Arxiu d'etiquetes: marine arthropods

Horseshoe crabs: “living fossils” among arthropods

Xiphosurans or horseshoe crabs are probably the most ancient living arthropods known nowadays. These prehistoric-like, marine and currently scarce organisms related to arachnids have survived since ancient times without suffering almost any obvious change…until now. In this article, we will introduce you the main traits of these arthropods, as well as their current threats.

What are xiphosurans?

Xiphosurans (from Ancient Greek xíphos ‘sword’ and ourá ‘tail’), commonly known as horseshoe crabs, are a group of marine arthropods dating from as far back as the Ordovician (485,4 ±1,9 – 443,8 ±1,5 myr), in the Palaeozoic Era. Originally, they represented an important fraction of marine fauna; however, their number is extremely scarce nowadays, with only 4 extant species belonging to one order (Limulida) being the sole survivors of a once great radiation. The rest of members are fossils.

To know more about fossils: Knowing fossils and their age“.

Limulus polyphemus or Atlantic horseshoe crab. Source: Public Domain.

Current xiphosuran species are considered ‘living fossils as they haven’t undergone obvious morphological changes in comparison with Carboniferous and Triassic fossil forms. Moreover, they are the only ones that survived different extinction events.

Xiphosurans’ place on the tree of life

Like pycnogonids or sea spiders, xiphosurans’ place on the tree of life has been widely discussed. Until recently, xiphosurans were classified along with the eurypterids or sea scorpions (currently extinct) forming the Merostoma group, because they seemed to share some morphological traits. However, some deeper analyses showed that these organisms weren’t related, so ‘Merostomata’ is considered an artificial group nowadays.

To know more about pycnogonids: ‘Spiders from the deep sea: Pycnogonida’.

Eurypterus, the most common eurypterid fossil and also the first described genus. Author: Obsidian Soul, CC.

Currently, the most accepted stance is that xiphosurans constitute a class of arthropods by themselves (Xiphosura) inside the superclass Chelicerata (subphylum Cheliceromorpha). Moreover, they are classified within the euchelicerates along with two more classes: Arachnida and Eurypterida.

And above everything…despite their appearance and their marine habits, they’re NOT related to crustaceans!

Source: Tree of Life Web Project.

External and internal anatomy

As most of modern cheliceromorphs, the xiphosurans have the body divided in two parts or tagmata (prosoma and opisthosoma), the head not differentiated from the thorax, and antennae and mandibles absent. However, the defining trait of cheliceromorphs is the presence of chelicerae, a pair of modified preoral appendages mostly linked to feeding functions. In spiders, the chelicerae form fangs that most species use to inject venom.

Modern xiphosurans reach up to 60 cm in adult length, but their Paleozoic relatives were usually smaller, sometimes as small as 1 to 3 cm long. Dorsally, their body is covered with a not-segmented tough chitinous cuticle divided in two articulated parts more or less equivalent to the prosoma and the opisthosoma:

Dorsal view. Modified picture, original photography property of Didier Descouens, CC.

Now, let’s see the main anatomical traits of living xiphosurans (Limulida):

Tagmata: prosoma and opisthosoma

In the prosoma, the cuticle has three ridges: one median ridge and two lateral ridges. In the anterior part of the median ridge there are located two small ocelli, while in the external surface of lateral ridges the are located the compound eyes. The cuticle extends laterally towards the opisthosoma forming a kind of wings called genal spines. Ventrally and anteriorly, the cuticle forms a wide triangular area known as hypostome where some sensorial organs are located (e.g. ventral ocelli and frontal organ).

Opisthosomal segments appear fused in the modern xiphosurans (on the contrary, they are differentiated in all specimens of the order ‘Synziphosurina’, currently extinct); however, opisthosomal segments can still been distinguished by the lateral opisthosomal spines and the dorsal fossae (a total of 6 pairs, corresponding to the 6 segments of the opisthosoma). The opisthosoma terminates in a long caudal spine, commonly referred to as a telson.

Dorsal view. Modified picture, original photography property of Didier Descouens, CC.

Appendages

Xiphosurans have 6 pairs of prosomal appendages: one pair of chelicerae to capture preys (or other food particles) and 5 pairs of walking legs. Xiphosurans’ legs have a double function: besides allowing them to walk and swim, the legs’ bases have hard and sharped teeth to grind the food (gnathobase). These special bases make contact medially forming a duct (endostome) through which food is transported to the mouth. All legs end in well-developed pincers, except the first pair in males. In both sexes, the last pair has an organ called flabella they use to analyse water composition.

In the prosoma, we can also see a pair of appendages morphologically related to the first pair of opisthosomal appendages: the chilaria. These appendages, which are thought to be vestiges of the first opisthosomal segment’s legs, act as block for food to not escape behind the last pair of moving legs.

Prosomal appendages (ventral view). Modified picture, original photography property of Wayne marshall, CC on Flickr.

The opisthosoma has 6 pairs of modified appendages: one pair of genital operculum more or less fused and 5 pairs of book gills to breath under water, which are protected by the operculum flaps.

Opisthosomal appendages (ventral view). Modified picture, original photography property of KatzBird, CC on Flickr.

A very special circulatory system

Despite being arthropods, xiphosurans have a well-developed circulatory system with ‘veins’ and ‘arteries’ that resemble of which of more complex organisms. Their blood contains two cellular types: amebocytes, equivalent to leucocytes, and cyanocites, equivalent to erythrocytes but with hemocyanin instead of hemoglobin. When the hemocyanin is attached to oxygen molecules or is exposed to air, xiphosurans’ blood acquires a characteristic blue tonality.

The blue liquid is its blood!. Author: Dan Century, CC on Flickr.

Biology

Reproduction and life cycle

During mating seasons, horseshoe crabs move to shallow waters, shore of beaches and estuaries in massive groups. Males climb onto the back of females, gripping them with their rudimentary first pair of pincers; then, females move to the shore with a male on their backs while looking for a good place to bury the eggs. Females usually lay from 200 to 300 not fertilised eggs. Finally, males cover the eggs with sperm (external fertilization).

Author: U.S. Fish and Wildlife Service Northeast Region, CC on Flickr.

After hatching, xiphosurans go through two pelagic larval stadiums before reaching the adult form linked to the substrate: trilobite larvae, which has the opisthosomal appendages uncompletely developed and a short telson, and prestwiquianela larvae, which has all the appendages completely developed. They have a 20 years life span.

Ecology and distribution

All living horseshoe crabs live in shallow marine waters, despite some of their fossil relatives also inhabited freshwaters and brackish water habitats. They usually live on sandy and silty substrates at 3-9 metres deep. They’re specialized on digging, for what they use both the margins of their though cuticle and the first four pairs of moving legs; at the same time, they use the telson to lift the opisthosoma so that the fifth pair of moving legs could analyse and filter the surrounding water.

When swimming, they do it upside down, as we can see in the following video property of Wayne Brear:

Horseshoe crabs are generally predators of different species of annelids, molluscs, as well as of other groups of benthic invertebrates. However, they can also feed on algae.

Current diversity of xiphosurans is represented by 4 species, all of them belonging to the order Limulida: Limulus polyphemus (Atlantic coast of North America), Tachypleus tridentatus, Tachypleus gigas and Carcinoscorpius rotundicauda (Indo-Pacific coast).

Rough distribution of the 4 living xiphosuran species. Source: Charmichael & Brush, 2012.

What is their current conservation status?

Humans have done it again. Despite of been living on Earth since prehistoric eras and even been survived to different extinction events, the horseshoe crabs are now more threatened than ever due to anthropic causes. Among the main threats horseshoe crabs must face we can list the ones that follow:

  • Habitat alteration: water temperature changes due to global warming, contamination and declining of quality of water, and destruction of shores and shallow water environments (essential for these organisms to accomplish mating). This is, probably, the most problematic threat.
  • They are used as baits for fishing industries.
  • They are used for biomedical purposes: xiphosurans’ blood is used to produce a bacterial endotoxin indicator called ‘Limulus amebocyte lysate’ (LAL). The amoebocytes of their blood react against some bacterial endotoxins, forming blood clots. So, the LAL test is used to detected different bacteria in a wide variety of materials. Currently, the way to extract blood from their bodies is quite invasive so, despite returning them to their native habitats after the extraction, they still experience a big mortality rate.
  • They are harvested for being used in researches about vision, endocrinology and other physiological processes.
  • They are captured to be commercialized as food: in some Asian countries, horseshoe crabs are served in traditional dishes and rituals, even though this not seems to be the major threat they must face.
  • They are commercialized as pets.
Blood extraction for LAL. Source: National Geographic/Getty Images.
Dish based on horseshoe crab in Si Racha (Thailand). Author: Marshall Astor, CC.

There exist scarce data about the conservation status of the living species of xiphosurans. The most part of information comes from the American species Limulus polyphemus, which is now classified as ‘Vulnerable‘ and with a decreasing population trend since the last 100 years by the IUCN.

Recently, horseshoe crabs have been recognized as important components of benthic food webs, and their eggs supplement the diet of migratory shorebirds along the Atlantic coast of the USA. So, there is considerable interest in propagating and restoring horseshoe crab populations to support these valuable economic, biomedical, ecological and cultural services.

.           .           .

Solving the phylogenetic relationships of a group mainly composed by extinct organisms is not a piece of cake. But now that we start to glimpse them, we are slowly condemning these organisms to their disappearance. Nor the living fossils are safe from the sixth extinction!

References

  • Carmichael, R. H. & Brush, E. (2012). Three decades of horseshoe crab rearing: a review of conditions for captive growth and survival. Reviews in Aquaculture, 4(1): 32-43.
  • Chacón, M. L. M. & Rivas, P. (2009). Paleontología de invertebrados. IGME.
  • Grimaldi, D. & Engel, M. S. (2005). Evolution of the Insects. Cambridge University Press.
  • Marshall, A. J., & Williams, W. D. (1985). Zoología. Invertebrados (Vol. 1). Reverté.
  • Pujade-Villar, J. & Arlandis, J. S. (2002). Fonaments de zoologia dels artròpodes (Vol. 53). Universitat de València.
  • The IUCN Red List of Threatened Species: Horseshoe crabs.
  • Xifosuros: Animales de la realeza. Boletín Drosophila.

Main photo property of Didier Descouens, CC.

Spiders from the deep sea: Pycnogonida

Biodiversity is surprising. Did you know that there exist sea spiders? Pycnogonida, also known as “sea spiders”, form one of the strangest groups of arthropods that have ever existed. They belong to the subphylum Chelicerata (along with spiders) and are a part of the invertebrate fauna from seas and oceans all over the world. Although their number is so scarce and they camouflage so well with the environment they are very difficult to see!  

Do you want to know how to find and recognize them? Keep reading!

So…What are Pycnogonida?

Pycnogonida (from the Greek pykno = ‘lots of’ or ‘thick’ and góny = “knees”), also known as Pantopoda (‘totally made of legs’), is a class of marine benthic arthropods (benthos = organisms that live in association with the sea floor). It belongs to the subphylum Chelicerata, group that also includes the ‘true’ chelicerates or Euchelicerata: arachnids (spiders, scorpions, ticks and mites),  xiphosurans (a relict group of organisms commonly known as horseshoe crabs) and the extinct eurypterids (sea scorpions).

There are about 8-9 families, 86 genera (some of them are fossils) and up to 1000 species of Pycnogonida described worldwide, and all of them are a part of an unique living order: Pantopoda.

antarctic
Pygnogonids from the Anctartic ocean; this specimen had a lenght of about 30cm (Picture: Keith Martin-Smith).

Pycnogonida live in almost all marine habitats to almost any depth and latitude from the equatorial to Polar Regions both in coastal areas and in the abyssal plains about 6000m depth (although they appear more usually in the Mediterranean Sea, the Caribbean Sea, and Arctic and Antarctic oceans). Although its cryptic appearance and its capacity for camouflage with the environment make them difficult to see at the first sight.

Sea-spider
Pycnogonida from the species Nymphon gracile (Picture: Christophe Courteau)
seaspider
Pycnogonida from the genus Pseudopallene (Picture: Claudia Arango)

Firstly, they were classified as arachnids because of its resemblance to spiders. Due to new anatomical and biological studies, they were reclassified as a new group inside chelicerata which was been related with arachnids. The possibility they were an ancestor of all chelicerates has also been considered, so they will form a very ancient group.

TOL web
Phylogeny of Arthropoda from the Tree of Life Project (Source: tolweb.org)

External anatomy

Pycnogonida is a morphologically varied group of organisms with a wide range of sizes: from species that barely reach a few centimeters (which are commonly located in coastal benthos) to those that reach 50-70cm lenght (which tend to appear in abyssal depths).

divrsidad_morfológica
Different species and their morphological differences (Cano E., 2015)

They have a spider-like body divided into two main parts: prosoma (head or cephalon + thorax) and opisthosoma (abdomen). On their head they feature a proboscis, that is, an organ that allows them to suck fluids from soft-bodied invertebrates; they also display 4 eyes on a tubercle and three pair of appendices: a pair of chelifores, a pair of palps and a pair of ovigerous legs (or ovigers), these last being exclusive to pycnogonids. Pycnogonids mainly use ovigerous legs for a self-cleaning function, but these structures seem to be more well developed on males in order to carry the eggs (and even the larvae when they hatch).

Pycngonida usually have 8 legs (even they sometimes suffer polymerization of their body segments and get one or two more pairs of legs, having 10-12 legs in total) which can be as thick as the rest of their body. This usually make them to look like if they were made only of legs (in fact, the term Pantopoda means ‘totally made of legs’).

Anatomía externa de los picnogónidos (Fuente de la imagen: http://sprott.physics.wisc.edu/pickover/pycno2.gif)
External anatomy of Pycnogonida (Image source: http://sprott.physics.wisc.edu)

Internal anatomy

Pycnogonida are very strange creatures on their inside: they lack respiratory system (since a very thin cuticle covers their body, the gas exchange takes place through its surface) and excretory system; they have a reduced circulatory system and the nervous system is composed only of a simple brain and two ventral nerve cords. Gonads are located in the prosoma and they extend in the leg cavities; in most species, genital openings are also located on the legs.

Where and how do they live? 

As we said above, Pycnogonida form a group widely distributed in seas and oceans from all over the world. Whether they are located in the deeps or on the surface, they are always a part of the benthos.

img_fotosMiniatura_esquemaEcosistemas
Mainly biological components of marine ecosystems (Picture by Castro and Huber, 2007).

After a courtship that is still unknown, both males and females release their gametes in to the environment, where the fertilization occurs. Once fertilized, the eggs are gathered by the male, which will take care of them. To pick them up, it binds them together and sticks them to its body by secreting a sticky substances.

Sea-spider-male-carrying-eggs
Male of the species Nymphon gracile taking care of a bunch of eggs sticked to its ovigerous legs (Picture by Christophe Courteau)
73397_orig
Tanystylum duospinum (© 2005 California Academy of Sciences, CC)

After hatching, it emerges a free-living larva known as protonymph, which will reach the adulthood by suffering a metamorphosis process (are you interested on metamorphosis? Click here to learn more about it!).

They became carnivorous once they grow; they use chelifora to capture and chop soft-bodied organisms and then use their proboscis to suck their fluids (as spiders do). Generally, they feed on benthic or sessile organisms, like anemones, sponges and bryozoans.

sea on a briozoan
Pycnogonida from the genus Pseudopallene on a bryozoan (Picture by Claudia Arango).

NOTE: Pycnogonida are difficult to see at the first sight, but if you are curious people and you like diving, I encourage you to look into dense algae and sea phanerogams extensions. You will see one if you are lucky!

But it will be more easy to identify them if we have seen them in motion before (video from the Youtube channel Cloud. Tube):

Curiosities of the group

Pycnogonida form a very curious group, both for the morphological external traits that characterize its species and for its biological traits:

  • Some species develop a parasitical larval stage that remains in a latent state inside other organisms (e.g. corals) until they reach adulthood, when they leave these organisms and adopt a free-living form.
  • The polymerization of their legs due to the increment of corporal segments seems to be an exclusive phenomenon among arthropods.
  • Many species of Pycnogonida lose their legs by a process called autotomy, that is, the separation of a body part by self-amputation (e.g. the loss of the tail in lizards).
  • Pycnogonida is almost the only group in which the parental generation takes care of its descendants. The male is the one responsible for protecting, cleaning and oxygenating the eggs, even after hatching.

.         .         .

Nowadays, Pycnogonida is a fairly unkown group of organisms on many levels: they are phylogenetically old, scanty and often located so deep in the sea they turn out to be difficult to study. Moreover, no benefits for humans are been found from this group beyond its importance in terms of biodiversity. Unfortunately, this fact usually condemn strange organisms to oblivion.

And you, what do you think about this? Do you think is essential for us, humans, and for all life forms on Earth to preserve biodiversity?

References

  • Australian Government. Department of Environment: Australian Antarctic Division. Web: http://www.antarctica.gov.au/science/cool-science/2010/sea-spiders-provide-insights-into-antarctic-evolution.
  • Bamber, R. N. & A. El Nagar (Eds.) 2014. Pycnobase: World Pycnogonida Database. Accesible (2014) en: http://www.marinespecies.org/pycnobase/
  • Biodiversidad, taxonomía y biogeografía de los Artrópodos de México. Vol. III, Edición: 2002, Capítulo: Pycnogonida (por Tomás Munilla), Publisher: Universidad Nacional Autónoma de México, Editores: J. Llorente y J. Morrone, pp.215-22.
  • Blaxter J., Douglas B. (1987). Advances in Marine Biology, Volumen 24. Academic Press.
  • Cano E., López P.J. (2015). Clase Pycnogonida, Orden Pantopoda. IDE@-SEA, nº 22 (30-06-2015): 1-13.
  • Padilla F., Cuesta A. Zoología Aplicada. Ediciones Díaz de Santos, 2003.
  • Fauna marina circalitoral del sur de la Península Ibérica: resultados de la campaña oceanográfica “Fauna I”. Editorial CSIC – CSIC Press, 1993.

Main image: Colossendeis megalonyx from the deeps of the Anctartic ocean by Norbert Wu/Minden Pictures/FLPA.

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