Located at the crossroads of several major biogeographic provinces connected by major seasonal currents and upwellings, Socotra’s marine biodiversity is rich and is characterized by a unique mixture of species from the following regions - the western Indian Ocean, the Red Sea, Arabia, East Africa and the wider Indo-Pacific. It is among the richest in the Western Indian Ocean, rivaling other much larger coastal areas.

Marine Biogeographic Areas of the North-western Indian Ocean. AG – Arabian Gulf; AS – Arabian Sea; ASC – Arabian Sea Coast; GOA – Gulf of Aden; GOO – Gulf of Oman; RS – Red Sea; SCC – Somalia Coastal Current; IWP – Indo-west Pacific. Base map adapted from UNEP-WCMC (2001). Biogeographic zones adapted from Klausewitz (1972, 1978), Sherman and Duda (1999), Kemp (1998), Wilson (2000) and Klaus and Turner (2004)

.The numbers of species of hard corals and fish are comparable to those of the Red Sea, despite the small size of the archipelago. There is a high degree of faunal similarity and biogeographic affinity between Socotra and the Southern Arabian region across most groups, although there is also a strong Indo-Pacific influence at Socotra. Different taxa show different affinities to the regions. For the corals, most occur across much of the Indo-west Pacific region, others are more widespread in the Western Indian Ocean, but some 20% were not previously known from Arabian seas, and a few were previously known only from the Red Sea or Arabian Gulf. The Archipelago may provide the crucial "link" for some marine species in maintaining their distribution governed by the duration of their planktonic larval stages.

The Oman Anenome Fish Amphiprion omanensis, previously known only from the Arabian Sea coast of Oman, and the Threespot Dascyllus Dascyllus trimaculatus, a widespread Indo-Pacific species, photographed on the north coast of Socotra Island.

There is a wealth of habitat diversity around the islands – coastal cliffs, rocky coves, cobble and sandy beaches, mudflats, lagoons and coral reefs. Some of the coastal cliffs continue underwater, their sheer sides occasionally poked by caves. Others fall away to a flat limestone plain fronted by ancient coral reef, some of which now lies above sea level. In places more sheltered from the monsoon winds, waves and currents; small patches of tropical coral reef, tranquil shallow lagoons and mudflats occur. Further offshore, the open sea is influenced by the seasonal onset of eddies and gyres, with major effects on productivity, dispersal and community structure.

Beaches range from small sand or cobble patches scattered among the boulders, fossil reefs and cliffs, to long expanses kilometres in length. The longest continuous sandy beach is located on the north coast at Ghubbah, where it extends over 20 km and providing the main local nesting site for Loggerhead Turtles Caretta caretta. Cobble beaches occur along parts of the Noged Plain on the south coast, on the north coast near Hadiboh, Qadheb, Ras Qadama and south of Qalansiyah and Shu’ub in the west.

Mudflats occur at the mouths of wadis, notably at Riy Di Hamari, Di Lishah, Khawr Girmah, Ditwah lagoon and Shu’ub. Some mudflats experience daily tidal influence, while others are separated from the sea for several months.

Major dune systems, deposited by strong wind, can be found on both the south and the north (pictured) coasts.
Sand bar on the seaward edge of Ditwah lagoon near Qalansiyah. In the shallow waters of the lagoon, seagrass form expansive patches, providing nursery grounds, shelter and nutrition for other species. At least seven seagrass species occur around the islands.		Cobble beach on the south coast (Noged) with extensive limestone plateaus as backdrop. Such beaches are often carpeted with macro-algae following the SW monsoon season in summer when intensive upwelling enriches the coastal waters with nutrients from the ocean depths.

Rocky shores of varying form – cliffs, fossil reef terraces and boulders – occur around much of the coast. Occasional rock pools provide varied micro-habitats. In the sublittoral zone, benthic community structure corresponds closely to exposure to upwelling. The more exposed south coasts support the most diverse and productive algal communities while the north coast support numerous coral communities. The more palatable algal species and their attached micro-fauna are grazed by rabbitfishes, surgeonfishes and parrotfishes, among other herbivores.

Sea cliffs on Samha Island, with fresh water seepage and bands of inter-tidal organisms including barnacles, oysters and algae.		On the sides of intertidal boulders, Sabellariid polychaete worms construct tiered colonies from where they filter-feed, trapping small particles that wash past their home.

Coral reefs around the islands do not compare with the large reefs of other tropical areas of the Indian Ocean in extent, or indeed with the large fossil reef terraces that fringe sections of the coast. Limitations on reef development are common in extreme environments and may include variability in the intensity of upwelling in the Arabian Sea during the Holocene, competition with macro-algae, predation, undersea seepage of freshwater through porous limistones, and bioerosion especially after major coral bleaching events.

Nevertheless, though small and relatively inconspicuous, present-day coral reefs are composed of unique coral communities boasting high species diversity. They support a rich associated fauna, the marine equivalents of ‘oases’ surrounded by submarine sand ‘deserts’.

Fossil terraces such as these on the south coast of Socotra, signify extensive reef development in the distant past, probably some 125,000 years ago.
A flourishing coral community on Abd al Kuri, with plating Montipora spp. (foreground) and massive Porites and Lobophillia hemprichii (centre and back), with mixed schools of triggerfishes and surgeonfishes.
Many sessile marine invertebrates employ a kind of chemical warfare in defence and to conquer new territory. The red sponge, possibly a species of Dysidae, is slowly killing the surrounding Porites coral.		Butterflyfish are reef-associated species, often inhabiting areas of high diversity of hard and soft corals, and are sometimes used as indicators of reef health. Interestingly, Chaetodon auriga (lower left of photo) was previously known only from the Red Sea. Its occurrence around Socotra is an extension of its distribution range (J. Kemp, pers. comm..). Chaetodon fasciatus (top in photo) is known from the Red Sea and Gulf of Aden.
Reef cuttlefish use Socotra’s coral bommies, such as massive Porites (pictured), as a kind of ‘lek’, a site attractive to females and used by males for courtship. The female cuttlefish will deposit her fertilized egg case in a crevice on the coral.		Crown-of-thorns seastars are voracious predators of reef-building corals across the Indo-Pacific region. Luckily, these remain uncommon around Socotra.

Threatened Marine Species

At least four marine turtles roam the seas around the islands. The Loggerhead Turtle Caretta caretta regularly nest near Ghubbah on the north coast of Socotra Island, from May to September. The Green Turtle Chelonia mydas has its largest nesting site on Abd al Kuri and also occasionally nests on the south coast of Socotra. Hawksbill Turtles Eretmochelys imbricata and the rarer Olive Ridleys Lepidochelys olivacea also occur around Socotra, although no nesting has yet been confirmed.

To afford protection for the endangered turtles and to minimize habitat destruction from infrastructural development, the main Loggerhead Turtle nesting beach near Ghubbah is designated as National Park. A monitoring program by tagging nesting Loggerhead Turtle during the southwest monsoon season has been ongoing since 1999. The hundreds of tagged turtles have given valuable information on the species’ foraging and nesting patterns, and raised awareness for their conservation.

A female Loggerhead Turtle nesting near Ghubbah.		Turtle hatchlings found on the north coast of Socotra.

Numerous marine mammals do occur around the islands although confirmed records are few. At least six dolphin species are known to be present in the area, and ten whale species migrate to or through the region, including at least two globally threatened species: the Sperm Whale (Vulnerable) and the Blue Whale (Endangered).

Whale populations in the NW Indian Ocean, as elsewhere, are threatened by a wide and growing variety of impacts. Each year, several carcasses of Sperm Whales get washed ashore at Socotra, perhaps victims of encounters with industrial fishing boats, pollution or other misadventures. Photograph by Malek Abdul Aziz.