marine-type ecosystems, some fixed to some substrate and others free in marine currents. Likewise, there are species that are typical of tropical temperatures, while there are others that can be found in cold-temperature waters.

Article index

  • one

    Characteristics

  • two

    Taxonomy

  • 3

    Morphology

    • 3.1

      – External anatomy

    • 3.2

      – Internal anatomy

  • 4

    Classification

    • 4.1

      Comatulida

    • 4.2

      Cyrtocrinide

    • 4.3

      Bourgueticrinide

    • 4.4

      Isocrinide

    • 4.5

      Hyocrnida, Millecrinida and Encrinida

  • 5

    Reproduction

    • 5.1

      Asexual reproduction

    • 5.2

      Sexual reproduction

  • 6

    Nutrition

  • 7

    Featured species

    • 7.1

      Lamprometa palmata

    • 7.2

      Stephanometra indicates

    • 7.3

      Tropiometra carinata

    • 7.4

      Clarckomanthus alternans

  • 8

    References

Characteristics

Sea lilies fall into the category of multicellular eukaryotic organisms. According to these characteristics, the genetic material of these animals is organized and packed inside a cellular structure called the nucleus.

They are made up of various types of cells, which have undergone a specialization process that has allowed them to fulfill specific functions, such as reproduction, nutrition and tissue repair, among others.

Likewise, sea lilies have been characterized by presenting totipotent cells. This implies that in the adult individual, their cells still retain the ability to diversify, transform and specialize in the various types of tissues that make up these individuals. This is helpful because it allows them to regenerate lost limbs and even regenerate entire individuals from fragments.

These types of animals are dioecious, that is, they have separate sexes. There are individuals with male gonads and individuals with female gonads. They present, in most species, sexual reproduction, although under certain conditions they can reproduce asexually.

In this same order of ideas, the crinoids present external fertilization, because it occurs outside the body of the female; Indirect development, because after being born they must undergo a metamorphosis until they reach the appearance of an adult individual and they are oviparous because they reproduce by means of eggs.

Taking into account their embryonic development, crinoids are classified as triblastic, coelomate and deuterostomate. This means that they present the three known germ layers: ectoderm, mesoderm and endoderm, which generate all the tissues of the adult animal.

In this sense, crinoids also have an internal cavity called coelom and an embryonic structure (blastopore) that simultaneously gives rise to both the mouth and the anus.

Finally, sea lilies present radial symmetry, since their organs are located around a central axis. In their larval stage they present bilateral symmetry.

Taxonomy

The taxonomic classification of crinoids is as follows:

Domain: Eukarya.

Animalia Kingdom.

Phylum: Echinodermata.

Subphylum: Pelmatozoa.

Class: Crinoidea.

Morphology

– External anatomy

The body of crinoids is made up of a cup-shaped structure, called the calyx, and an elongated structure known as the peduncle, through which they can be attached to the substrate.

Body (chalice)

It is made up of several rings (up to 3) that continue with plates that are fused. In addition, it has a central disk, from which several arms (generally 5, there can be up to 200) detach. These begin to fork practically from their point of origin.

Each branch of the arm or tentacle is known as a pinula. This is nothing more than a kind of thorn with a rigid texture that forms a kind of comb on each arm of the crinoids. The pinnules give the tentacles the appearance of feathers, which is why these animals are also known as feathered stars.

Schematic of the external anatomy of the sea lilies. Source: British Encyclopedia [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]

The body of crinoids has two surfaces, one oral and the other aboral. The location of both constitutes a distinctive element of this class, since the aboral zone is oriented towards the substrate, while the oral zone is located on the upper edge of the central disc, oriented outwards.

The oral surface is covered by a membranous organ known as the tegmen. In this they open the mouth, which has a central position, in the middle of the disk; and the anus that is to the side, between two arms. The tegmen also presents a series of pores that are known as aquifer pores, which, as a whole, supply the function of the madreporite of other echinoderms.

The tegmen also has a series of grooves called ciliated grooves or ambulacral grooves. These are lined by a ciliated epithelium and extend from the animal’s mouth to the arms. It fulfills functions in the process of feeding the animal.

Peduncle

It is a cylindrical structure, analogous to the stem of plants that allows the crinoids to remain fixed to the substrate. This is made up of several discs that are articulated with each other through ligaments.

Inside it presents a cavity or central channel through which neural tissue runs. In its final part, the peduncle branches into a series of extensions, like short tentacles that are called cirrus. The main function of these is to keep the sea lily fixed to the substrate on which it rests.

– Internal anatomy

Nervous system

The nervous system of crinoids is made up of a large number of nerve fibers that are distributed throughout the body of the animal. These nerves originate from a single main ganglion, which functions as the brain.

This ganglion is located in the aboral area of ​​the calyx. In turn, it originates nerves that go to the cirrus and arms of the crinoid. At the terminal end of the arms, the nerves branch out again, giving rise to the so-called brachial nerves.

Digestive system

Sea lilies have a digestive system that is made up of an oral cavity, esophagus, intestine, and anus.

The mouth opens to the oral cavity, which communicates directly with the esophagus, which is short in length. Then there is the intestine, which is not linear in shape, but curls up and turns into the anus, which is where the digestive tract finally ends.

Respiratory system

Crinoids do not have a proper respiratory system, but rather breathe through the so-called ambulacral system.

Circulatory system

The circulatory system of sea lilies is quite rudimentary. Each arm has two radial vessels that originate from an oral ring found in the central disc of the calyx.

Classification

The class Crinoidea currently includes a subclass: Articulata. This in turn is classified into seven orders, of which two are extinct.

Comatulida

This order encompasses the largest percentage of sea lilies known today. They are characterized because they are not fixed to the substrate, but can move freely through water currents.

Cyrtocrinide

It is made up of lilies that remain fixed to the substrate. These are characterized by having a short column and short and very robust arms. They are very old, since there are fossil records from the Jurassic period.

Bourgueticrinide

They are lilies that are fixed to the substrate. They have a long stem from which about five arms that are feather-like come out. They had their origin in the Triassic period and have been preserved to this day. It is made up of five families.

Isocrinide

The lilies of this order are characterized by presenting a heteromorphic stem. They also have a shallow calyx. They are fixed to the substrate.

Hyocrnida, Millecrinida and Encrinida

There were three orders that are currently extinct.

Reproduction

Sea lilies have two types of reproduction: sexual and asexual. The difference between the two is that one presents fusion of sexual gametes and the other does not.

Asexual reproduction

In this type of reproduction, an individual can give rise to its descendants without the need for any other individual of the same species to intervene.

Asexual reproduction is not common or regular in crinoids, but only occurs when the animal experiences some tension due to feeling some threat from the external environment.

When this happens, the animal can detach itself from one of its arms or from the chalice. Later, from these fragments, a new individual may develop.

This happens thanks to the fact that the cells of the crinoids maintain their totipotency. This is nothing more than the ability of some cells to differentiate, diversify and transform into any type of tissue.

Because the cells of the crinoids retain this property, they can transform into the tissues that make up these animals and thus generate a new one. It is important to note that this new individual is exactly the same as the one who gave rise to it.

Sexual reproduction

This type of reproduction involves the union of male sex cells and female sex cells. Reproduction of a sexual nature carries an advantage over asexual.

This is because it is related to genetic variability, which is closely linked to the survival of different species over time, as a result of adaptation to the changing environment.

The cells that originate the gametes are located in the pinnules of sea lilies. When the organism has reached sexual maturity, the pinnules tend to swell.

In the case of lilies with male gonads, the sperm are released to the outside through a pore, while in female lilies, the pinnules break and the ovules are released.

Fertilization is external, so it occurs outside the female’s body. When this occurs, eggs are formed, which develop very close to the female, so the first stages of the development of the offspring occur near the female.

It is important to note that sea lilies have an indirect development, so that the young that hatch from the eggs do not have the characteristics of adult individuals, they are known as planktonic keg larvae. It must undergo a series of changes until it reaches maturity.

Nutrition

Many tend to mistake sea lilies for plants due to their morphology. However, these belong to the animal kingdom and as such are considered heterotrophic organisms.

According to the way they feed, sea lilies can be carnivores or, in most cases, suspensivores.

Species that are carnivorous feed on zooplankton, as well as microscopic organisms such as diatom algae and others such as actinopods, small crustaceans and even larvae of some invertebrates.

On the other hand, in the case of species that are suspensivores, feeding is given by the capture of food particles that are suspended in the water currents.

Regardless of the type of food that the different species of sea lilies have, the food is captured by the animal’s arms, which are impregnated by a kind of mucus in which the food is trapped.

Subsequently, the food passes into the oral cavity where it begins to be processed thanks to the action of digestive enzymes. Then it passes to the esophagus and from there to the intestine, which is the place where the absorption of the nutrients that have already been processed takes place. 
Finally, the digestion waste is released through the anus of the animal.

Featured species

Currently only about 600 species of sea lilies persist.

Lamprometa palmata

It is the only species in the genus Lamprometa. It is characterized by presenting a barbed structure that resembles a comb in the terminal segments of its pinules. It has a protective function. It can also be located in shallow waters 1 meter deep, to deeper waters of almost 80 meters.

It is common to find them fixed to hard and resistant coral structures, as well as rocks.

Stephanometra indicates

It belongs to the Mariametridae family. It is generally found hidden in coral reefs, for example under corals. It feeds on suspended particles in water currents. It is an animal with nocturnal habits, since during the day it is lethargic, but at night it opens its arms and expands them.

Specimen of Stephanometra indica. Source: Anne Hoggett [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)]

Tropiometra carinata

It belongs to the Tropiometridae family. It is characterized by having ten arms, which have pinnules that have the appearance of feathers. Also, they are bright yellow. They can move slowly making use of elongated appendages called cirrus, as well as their arms.

Clarckomanthus alternans

It is a species of sea lily that belongs to the Comatulidae family. In this species specimens have been found that have only ten arms and others that have up to 125. Likewise, they can be located flush with the surface and more than 85 meters deep.

References

  1. Brusca, RC & Brusca, GJ, (2005). Invertebrates, 2nd edition. McGraw-Hill-Interamericana, Madrid
  2. Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition
  3. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill.
  4. Mladenov, P. (1987). Reproduction and development of Marine Invertebrates of the Northern Pacific Coast. University of Washington.
  5. Mironov, A., Améziane, N. and Eléaume, M. (2007). Deep-sea fauna of European seas: An annotated speciescheck-list of benthic invertebrates living deeper than2000 m in the seas bordering Europe. Invertebrate zoology. 11 (1).
  6. Rupert, E. and Barnes D. (1996). Invertebrate zoology. McGraw-Hill-Interamericana
  7. Vargas, P. (2012). The tree of life: systematics and evolution of living beings. Impulso SA

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