Saturday 19 January 2019

Diadema setosum: An invasive alien Sea Urchin found on the Mediterranean coast of Israel.

Invasive organisms (organisms introduced to an environment from elsewhere, usually by Human actions) are of concern to environmentalists, as a lack of adapted predators, parasites and pathogens can lead to explosive population booms and subsequent dominance of the new environment. In marine environment Echinoderms (Sea Urchins, Starfish, etc.) are particularly worrying, as their grazing can rapidly deplete habitat forming organisms such as Corals or Seaweads, leading to changes that impact a wide range of other organisms.

The Sea Urchin Diadema setosum is widely distributed in shallow waters in the Indian Ocean and West Pacific. In 2006 a specimen of this Sea Urchin was collected from the Kaş Peninsula in Anatolian Turkey, since when there have been sightings of the species in the Aegean Islands of Greece, and along the coast of the Eastern Mediterranean as far as Lebanon. Since 2016 there have been several reported sightings of Diadema setosum on reefs off the Mediterranean coast of Israel, though an extensive survey by BioBlitz Israel was unable to find any evidence of this.

In a paper published in the journal Zootaxa on 11 October 2018, Omri Bronstein and Andreas Kroh of the Geological-Paleontological Department at the Natural History Museum Vienna report the first confirmed sightings of Diadema setosum on the Mediterranean coast of Israel.

The first evidence presented by Bronstein and Kroh comes from a video made by recreational divers during a night dive at the Gordon Beach site, on the coast of Tel Aviv, on 10 November 2016. This shows a long-spined black Sea Urchin, with a pattern of five white spots, as well as a bright orange ring surrounding the distal margins of the periproctal cone (anus), with white interradial lines connecting the apical disc to the white spots, a pattern considered to be diagnostic of Diadema setosum.

Specimen of Diadema setosum sighted on 10 November 2016 at a depth of 6-8 m off the coast of Tel Aviv, Israel. Adi Gvir in Bronstein & Kroh (2018). 

This sighting prompted a renewed hunt for the species around Gordon Beach by BioBlitz Israel, eventually resulting in the discovery of a single adult specimen in a rock crevice at the Gordon Caves site (near Gordon Beach), from which a single spine was collected.

Specimen of Diadema setosum sighted on 17 June 2017 at a depth of 3.5 m off the coast of Tel Aviv, Israel. Bronstein & Kroh (2018).

The spines of all Sea Urchins are covered on scales called verticillations, which come off in any wound caused by the spines, making the injury more painful and unpleasant, and therefore making Sea Urchins less attractive to predators. The verticillations of Diadema setosum are longer and less flared than those of other members of the genus Diadema, another diagnostic feature for the species.

Scanning electron microscope image of Diadema setosum specimen from Gordon Caves, in longitudinal view showing the characteristic diadematoid verticillation. Bronstein & Kroh (2018).

Previous genetic studies of Diadema setosum have suggested that the species is made up of two clades (a clade is a group comprising all the organisms sharing a single common ancestor), which are likely to have split during the Miocene, about 2.5 million years ago. Of these, Clade a is found around much of the margin of Indian Ocean and west Pacific, while Clade b is found around the Arabian Peninsula and from the Red Sea south to Zanzibar. Analysis of the two known Mediterranean specimens (from the Kaş Peninsula and Tel Aviv) shows that these belong to Clade b, supporting the idea that the species has jumped from the Red Sea to the Mediterranean through the Suez Canal.

The genetic analysis also showed that the two specimens were very close to genetically identical, suggesting a very small founder population within the Mediterranean, which has spread slowly around the eastern end of the sea, while maintaining a very low population level, though whether the population will remain stable at such a level in future is uncertain.

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