Encope emarginata (Leske 1778)

Encope emarginata, a notched sand dollar, is a marine echinoid ranging the western Atlantic ocean. They are best known for their bioturbation in the sediment, relationship with crabs, and their widespread distribution.

Description

E. emarginata has a thick test, or shell, that often remains intact and preserved.^[1] Tests are oval-shaped, centrally domed, typically greenish-brown colored, and have 6 lunules, or notches, as well as large bowed petaloids ^[2] Young E. emarginata can be mistaken for its sibling, E. michelini, because of the presence of open lunules as juveniles, although closed as adults.^[3]

Basic anatomy of E. emarginata

Habitat and Distribution

Notched Sand Dollar found on coast of Brazil by @fernandas via iNaturalist

Mostly found in subtropical and tropical waters of the Caribbean Sea, Gulf of Mexico, and southwestern Atlantic Ocean, E. emarginata ranges from Belize to Argentina.^[4] Common on the South America Atlantic coastline, they are one of the only extant echinoids found on the coast of Rio Grande do Sul (the other being Mellita quinquiesperforata) and the only one south of the La Plata river.^[5] Area inhibited by Encope emarginata is below the action of the normal wave regime, thus the fossils of this species is only truly disturbed during extreme coastal weather.^[5]

E. emarginata inhabits sandy, muddy sediments of coastal waters.^[4] Although a marine species,^[6] they have also been found in estuaries and river mouths.^[7] The waters they live in are typically shallow, as deep as 20 meters.^[8]

Taxonomy

These sand dollars were originally named Echinodiscus emarginatus,^[9] but have since been transferred to the Encope genus within the Mellitidae family.^[6] Encope is thought to be the most diverse genus among sand dollars, containing 7 extant species.^[1] Fossils can be dated back to the Pliocene or Pleistocene, up to 5 million years ago.^[1]

There is ongoing research suggesting that two populations of E. emarginata may emerge as separate species as a result of a barrier created by upwelling, a process known as allopatric speciation.^[4]

Bioturbation^[10]

E. emarginata is well-known for its bioturbation within the sediment, especially its effect on the spatial distributions of phytoplankton and other small benthic organisms. Moving at a maximum rate of 15 cm per hour, these sand dollars typically work through surface-layer sediments. E. emarginata has shown to have significant impacts on the concentrations of Chlorophyll-a within the sediment, as well as causing a greater variance in microphytobethic and meiofaunal populations due to the sand dollars' foraging behaviors. The bioturbation of this echinoid is thought to create top-down effects, enhancing sediment heterogeneity and maintaining benthic biodiversity.

E. emarginata in the Natural History Museum in London, England

Relationship with Ectosymbiotic Crabs

Genus Dissodactylus are commensal or parasitic crabs found to live on the bodies of irregular echinoids, such as E. emarginata. There is ongoing debate whether the size of E.emarginata influences the number of crabs found on it, where one study found a positive correlation^[11] and another found no relationship.^[12] The crabs are thought to consume the spines of the sand dollar, but additional research is necessary to determine if the Dissodactylus are truly parasitic.^[11]

References

Encope emarginata est une espèce d'oursins plats de l'ordre des Clypéastéroïdes et de la famille des Mellitidae.

Description

Ce sont des oursins plats, d'où leur surnom anglais de sand dollars, du fait de leur ressemblance avec une grosse pièce. Leur forme est arrondie mais aplatie à l'arrière, avec un test (coquille) perforé d'une longue lunule anale antérieure et portant cinq encoches radiales arrondies au bout des pétales ambulacraires. Le corps est couvert de radioles (piquants) fines et courtes formant un tapis mobile permettant la progression dans le sable, de couleur orange à violet sombre en passant par le brun. La bouche, très réduite, occupe une position centrale sur la face inférieure (où elle est suivie par l'anus), et la lanterne d'Aristote (appareil masticateur) est modifiée en « moulin à sable » plat^[1]. Les cinq pores génitaux sont situés au milieu de la face aborale (le système apical), avec le madréporite. L'intérieur du test est consolidé par des cloisons délimitant des loges.

Habitat et répartition

Ces oursins se trouvent principalement dans le sud de la Mer des Caraïbes, jusqu'au Venezuela. Ils sont particulièrement courants à Panama.

Ce sont des oursins fouisseurs, qui vivent enterrés dans le sable ou la vase, qu'ils filtrent pour en retirer les nutriments dont ils se nourrissent.

• Test naturalisé au Natural History Museum de Londres.

Références taxinomiques

• (en) Référence World Register of Marine Species : espèce Encope emarginata (Leske, 1778)
• (fr) Référence SeaLifeBase :
• (fr+en) Référence ITIS : Encope emarginata (Leske, 1778)
• (en) Référence Catalogue of Life : Encope emarginata (Leske, 1778) (consulté le 21 décembre 2020)
• (en) Référence uBio : Encope emarginata (Leske, 1778)

Notes et références

Encope emarginata is een zee-egel uit de familie Mellitidae.

De wetenschappelijke naam van de soort werd in 1778 gepubliceerd door Nathanael Gottfried Leske.

• Kroh, A. (2010). Encope emarginata (Leske, 1778). In: Kroh, A. & Mooi, R. (2010) World Echinoidea Database. Gebaseerd op informatie uit het World Register of Marine Species, te vinden op http://www.marinespecies.org/aphia.php?p=taxdetails&id=513255.