Discover Life -- Shorefishes of the Tropical Eastern Pacific -- Zoogeography
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Works on the region's zoogeography

Charles Darwin's name inevitably appears in any historical account of marine zoogeography. In The origin of species by means of natural selection (Darwin, 1872, p 279) he briefly discussed two key topics of modern debate: (i) the close relationships between fishes on the two sides of the isthmus of Panama, and (ii) the significance as a barrier of the 4,000-7,000 km wide expanse of ocean separating the west coast of the Americas from the islands of the central Pacific, a barrier that we know as the Eastern Pacific Barrier (EPB). Early works on global patterns of marine zoogeography [ Zoogeography of the sea by Ekman (1953), and Marine Zoogeography by Briggs (1974)] emphasized the importance of isolation produced by the EPB, as have studies of the TEP's shorefishes (Briggs 1961, Rosenblatt 1967, Rosenblatt et al. 1972) and other groups of organisms (e.g. echinoids - Mayr 1954, and molluscs - Vermeij 1987). Springer (1982) examined the zoogeographic affinities of shorefishes found on the Pacific Plate, which extends from the northwestern Pacific to the northern part of the TEP, and on which Clipperton, the Revillagigedos and the Baja Peninsular are situated. Aspects of the biology of the pelagic larval stage of reef fishes that may affect the potential for colonization of the TEP by central Pacific fishes have been addressed by Leis (1984, 1986), Victor ( 1987), and Victor et al. (2001).

Realization of the close relations of the TEP shorefish fauna to that of the tropical western Atlantic began with Gunther (1868), who proposed that the two regions once were connected. Discussion of the evolutionary significance of that relationship has continued ever since - Jordan (1905, 1908), Rosenblatt (1967), Briggs (1970), Thresher (1991), Robertson (1998).

A considerable amount of attention also has been given throughout the past century to the zoogeography of the distributions of endemic species within different parts of the TEP, particularly the relationship of the fauna of the Gulf of California to that of the rest of the region - Gilbert and Starks (1904), Meek and Hildebrand (1928), Hubbs (1952, 1953), Briggs (1955), Springer (1959), Walker (1960), Rosenblatt (1963), Rosenblatt and Walker (1962), Stephens (1963), Thomson and Gilligan (1983), Hastings (2000).

Grigg and Hey (1992) assessed the geological history of the EPB, while Veron (1995) and Glynn and Ault (2000) provide modern zoogeographic overviews of coral reef development in the TEP; the latter incorporate much general information that is relevant to the zoogeography of the region's shorefish fauna.

Recently, genetic analyses have begun to be applied to zoogeographic questions about the TEP shorefish fauna: the degree of ongoing connections with the central Pacific (Rosenblatt and Waples 1986, Bowen et al. 2001, Colburn et al. 2001); the history of connections between the TEP and west Atlantic broken during the rise of the Isthmus of Panama (Gormon and Kim 1977, Gormon et al. 1976, Vawter et al. 1980, Tringali et al. 1999, Lovejoy and Collette, 2000, Reed et. al., 2001) and of more recent connections between populations of warm temperate species currently restricted to the north and south of the TEP (Grant and Bowen, 1998); as well as relationships between geographically separated populations of the same or sister species within the TEP (see Lessios et. al. 1995, Muss et. al. 2001, Riginos and Nachman, 2001, and Riginos and Victor, 2001).

The coastal marine environment and habitats of the region

The marine environment of TEP is both very dynamic and variable geographically. In fact it must have one of the most dynamic coastal environments of any tropical region due to the regular influence of El Niño or ENSO events; the TEP is more strongly affected by that phenomenon than any other tropical region. Glynn and Ault (2000) have summarized the characteristics of the marine environmental regime in the TEP and how it affects coral reef development there, and much of the following is drawn from their article:

Current systems: The northern and southern limits of the region are defined by the points at which the cold California and Peru Currents turn westward. Conditions of the two largest groups of oceanic islands (the Galapagos in the south, and the Revillagigedos in the north) are strongly influenced by those cold currents and the shorefish faunas of both archipelagos contain species whose distributions are concentrated in the warm temperate provinces. Within the region the main coastal current is the Costa Rican Coastal Current, which flows northward from that country and into the mouth of the Gulf of California. Currents are more complex in the equatorial section, with a large anticlockwise gyre in Panama Bight and the Panama Current flowing southwest towards the Galapagos. The main eastbound current entering the region the North Equatorial Counter Current (NECC) from the central Pacific arrives in the equatorial part of the TEP, between 0-5 degrees N. The western side of the Galapagos is influenced by the eastbound Equatorial Undercurrent which surfaces there and produces cold upwelling conditions. During ENSO events the NECC strengthens and widens, and produces a surge of relatively hot water from the central Pacific that first hits the Galapagos and the equatorial part of the TEP. That surge then spreads northwards and southwards along the continental shore, and can affect virtually the entire region as well as the temperate regions to the north and south of the TEP.

Shoreline habitats: The continental shelf of the TEP is relatively narrow, averaging about 25km wide and broadening to about 150 km in only two places, the Gulf of Panama and the Gulf of Guayaquil (Ecuador). Much of the coastline is soft bottom, with large areas of sandy beaches, mangroves and estuaries. The largest belt of mangroves is in the equatorial section, around Colombia. There are two large sections of such sandy shoreline, one 370 km long at the southeastern edge of the Gulf of California between Topolobampo and Mazatlán and another 1,000 km long between the Gulf of Tehuantepec (southern Mexico) and El Salvador. There are two major continental areas of rocky shore with an abundance of near-shore islands that provide this same type of habitat - one in the Gulf of California (the western, northern and south-east parts) and one between Costa Rica and Panama. The oceanic islands all rise abruptly out of deep water and contain little soft bottom habitat. Clipperton is the only site in the region that consists exclusively of coral reef habitat.

Variation in rainfall and river input: Most of the TEP's continental coastline north of Costa Rica is relatively dry, although there are some small areas with moderate rainfall, such as around Puerto Vallarta at the SE corner of the Gulf of California. The highest rainfall and river discharge areas are restricted to the equatorial section (10 degrees N to 4.5 degrees S), which contains two main areas of high coastal rainfall, one between eastern Costa Rica and western Panamá and the other (larger) area between eastern Panama and northern Ecuador (see Garcia et al. 1992 and Glynn and Ault, 2000). Two of the TEPs largest estuarine systems with high freshwater outflows are situated in the Gulfs of San Miguel (eastern Panama) and Guayaquil (Ecuador). The third large estuary, in the Gulf of Fonseca (shared by El Salvador, Honduras and Nicaragua), is a high-salinity system in a low-rainfall area.

Coastal upwelling systems: Large sections of the coastline of the TEP are affected by upwellings. There are three large, seasonal (winter) continental upwellings - one at the Gulf of Tehuantepec in southern Mexico, one at the Gulf of Papagayo at northern Costa Rica and one at the Gulf of Panama, which affects eastern Panama and Colombia. Low temperatures and high nutrient levels that exist during these upwellings inhibit coral reef development, notably in a large part of the equatorial section of the TEP. In the Gulf of California reversing seasonal winds produce upwelling on the west coast in autumn and on the east coast in spring. Seasonal plankton blooms produced by shifting wind-driven circulation are strongest in the autumn and winter. The western Galapagos also has a regular upwelling. These upwellings may have intensified during glacial periods.

Coral reef development: Opportunities for coral reef development in the TEP are limited, by generally unfavorable conditions in the narrow coastal zone- sedimentation and freshwater input from river runoff, and large areas of upwelling with cold water and nutrient enrichment. However the regular impact of ENSO events likely is the strongest limiting factor on coral reef development in the TEP. ENSOs occur at 4-9 year intervals, and at intervals of about 1-3 centuries there is an intense ENSO like the event of 1982-83, which produced mass coral mortality throughout much of the region. ENSOs are a major reason why the TEP has only a few, small coral reefs, and a small coral fauna that includes few endemic species. The only significant coral reef in the entire TEP is at Clipperton Atoll, a 4km2 reef 1,100km offshore from the Mexican coast, well away from coastal influences and at the fringe of the zone of greatest impact from ENSO-produced high water temperatures.

Varying conditions in the three climate zones: The equatorial section of the TEP, from Costa Rica to Ecuador, is the most complex part of the region in terms of its nearshore currents, variation in conditions due to upwelling and non-upwelling areas, the degree of impact from ENSOs, and variation in the mixture of large scale habitat types (rocky and soft-bottom shorelines, presence of large, active estuaries and areas of mangroves, the presence of complexes of nearshore islands). The subtropical part of the region between northern El Salvador and the SE corner of the mouth of the Gulf of California is much more homogeneous, with a relatively straight coastline with very few islands (other than in the Fonseca estuary), a simple current system, dry coastal conditions, a single upwelling system, and an abundance of soft-bottom shoreline. The Gulf of California, which constitutes the subtropical part of the region, offers a complex and dynamic set of environmental conditions (see Castro-Aguirre et al. 1995). The range of sea surface temperatures increases greatly towards the northern Gulf (to as much as 28 degrees C), where the tidal range also is much more extreme than at the mouth of the Gulf. The separation of the areas with different temperature regimes in the upper and central Gulf is enhanced by a zone of permanently cold water around the midrift islands. The range of sea temperatures in much of the Gulf is similar to that in the warmer parts of the Californian Province, and led Briggs (1974) to include the Gulf as part of that biogeographic province. Sea temperatures differ on the two sides of the Gulf, with sites on the east coast having higher temperatures than sites at similar latitudes on the west coast of the Gulf (e.g. 22-28 C at Mazatlán vs 14-26C at La Paz). During spring and summer sea surface temperatures in the Gulf are about 10C higher than at locations at the same latitude on the Pacific coast of Baja. Conditions at the tip of Baja are sufficiently subtropical (the temperature range is only 9 degrees C) for it to provide a small outpost of coral development, with many tropical fishes found there but nowhere else in the Cortez Province. Geographically the Gulf is complex with many nearshore islands along the western coast, and a group of islands straddling the junction between the upper and lower Gulf. There are localized upwelling areas on both east and west coasts. Salinities of estuaries ranges from high in the north to more variable in the south where they are fed by flowing rivers (the Colorado estuary at the northern limit of the Gulf is no longer river fed). The strong environmental gradients within the Gulf are reflected in differences in the distribution of shorefishes within the Gulf that are sufficiently large for ichthyologists to have described distinct biogeographic subdivisions of the Cortez Province (Walker 1960, Thomson et al. 1979).

Zoogeography of the region's shorefish fauna

Compared to the tropical area in the Indo-Malayan global center of diversity, the eastern Pacific has a relatively impoverished shorefish fauna. Here we cover 1,195 species, 1,087 of which are restricted to inshore habitats, and 978 of which are entirely or largely restricted to the TEP (the remainder are Californian or Peruvian Province species that enter the fringes of the TEP). However, with 870 species of endemics the TEP has the highest rate of endemism (79.3% of the resident shorefishes) of any tropical region of its size.

The reasons for this relative impoverishment are complex, but several are major contributors. One involves the size of the region. Although the mainland coastline spans nearly 33 degrees of latitude, it is relatively straight and has a fairly narrow continental shelf. Further, there are only a few offshore islands, which, between them, provide <2% of shorefish habitat in the region, with 97% of that habitat occurring in only one island group, the Galapagos. The largest such island areas (the Revillagigedos and the Galapagos), which together contain 99% of the insular habitat in the region, are at the northern and southern limits of the TEP. Hence they have cooler temperatures that likely limit the ability of tropical species to survive. Thus the geography of the TEP is fairly simple and the amount of shoreline habitat is relatively small and most of it is continental. In contrast the sister region of the TEP, the Caribbean, is much more complex geographically, with large areas of continental shelf and many offshore islands. There is also a significant reduction in the abundance and diversity of coral reef habitats in the TEP compared with the Caribbean and the remainder of the Indo-Pacific. Our region contains only about 25km2 of structural coral reef, and only about 45 species of reef building corals are present. Most of the coral reefs are constructed by very few species of corals. In addition there are no extensive seagrass beds such as those found in association with most areas of coral reef in the Caribbean and some parts of the Indo-west Pacific. In contrast the Caribbean has some 20,000km2 of coral reefs and vast areas of seagrass. Last, but not least, the region is isolated by about 4,000-7,000km of deep water from the reefs and atolls of the Central Pacific, which are the nearest contemporary source of immigrant shorefishes; only about 7% of the shorefishes found in the tropical eastern Pacific seem very likely to have arrived there from the central Pacific in the very recent past. Many of the 86 circumtropical species that occur in the TEP may simply be derived from populations split from those in the Caribbean by the rise of the Central American isthmus. About 1/3 of the 65 circumtropical species found on both sides of that isthmus likely fall into this category, because they are either absent from or rare in the central Pacific.

ENSOs are thought to enhance immigration of shore organisms from the central Pacific to the TEP because the surge of water doubles the velocity of the NECC and cuts in half the the time needed to transit the 4,000-7,000 km of open ocean separating the TEP from the central Pacific islands. However, while that surge does bring that transit time to within the range of the duration of the pelagic larval stage of many reef fish species, whether this actually results in enhanced immigration that has permanent effects on the composition of the TEP fish fauna is unclear. In recent years an increasing number of transpacific fishes have been reported in the Galapagos. While that may be due to enhanced immigration during El Niño events, those sightings may also reflect increases in the amount of diving activity there during the past 3 decades. At present there is little evidence that ENSO currents do more than bring the odd vagrant of a few species rather than help add new resident species to the TEP shorefish fauna. ENSO events do affect the shorefishes of the TEP, most noticeably in the Galapagos. In those islands conditions normally range from warm temperate in the south (due to the cold Peru current) and west (due to a cold upwelling) to equatorial in the north. ENSO's produce tropical conditions throughout the islands, with large increases above normal equatorial temperatures. The 1982-83 event produced mass mortality of resident shorefishes and other marine life in the Galapagos, but also produced influxes of species from the mainland that normally are rare in the archipelago (Grove 1984, 1989). One of the shorefishes that was known only from the Galapagos, the Galapagos damsel, Azurina eupalama, has not been seen in the islands since that event and may have become extinct. Perhaps however, it has a refuge in offshore islands to the south of the Galapagos. During the 1997-98 ENSO range extensions were again noted, but this time involving not only mainland species at the Galapagos but also insular species between islands and to the mouth of the Gulf of California at the other end of the region (see Ruttenberg 2000 and Victor et al. 2001). In addition there were changes in the abundances of species already present at the Galapagos (Ruttenberg 2000). While conditions in the TEP are unfavorable for the development of coral reefs and the maintenance of a significant coral fauna, what effects such conditions have on the size and composition of the region's fish fauna is unclear. That fauna is fairly similar in size to that of the TEP's sister region, the Greater Caribbean, both overall and in terms of the reef-fish component (Robertson 1998, and see Smith 1997 for information on the Caribbean fauna). Apparent differences in the size of the reef-fish faunas, with the TEP thought to have ~20% fewer species, were attributed to the lack of coral reef development in the TEP. However, part at least of such differences are due to lower levels of sampling in the TEP, which has been partly rectified, especially over the past decade. At present there is little suggestion that ENSO effects on the region's shorefish fauna extend beyond relatively minor impacts on local distributions and immigration from the central Pacific.

Relationships with other tropical faunas

The tropical Eastern Pacific shorefish fauna has a strong relationship with the Western Atlantic shorefish fauna, but also significant levels of endemism, particularly at the species level. This pattern is the result of the history of isolation of the region. For perhaps as much as 65 million years (Grigg and Hey 1992) the shoreline biota of the TEP has been isolated from that of the central and western Pacific by the world's largest open ocean barrier to connections with the shore faunas, the "Eastern Pacific Barrier". That barrier currently is 4,000-7,000 Km wide and apparently has been about as wide as this throughout that period. The slow northward movement of the Line Islands, which brought them into the region of the eastbound North Equatorial Counter Current about a million years ago, likely has facilitated some eastward immigration of marine shore organisms. Isolation of the region increased with the final closure of the isthmus of Panama, about 3 million years ago (Coates and Obando 1996), although separation of the tropical biotas of the eastern Pacific and Caribbean began as much as 10 million years ago as the central American isthmus gradually formed (Lessios, 1998). The long-term western barrier and the more recent, and much more complete, eastern barrier have given the fish fauna of the TEP its distinctive characteristics. Rosenblatt (1967) noted that while the shorefish faunas of the TEP and the tropical western Atlantic are both dominated at the genus level by circumtropical genera (over 40% in each case), about 25 % of the genera found in the american tropics occur only on both sides of the Central American Isthmus, and 35% of the eastern Pacific genera are such neotropical genera. Among the 362 TEP genera he considered, 14% are endemic to the region. Our figures for the 467 genera of non-introduced fishes included here are much the same: 44% are circumtropical, 30% are neotropical (or also found in the eastern Atlantic) and 16% are endemic.

At the species level regional endemism is much higher, presumably because the isthmus rose so relatively recently that new taxa that have arisen have not had sufficient time to accumulate differences that would warrant their classification as new genera, and extinctions need a long time to change the genus level composition of a fauna. Of the 1,195 species included here 72.8% are endemic to the TEP. Most of the remaining species either have circumglobal distributions (7.1 %) or also occur elsewhere in the Indo-central Pacific (8.7%). While 79 species occur on both sides of the Central American isthmus, most of those are circumtropical species and only 1.1% of the fauna consists of neotropical species that occur naturally on both sides of the central American isthmus (another 0.9% have been introduced from the west Atlantic, either intentionally or by making there way through the Panama Canal). 9.3% of the TEP fauna consists of species whose distributions are concentrated in the temperate regions to the north (the California Province - 51.4%) and south (Peruvian Province - 36.9%, both provinces 11.7%).

The low percentage of species shared with the Western Atlantic is somewhat deceiving as there are numerous species pairs composed of close relatives that occur on either side of the Central American isthmus. The members of these pairs are very similar in appearance and are often referred to as geminate or twin species. The spadefishes of the genus Chaetodipterus present a good example of this phenomenon; C. zonatus of the Pacific is nearly identical to the Atlantic, C faber. Similarly, the sergeant major Abudefduf troschelii looks virtually identical to its west Atlantic sister, A. saxatilis. Numerous examples of similar twins occur in many other families, and all six members of the snook genus Centropomus are paired with look-alike twins in the Caribbean. The high number of shared genera and occurrence of twin species reflects the common ancestry of the Eastern Pacific and West Atlantic faunas, and their recent separation by final closure of the uplifting Central American landbridge.

As well as being relatively small in size compared to the fauna of the Indo-West Pacific, the shorefish fauna of the Eastern Pacific tropics has a special "flavour" imparted by its large number of endemic species and the unique faunal "mix" of fishes. Many coral reef families that are well represented in the Indo-West Pacific have few species in our region, although the relative abundance of species in major reef-fish families is fairly similar to the relative abundance in the fish fauna of the Caribbean, which is rich in coral reefs. However, several families have undergone a tremendous radiation in the eastern Pacific; these include the Sciaenids, with 78 species of primarily soft-bottom fishes, and, to a lesser extent, the Gobiesocids, with 41 species that live mainly in rocky habitats. Our region also has a large indigenous representation among the blennioid families - the reef-dwelling Labrisomidae and Chaenopsidae (37 and 33 species, respectively), and the soft bottom Dactyloscopidae (23 species). All three of those families are well represented in the western Atlantic shorefish fauna but absent from the remainder of the vast Indo-central Pacific. In fact the TEP represents a global hotspot of diversity for those five families: 30-50% of the members of each family are endemic to the TEP, which collectively provide 23% of the region's endemic shorefishes.

The TEP fauna includes about 8% vagrants. Two thirds of these are from the temperate areas to the north and south of our region. The remainder probably originated in tropical areas of the central Pacific, although one at least (Cyclichthys spilostylus) must have come from the western Pacific, as it is not known from the central Pacific.

Since 1914 the Panama Canal has provided opportunities for partial reconnection of the shorefish faunas of the TEP and tropical Atlantic, by species that can tolerate the freshwater environment of the canal. However, there have been surprisingly few cases (7 species) in which Caribbean fishes have entered the TEP by this method (McCosker and Dawson, 1975), and it is unclear whether any of those have successfully spread beyond the Pacific entrance to the Panama Canal. Of three west Atlantic fishes, two silversides and one shad, that were intentionally introduced into the TEP, only the shad seems to have become established.

Zoogeographic subdivisions of the TEP

The number and limits of major zoogeographic subdivisions or provinces within the tropical eastern Pacific has been a matter of debate for 50 years (Hubbs 1952, Briggs 1955, Springer 1959, Stephens 1963, Briggs 1995, Hastings 2000). Here we follow Hastings (2000) and include three major continental provinces, with these divisions being based on the distribution patterns displayed by species of rocky-shore fishes. The two large stretches of soft-bottom shoreline in the SE Gulf of California and between southern Mexico and El Salvador define the limits of faunal provinces for the regions reef fish fauna (Springer 1959, Walker 1960, Rosenblatt and Walker 1962, Stephens 1963, Rosenblatt 1967, Hastings 2000). The northern or Cortez Province includes southern Baja California, and the central and southern Gulf of California. On the eastern coast of that Gulf that province extends as far south as Topolobampbo, Sinaloa. The Cortez province is isolated from Mexican Province to the south by the Sinaloan Gap, a 370 km stretch of sand and mud coast between Topalabama and Mazatlán. The latter province extends down to the northern edge of Gulf of Tehuantepec, in southern Mexico. The Cortez and Mexican Provinces both have endemic species, with a high level of endemism in the Cortez Province. Springer (1959) proposed the name "Pacific Central America Faunal Gap" for the area between the Gulf of Tehuantepec and the Gulf of Fonseca, which is shared by El Salvador, Honduras and Nicaragua. He noted that this 1200km stretch of coast was apparently devoid of rocky reefs, consisting instead of either sand, mud, or mangroves, and that it coincided with the distributional limits of certain species of clinid fishes (now grouped in Labrisomidae). More recent observations show that El Salvador has rocky shore in the Gulf of Fonseca and along its Pacific coastline northwards to around Acajutla, near the border with Guatemala. Hence here we use Acajutla to define the southern edge of the Central American Gap, which reduces to 1,000km in width. The southern or Panamic Province extends from El Salvador southward to about Cabo Blanco, in northern Peru. The status of the oceanic islands in subdivisions of the TEP is discussed below.

Continental and island components of the regional fauna

Shorefishes endemic to the continental shoreline of our region constitute 55.8% of the 870 regional endemics and island endemics another 11.4%; the remaining 32.8% are found on both the continent and at least one island. Cortez Province endemics represent 9.3% of the 486 continental endemics, Mexican Province endemics 2.9%, and Panamic Province endemics 29.6%; 9.9% are shared by the Cortez and Mexican Provinces and 10.3% by the Mexican and Panamic Provinces, while 38% occur in all three provinces. Rates of endemism within each province (the % of the inshore fishes represented by local endemics) range from 2.3% for the Mexican Province, to 6.9% for the Cortez Province and 18.2% for the Panamic Province. The level of endemism of each continental province would rise if calculations took into account (i) geographically separated subspecies of a few widespread species (principally of blennioid fishes), (ii) species that are principally mainland forms and that have been recorded at only one island, often apparently as vagrants, and (iii) species that are common and distributed in one province, but are rarely encountered in a neighboring province.

The volcanic Galapagos Archipelago, lying 930 km off the coast of Ecuador, is composed of 13 major islands and numerous small islets. The Galapagos fauna, containing approximately 348 shorefish species, is an blend from the tropical Eastern Pacific (74%), temperate South America (5.8%), and Indo-central Pacific (20.2%), and there is a significant percentage of endemism, currently estimated at 34 (9.8 % of the fauna), while another 18 are island endemics that also occur on other oceanic islands). The faunal complexity of the Galapagos is partly explained by its relatively large size (shoreline habitats in this island group represent 95% of the shorefish habitat at the region's offshore islands) and diversity of habitats (which include mangroves, extensive sandy areas, rocky reefs, and (prior to the 1982-83 El Niño event) a few small areas of good coral growth. There is also a complex interaction of warm tropical seas, in the northern part of the islands, and cold waters of the Peru or Humboldt Current in the southern part and cold upwelling on the west side of the islands, which provides the potential for the occurrence of warm temperate as well as tropical species.

Malpelo Island is situated 380 km off the Colombian coast. This rocky outpost is volcanic in origin, largely devoid of vegetation, and rises steeply to an elevation of approximately 450 m. The main island is 2 km long and about 1 km at its widest point. In addition, there are a number of rocky outcrops and pinnacles in the immediately adjacent waters. About 213 species of shorefishes have been recorded at and around the island, including five endemics (2.3% of the fauna). Most of the fauna is eastern Pacific mainland in composition, although eight species are shared only with the Galapagos and/or Isla del Coco. Malpelo has a very dynamic environment - it is affected by the seasonal upwelling of the Gulf of Panama and lies directly in the path of the eastward ENSO surge.

Isla del Coco lies about 560 km northeast of the Galapagos and about 495 km southwest of Costa Rica. It is a small, steep-sided, well vegetated island with a land area of about 47 km2. Some 235 shorefishes are known from the island, of which about 72% are eastern Pacific endemics, while the rest are either Indo-central Pacific or circumglobal species. The Cocos fauna includes 12 species of named shallow marine endemics (5.1% of the fauna). Plus 16 known only from Cocos and other offshore islands. In addition there is one undescribed shallow water endemic, 8 deep-water endemics and 3 freshwater endemics.

Tiny Clipperton Island is the only true coral atoll in the eastern Pacific and the only site in the TEP at which all habitats are formed by coral reef. This small reef is only about 3 km in diameter and lies about 1,075 km from the mainland, southwest of Acapulco, Mexico. The 102 species of shorefishes of this remote outpost include a much higher percentage of transpacific species (~50%) than any other island in the eastern Pacific, as well as 7 species and 2 subspecies of endemics, plus 8 insular endemics known from other islands.

The Revillagigedos Islands, consisting of three small volcanic islands and an isolated rock spread several hundred kilometers apart on an east/west axis, lie about 405 km south of the tip of Baja California. Although the shorefish fauna has not been throughly investigated, at least 177 species occur there, including 17 endemics (9.6% of the fauna) and 9 other insular endemics shared with other islands. That fauna likely is significantly larger than this figure because these islands offer a larger and more complex set of habitats scattered over wider area than do equally isolated but single islands like Cocos and Malpelo, which have better known and larger faunas.

An oceanic island Province?

Due to their relatively high level of endemism the Galapagos Islands have sometimes been considered to represent a fourth province of the tropical eastern Pacific (e.g. Briggs 1974, 1995). The status of the other oceanic islands has been left largely hanging, with some assigned to the nearest continental province. This focus on the Galapagos is not surprising, given their large size (they contain ~95% of the island habitat in the TEP), variety of environments and abundance of endemics and other species of shorefishes. However, a case can be made for an Oceanic Island Province that encompasses all five offshore islands and archipelagos. Each has its own endemics (varying from 2.3-10.3 %); 46.5% of the island endemics occur at islands other than the Galapagos; 24% of the insular endemics occur on multiple islands; and each island shares a significant number of insular endemics with at least one other island. Further, the density of endemics (no. per Km2 of habitat) is much greater at any of the islands than on the mainland and the percentage of endemics in the fauna of any island and the five islands collectively is at least as great as that at the Mexican Province. The aggregate rate of endemism in the island fauna (endemics as a % of the inshore species) is 19.5%, which is greater than that of any of the three mainland provinces (2.3-18.2%).

Besides their patterns of endemism the faunas of each of the islands have other shared characteristics that imbue the combined island fauna with distinctive characteristics that set it apart from the fauna of any of the continental provinces: For example, transpacific species are much better represented in the islands' faunas than in any part of the continental fauna. Further, soft bottom species are much more abundant in the provincial continental faunas than the island faunas. Collectively the shallow habitat available to shorefishes at the oceanic islands represents less than 2% of the shorefish habitat in the entire tropical eastern Pacific. However, despite the small size of those islands their fish faunas together include 44% of the region's endemic fishes and 50% of the entire regional fauna considered here. Insular endemics are four times more abundant relative to the amount of habitat available than are continental endemics. This difference presumably is largely due to the isolation of the islands (from each other as well as from the mainland) having promoted the development of island endemics. There are other differences between the island and continental faunas of the TEP that reflect differences in their marine environments. The oceanic islands are small and rise abruptly from great depths. The two largest groups of islands (the Galapagos and the Revillagigedos) have dry climates. As a result both the amounts and diversity of shallow soft-bottom habitats are low. Not surprisingly therefore the island fauna has relatively few species that are restricted to these habitats: such species constitute 56.3% of the coastal species found on the continent, but only a third of those found at the islands, and while soft-bottom species restricted to the continent represent 40.7% of the continental inshore fauna, the analogous figure for island species is 3.5%

Thus a reassessment of faunal relationships throughout the region, and particularly of the oceanic islands, is needed to define its biogeographical subdivisions through consideration of both patterns of local endemism and a variety of other characteristics of the shorefish faunas of different parts of the region. Here we treat each of the five oceanic islands as a separate entity.

Distribution of the fauna in climate zones

The ranges of the vast majority (83.8%) of the region's shorefishes include at least part of the equatorial zone. However, large proportions of the regional fauna also occur in the northern tropical zone between 10-23 N (64.6%) , and the northern subtropical zone between 23-28N (62.4%). Significant proportions of the regional fauna have also been recorded in the temperate zones outside our region to the north (40.1%, including 37.6% of the TEP endemic fauna) and south (34.3%, including 30.8% of the TEP endemic fauna). Many of the occurrences of temperate species into our region and by tropical species into the temperate areas probably are due to traffic by vagrants rather than the occurrence of resident populations in both temperate and tropical areas, with such traffic occurring during either El Niño events, when warm water flows from our region into neighboring temperate regions, or La Niña events when colder conditions prevail in our region.

Patterns of variation in species richness throughout the region

The TEP fauna includes 1,089 known species of shorefishes. As measured by overlap in ranges of different species the sections of the coastline with the greatest numbers of species (700-740) occur along Costa Rica and Panama. Species richness per section of coastline declines to the north and south of these two countries but slowly - the southern coast of Ecuador and the southern edge of the mouth of the Gulf of California still have about 600-630 species, and the tip of Baja California and the south-western Gulf of California have about 585. Even sites near the limits of the region (northern Peru, the central Gulf of California and southern Baja) are included in the ranges of ~450 species.

Looking at the pattern of variation in species richness in greater detail we find that the overall pattern described above, with the greatest richness in the Costa Rica/Panama area also occurs in many species-rich families (e.g the Muraenids, Engraulids, Clupeids, Ariids, Batrachoidids, Atherinids, Sciaenids, Blenniids, Gobiids, and Tetraodontids). Other families follow this same general pattern but with the richest zone spread more widely to the north (to central Mexico) and/or south (to Ecuador) of the Costa Rica/Panama section; these include the Urolophiids, Dasyatids, Ophichthids, Scorpaenids, Lutjanids, Haemulids, Dactyloscopids, and Cynoglossids. In a few families (Carcharinids, Serranids, Carangids, and Scombrids) richness is spread nearly uniformly throughout the three major provinces, without any obvious concentration. Other families have more idiosyncratic richness distributions: the Pomacentrids have slightly greater richness around the mouth of the Gulf of California and at the offshore islands (except Clipperton); in the Tripterygiids and Labrisomids richness is greatest in the northern part of the region, around the Gulf of California. Chaenopsids and Gobiesocids have two centers of richness - the Gulf of California and Nicaragua to Panama. Care should be taken in interpreting this numeric displays because some apparent centers of richness at the tip of Baja reflect the occurrence of vagrant Californian Province species in the northern edge of the TEP.

Regional patterns of richness among species with different biological attributes often follow the general pattern (greatest richness in Costa Rica/Panama) described above. This occurs for species resident in the TEP (not surprisingly vagrants are concentrated at the edges of the region), those living on sand and mud bottoms, or in the water column; demersal or bottom living species in general; species living inshore and those living offshore; l carnivorous and planktivorous species; and non-marine species. Other groups show somewhat different patterns: reef living species and marine species show a broad peak in richness between central Mexico and Colombia; omnivorous species have two peaks (Costa Rica/Panama and the Galapagos). The most atypical pattern is for transpacific fishes: while there is a peak of richness in Costa Rica and western Panama (about 146 species) the same number occur at the Galapagos and slightly lower numbers at the other offshore islands (123 at the Revillagigedos, 109 at Clipperton, 125 at Cocos and 110 at Malpelo). In terms of the percentage of the fauna represented by those transpacifics they are much more important components of the island faunas than the continental fauna: while they constitute 18.6% of the Costa Rica/Panama fauna their percentage representation in the oceanic-island faunas is 68% for Clipperton, 49% for the Revillagigedos, 41.7% for Cocos, 38.2% for Malpelo and 33.9% for the Galapagos.

The general pattern of latitudinal change in species richness described here for the TEP fish fauna (declining richness with increasing latitude) is an example of a general phenomenon for animals and plants in marine and terrestrial environments. For coastal marine organisms this has been ascribed to latitudinal variation in energy supply, as reflected in sea surface temperatures, and nutrient levels due to river inputs and upwellings (see. Macpherson, 2002, for a recent discussion). In the present case we need to consider the extent to which the regional pattern is influenced by differences in sampling effort at various parts of the TEP. While all countries have been sampled there has been a concentration of effort in two areas - the Gulf of California, and Costa Rica to Panamá. Despite the coincidence of these areas of enhanced sampling and species richness it seems likely that the sampling is not the primary factor involved. Those two areas also have features likely to support richer faunas, namely a diversity of environmental conditions, including: large groups of nearshore islands that offer a range of turbidity conditions; large, semi-enclosed bays; large highly active estuarine systems; substantial areas of both rocky shores and soft-bottoms; and areas with varying or different temperature and productivity regimes (e.g. Panama and Costa Rica both have areas with and without large seasonal upwelling systems). The Costa Rica - Panamá area, with the highest species richness in the TEP, also is in the region with highest sea surface temperatures and high levels of input of nutrients from rivers and upwellings.

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