Zoogeography of the Southern African Echinoderm Fauna

S. Afr. J. Zoo!. 1989,24(4) 311 Zoogeography of the southern African echinoderm fauna A.S. Thandar Department of Zoology, University of Durban-Westville, Private Bag X54001, Durban, 4000 Republic of South Africa Received 17 April 1988; accepted 28 March 1989 Pertinent features of the oceanography of southern Africa are reviewed and an analysis of the echinoderm fauna in relation to the genera] biogeographic regions and local faunistic provinces is given. The last such analysis appeared in 1923, based on fewer species (216) and long before Stephenson's comprehensive analysis of the distribution of the southern African marine biota. Over 400 species of echinoderms are currently known from southern African waters, south of the tropic of Capricorn. These comprise 17 crinoids, 99 asteroids, 124 ophiuroids, 59 echinoids and 108 holothuroids. The endemic component is the richest, accounting for at least 47% of the fauna, with the Indo-Pacific component, including those species restricted to the Indian Ocean, or specifically to the West Indian Ocean, making up 37% of the fauna. The remaining species are either cosmopolitan (3%), tropicopolitan (1 %), or shared with the Atlantic (6%) or Southern Oceans (2%), or are 'Other Foreign' species (4%). The distribution pattern along the coast supports the division of the southern African marine region into three faunistic provinces - tropical, subtropical and temperate. The echinoderm fauna appears to have had mostly an Indo-Pacific origin but evidence indicates that, once it became well established and isolated, there was a secondary development of an active evolutionary centre. Pertinente kenmerke van die oseanografie van Suider-Afrika word oorsigtelik behandel en 'n analise word gegee van die Echinodermata-fauna in terme van die algemene biogeografiese streke en plaaslike faunistiese provinsies. Die laaste soortgelyke analise, gebaseer op 'n kleiner aantal spesies (216) het in 1923 verskyn, lank voor Stephenson se omvattende analise van die verspreiding van die Suider-Afrikaanse marine biota. Meer as 400 spesies van Echinodermata is tans bekend uit Suider-Afrikaanse waters, suid van die Steenbokskeerkring. Dit sluit in 17 Crinoidea, 99 Asteroidea, 124 Ophiuroidea, 59 Echinoidea en 108 Holothuroidea. Die endemiese komponent is die rykste, en sluit ten minste 47% van die fauna in, terwyl die Indo-Pasifiese komponent, insluitende daardie spesies wat beperk is tot die Indiese Oseaan, 37% van die fauna behels. Die oorblywende spesies is of kosmopolities (3%), of tropikopolitaan (1 %), of word gedeel met die Atlantiese (6%) of Suidelike oseane (2%), of is 'ander vreemde' soorte (4%). Die verspreidingspatroon . langs die kus ondersteun die onderverdeling van die Suider-Afrikaanse mariene streek in drie faunistiese ) 0 provinsies, naamlik, tropies, subtropies en gematig. Die Echinodermata-fauna het blykbaar 'n hoofsaaklik 1 0 Indo-Pasifiese oorsprong gehad maar aanduidings bestaan dat, nadat die fauna goed gevestig en geisoleer 2 is, daar nietemin 'n sekondere ontwikkeling van 'n aktiewe evolusionere sentrum plaasgevind het. d e t a d ( Introduction The Indian Ocean was formed on its eastern side some r e 180 my a by the drifting away of Australia, Antarctica h The southern African subcontinent, lying south of s i l 23V2oS latitude forms a small landmass, narrowing and India while the Atlantic Ocean was formed on its b u gradually southwards. Since it is the meeting place of western side some 100 my a by the drifting away of the P two of the world's greatest oceans, it has received faunal South American continent. Shillington (1986) pointed e h t invasions from both sides, producing a highly complex out that the stress caused by the separation of the y Falkland Plateau from the eastern side resulted in a b fauna, with numerous endemic species. d Its coastline of over 4000 km is very exposed with no shallow (150 m) and narrow shelf (c. 12,5 km at Port St. e t large offshore islands to break the force of ocean swells. Johns), sloping steeply from its edge to the abyssal plain n a r Nevertheless the subcontinent offers a variety of marine at 4000 m. This steep slope, in combination with g habitats: from coral reefs to rocky shores, sandy beaches numerous gullies, canyons, ridges, submarine mountains e c n to mudflats, mangrove swamps to kelp beds, shallow etc., makes dredgings on the east coast very difficult, if e c estuaries to ocean depths in excess of 5000 m, narrow not impossible in places. Further, Flemming (in Hey i l r shelves to wide shelves and shoals, gentle slopes to steep dorn 1978) describes southward migrating marine dunes e d slopes, canyons to ridges and submarine mountains. on the east coast, some up to 8 m high, in water depths n u There are, in addition, tropical, subtropical, warm and greater than 50 m. Such massive sediment transport, y cold temperate waters as determined by the latitude of a according to Heydorn (1978) forms a powerful physical a w particular region and the prevailing ocean currents. barrier to most benthic organisms. Even the outer shelf e t Hence it is not surprising that over 400 species of a with its relict algal nodules and shell fragments is said to G echinoderms have so far been recorded from this region support only sparse populations of sedentary organisms t e and this is by no means the full extent of our knowledge and hence is unsuitable for the mostly mobile, benthic n i b of this group. echinoderms. Thus it is not surprising that only about a a S third of the echinoderms recorded from the east coast y Marine geology and bottom topography b have been taken from waters between 50-1000 m depth. d e It is well established that southern Africa was once part South and west of Knysna, however, is a submerged c u of the supercontinent of Gondwanaland, some 225 mya. plain, the Agulhas Bank, with a gentle slope (1:1000) d o r p e R 312 S.-Afr. Tydskr. Dierk. 1989,24(4) and fairly deep (190-250 m) and wide (c. 300 km) conti (Shillington 1986). Both Agulhas and Benguela currents nent shelf, the widest anywhere on the subcontinent. interact in the extreme south-west of the Cape Province. The west coast of southern Africa is characterized by Further, between Cape Peninsula and Cape Agulhas sand dunes, with occasional rocky outcrops. North of there is some injection of cold water derived from the 32°S latitude the coastline is regular and, except for West Wind Drift (Dietrich 1935; Clowes 1950). The Walvis Bay and Luderitz, devoid of significant embay mixing of water masses and the degree of upwelling are ments. However, south of this latitude, the coastline both variable and seasonal. Hence in the extreme south becomes irregular with several capes and bays, making west region of the Cape Province, especially in and the south-west Cape topographically different from the around False Bay, there are extreme temperature fluctu remainder of the area (Shannon 1985). Shillington ations with new species perhaps continuously evolving to (1986) states that the west coast resulted perhaps from adapt to these specialized conditions. Mass mortalities tension faulting. It may be due to this that its bathymetry often result amongst those species which cannot adapt to is so variable. According to Shannon (1985) the conti these temperature fluctuations. Hence it is not surprising nental shelf is narrowest (20 km) in southern Angola that about 10% of all the echinoderms so far reported (not truly part of the southern African region here from the temperate region of southern Africa are ende considered), but widest (180 km) off the Orange River. mic to False Bay where, according to Day (1970), high Generally, the western shelf is much wider than the summer temperatures of up to 23°C have been repor eastern shelf and gently sloping. Green mud, just ted. While this figure might just be a reflection of the offshore from Walvis Bay, at 50-150 m is said to reflect frequency of collections, at least some species may exist sterile areas owing to the abundance of hydrogen which will be found not to extend their range out of False sulphide and lack of oxygen (Kilburn & Rippey 1982). Bay. On the east coast, owing to the shearing action of the Currents and other environmental factors southbound Agulhas current, clockwise eddies are gen Brown & Jarman (1978) emphasize two environmental erated, occasionally giving rise to north-setting counter factors of paramount importance in determining the dis currents. While the origin of the latter is debatable and tribution of marine organisms: the degree of separation subjected to speculations (Mallory 1961; Bang & Pearse, of the continental landmasses and the temperature of the in Heydorn 1978) there is little doubt that they are water. In the southern hemisphere the continents are responsible for transporting warm temperate species widely separated, resulting in limited migration of northwards along the east coast, especially those with . species between them. The species thus shared between drifting, planktonic larvae. The complexities of the ) 0 southern African ocean currents become apparent when 1 the landmasses must have been transported passively by 0 local distributions of the various marine groups are 2 flotsam, etc. or have migrated during shallower times. d Thus each such continent has many endemic species, e.g. analysed. e t a c. 47% of the echinoderms occurring in southern Africa d ( Review of biogeography are endemic to this region. A high proportion of ende r e mic species has also been reported for other groups by The biogeographical history of southern Africa has h s i various workers (Day 1967; Griffiths 1974; Millard 1978; recently been well reviewed by Brown & Jarman (1978) l b Kilburn & Rippey 1982; Williams 1986; Gosliner 1987).

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