Echinothuriidae
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The Echinothuriidae are a family of
sea urchins Sea urchins or urchins () are echinoderms in the class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal zone to deep seas of . They typically have a globular body covered by a spiny p ...
in the order Echinothurioida. These echinoderms are typical of the Indo-Pacific these softer-bodied "leather urchins" are found from abyssal to shallow littoral zones in the ocean. While common in the Indo-Pacific these colourful urchins can be found across the world. Some individuals within Echinothuriidae such as species in the genus '' Asthenosoma'' are also known as "fire urchins" due to their bright colours and painful, venomous sting.


Description and characteristics

The order to which they belong, Echinothurioida, is defined by compound plates which cover the ambulacra and the peristome. The test is composed of thin and weakly calcified plates, not always continuous. The spines are attached to perforated and uncrenulated tubercles. Spines from the oral face end with a hyaline hoof, which allows walking on soft substrate. The jaw (Aristotle's lantern) has five aulodont teeth. These sea urchins have a disc-like body, more or less bulging, structured by a flexible test, which is nearly unique among sea urchins. Most species can grow quite big and live in deep seas, though some genera contain shallow species (especially '' Asthenosoma''). This family, Echinothuriidae seems to have appeared at the end of the
Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...
.


Habitat

They are most often found in the Indo-Pacific, although they have been observed worldwide, and are among the most common urchins inhabiting deep marine environments. While they are most common at great depths (more than 200m below the surface) the genus Asthenosoma has been known to inhabit shallow zones.


Interspecies interactions

Members of the Echinothuriidae have been observed to attached themselves to living and dead stony corals (Order:
Scleractinia Scleractinia, also called stony corals or hard corals, are marine animals in the phylum Cnidaria that build themselves a hard skeleton. The individual animals are known as polyps and have a cylindrical body crowned by an oral disc in which a mo ...
) allowing them a degree of protection from crustaceans and fish that feed on urchins. When the threat of predators is low they can be found assembling closely together which likely provides advantages for reproduction and feeding. The levels at which they seek refuge in the coral will also change based on predator presence as the tend to shift higher on the structures in the presence of decapod or crustacean predators, while an increase in fish will lead to an increase in urchins closer to the sediment. Like other echinoids, members of the Echinothuriidae, namely those belonging to the genus Araeosoma, have been known to prey on or consume the remains of
crinoids Crinoids are marine invertebrates that make up the class Crinoidea. Crinoids that remain attached to the sea floor by a stalk in their adult form are commonly called sea lilies, while the unstalked forms, called feather stars or comatulids, are ...
. Although the extent to which they are capable of successfully hunting the crinoids is unknown, experiments done in captivity present this predator-prey interaction as a definite possibility in nature. Gut contents have been found to also contain pieces of corals, crustacean exoskeleton,
bryozoans Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic animal, aquatic invertebrate animals, nearly all living in sedentary Colony (biology), colonies. Typically about long, they have a spe ...
,
polychaetes Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are m ...
,
foraminifera Foraminifera ( ; Latin for "hole bearers"; informally called "forams") are unicellular organism, single-celled organisms, members of a phylum or class (biology), class of Rhizarian protists characterized by streaming granular Ectoplasm (cell bio ...
,
sponges Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and ar ...
, and a variety of other invertebrate elements. The high presence of sediment in the gut along with these elements indicates a scavenger like feeding strategy as they pick up deceased material from the sediment.


Reproductive biology

Echinothuriidae is separate from other members in their grouping due to having a softer test comparatively. Morphologically they are different but reproductively it is hard to tell visually without investigation. The reproductive habit of echinothuriidae is debated, from pelagic lecithotrophs to direct development, sperm from shallow water echinoid specimens demonstrate a highly conserved morph with minimal variation from species to species. Much of echinothuriidae reproductive biology is restricted to a handful of deep-sea observations of abyssal deep-sea specimens. The deep sea echinothurrid acrosomal vesicles and sperm nuclei are morphologically different from other echinoderm reproductive structures, abyssal echinothuriids showcase far more elongate sperm heads compared to littoral species. Some members of the family, specifically those belonging to the genus Asthenosoma, produce the largest eggs of any known echinoid. The embryo and larva develop slowly while floating under the water surface. The larvae are completely lecithotrophic, meaning their yolk sustains them for the duration of their growth.


Taxonomy

The World Echinoidea Database recognises these genera:Kroh, A. (2015)
Echinothuriidae Thomson, 1872
In: Kroh, A. & Mooi, R. (2015) World Echinoidea Database.
* Subfamily Echinothuriinae Thomson, 1872a ** genus '' Araeosoma'' Mortensen, 1903b - 19 current species and two fossils ** genus '' Asthenosoma'' Grube, 1868 - six current species and one fossil ** genus '' Calveriosoma'' Mortensen, 1934 - two species ** genus '' Hapalosoma'' Mortensen, 1903b - four species * Subfamily Hygrosomatinae Smith & Wright, 1990 ** genus '' Hygrosoma'' Mortensen, 1903b - three species * Subfamily Sperosomatinae Smith & Wright, 1990 ** genus '' Sperosoma'' Koehler, 1897 - 11 speci
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** genus '' Tromikosoma'' Mortensen, 1903 - six species * genus '' Retzneiosoma'' Kroh, 2005 † "†" means an
extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...
taxon In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...
. Image:Araeosoma thetidis insitu.tif, '' Araeosoma thetidis'' (
New Zealand New Zealand () is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and List of islands of New Zealand, over 600 smaller islands. It is the List of isla ...
, 188 m depth) Image:Toxic Leather Sea Urchin - Asthenosoma marisrubri.jpg, '' Asthenosoma marisrubri'' Image:Deep sea Echinothurioid.jpg, '' Hygrosoma'' sp. (abyssal species from north Atlantic) Image:Sperosoma.jpg, '' Sperosoma'' sp. (abyssal species taken in Hawaii) Image:Expl0867 - Flickr - NOAA Photo Library.jpg, '' Tromikosoma'' sp. (3000 m deep, California)


References

{{Authority control Echinoderm families Extant Middle Jurassic first appearances