Larval jish dispersal and the East Pacijic Barrier
Jeffrey M. IA~ls (1)
The assumplion that Lndo-Pacifie elements of fhe East Pacifie reef ichihyofauna were established and are genetically connected with Central Pacifie conspecifics via pelagic dispersa1 across the East Pacifie Barrier is examined. Indirect evidence fram four areas supports fhe pelagic dispersa1 potential of ihe transpacifk fishes: presence of pelagic eggs, presence of pelagic larval or juvenile stages, size of pelagic stages, and duration of pelagic stages. The common oceanic occurrence of many of these larvae is further indirect support for pelagic dispersa1 potential. However, many non-transpacific reef fishes have similar characteristics. Direct evidence for cross-Barrier pelagic dispersa1 from distributional siudies is weak: a few pelagic staqes of transpacific fishes have been reported within the East Pacifie Barrier, but unequivocal crossings of a major porkon of the Barrier have no1 been documented. The vicariante alternative is supported by some indirect evidence [positions of some islands and submerged ridges, and fossil corals on submerged ridges). However, the East Pacifie coral reefs and fauna may be less than 5,000 years old, which, if true, favours pelagic dispersal. On available evidence, neither the dispersa1 nor the vicariante hypoihesis cari be confirmed or rejected, and some directions for further research are suggesfed. I LA DISPERSION ni POISSONSRT BARRIÈRE EST-PACIFIQUE On étudie l’hypothèse selon laquelle des éléments indo-pacifiques de l’ichtyofaune corallienne de l’Est-Pacifique se seraient établis dans cette région par le biais d’une diffusion pélagique à travers la barrière Est-Pacifique et seraient génétiquement relit2 aux éléments conspécifiques du Pacifique central. Des preuves indirectes provenant de quatre régions plaident en faveur de la possibilité de dissémination trans-pacifique des poissons: présence d’cpufs pélagiques, présence de stades pélagiques larvaires ef juvéniles, taille et durée des stades pélagiques. La présence fréquente de beaucoup de ces larves est une confirmation indirecte supplémentaire de l’hypothèse de dispersion. Cependant, de nombreux poissons coralliens non trans-pacifiques ont des caracièristiques semblables. La démons- tration directe du passage à travers la barrière par une étude des distributions n’est pas décisive: quelques stades pélagiques de poissons irans-pacifiques orzf ètè signalés dans la barrière E’st-Pacifique, mais SOJI franchissement franc et massif n’a pas été rapporté. L’hypothèse alternative de vicariante peut être soutenue par des observafions indirectes (posifion de certaines îles et de dorsales sous-marines, présence de coraux possibles sur ces dorsales). Toutefois, la faune et les récifs coralliens de l’Est Pacifique pourraieni ne pas avoir atteints l’âge de 5 000 ans, ce qui serait en faveur de la dissémination. A ce stade des études, ni la dissémination, ni la vicariante, ne peuvent être confirmées ou infirmées et quelques proposiiions d’approfondissemeni sont faites. MOTS-CLÉS : Larves de poisson - Vicariante - Zoogéographie - Océan Pacifique. (1) The Ausfralian Museum, P.O. Box A28.5, Sydney, N.S.W. 2000, Ausfralia. Océanogr. trop. 19 (2): 181-192 (1984). 182 J.-M. LF:IS INTRODUCTION (1981). Taxonomie revisions that have become available since 1972 for the other groups in Table 1 The strong Indo-Pacifie element in the reef fish confirm the speciflc identify of populations from fauna of the East Pacifie (as high as 24 %, either side of the East Pacifie Barrier (e.g. Myri- ROSENBLATT et al., 1972) is usually assumed 10 prisfis, GREENFIELD, 1974; Forcipiger, BURGESS, have arrived from, and to maintain genetic inter- 1978, ALLEN, 1981; Scarus, HANDALL and BRUCE, change with, the central Pacillc by larval dispersa1 1983; Ostracion, RANDALL, 1972; Canthigaster, ALLEN across the 6,500 km-wide deep-water gap called by and RANDALL, 1977; Diodon, LEIS, 1978). EKMAN (1953) the East Pacifie Barrier (BRIGGS, 1961; ROSENBLATT and WALKER, 1963; ROSENBLATT et al., 1972). However, neither documentation of the CURRENTS OF THE EAST PACIFIC BARRIER dispersa1 abilities of the fshes which occur on both sides of the East Pacifie Barrier (i.e. transpacific Given the limited swimrning abilities of the species) nor review of direct evidence of dispersa1 pelagic stages of most reef fishes, any pelagic dispersa1 across the Barrier by reef fishes is available. Little across the East Pacifie Barrier must depend largely is known of the biology of larval stages of reef on currents. Therefore, an understanding of the fishes, SO assumptions about their dispersa1 abilities currents of the area is a prerequisite to a discussion should be carefully examined. of dispersal. Here 1 present some new information and review Currents of the East Paciflc Barrier have been published evidence on dispersa1 abilities of larval reviewed by WYRTKI (1965, 1966, 1967) and and juvenile stages of the transpaciflc reef fishes specifically as they apply to cross-Barrier pelagic and how this relates to the question of cross-Barrier dispersa1 by ROSENBLATT et al. (1972). Briefly, the pelagic dispersa1 and the vicariante alternative. ‘In North Equatorial Current, a surface current, centred his excellent study of the biogeography of Lhe at about 150 N, flows to Lhe west as does the South Pacifie Plate, SPRINGER (1982) discussed the dispersa1 Equatorial Current which is centred at the surface vs. vicariante conlroversy, but only briefly mentioned at about 70 South (Fig. 1). The Equatorial Counter- the East Pacifie Barrier as an effective isolating current is a narrow, surface, eastward-flowing current mechanism. SPRINGER (1982) emphasized the import- centred a few degrees north of the equator (Fig. 1). ance of cladistic studies in interpreling zoogeo- The Equatorial Undercurrent is a subsurface (50- graphie data. However, while cladistic studies cari 300 m) eastward-flowing current centred on the be of great use when considering distributions of equator. Al1 these currents are subject to considerable populations of related species, they are of little temporal variation in speed both seasonally and at use when considering distributions of monospeciflc other time scales (such as El Nino events), and the populations, as 1 do here, unless intrapopulation Undercurrent completely disappears at Limes (FIRING differences cari be established. ROSENBLATT et al. and LUKAS, 1983). The Equatorial Countercurrent is (1972) considered other aspects of the problem of usually hypothesized to be the means by which cross-Barrier dispersa1 such as the direction of cross-Barrier pelagic dispersa1 is accomplished (e.g. migration, ecological interactions between endemic BRIGGS, 1961, ROSENBLATT, 1967). The Equatorial and transported species, and the taxonomie issues Undercurrent is another possible cross-barrier pelagic involved. dispersa1 route, but might be t,oo cool (120-200 C) for Because of the relative paucity of information on the larvae of coral reef llshes. There is considerable the larval and pelagic juvenile stages of reef fishes, exchange of water between these current systems: 1 have used information on congeners or confamilials for example, the Undercurrent loses water to the if no information on the transpacific species was North and South Equatorial Currents and becomes available. Caution is advised in su& cases, as progressively weaker as it moves eastward. errors may have been introduced. Also, because Recently, details of two eastward-flowing equa- little is known about developmental flexibility in torial currents in the South Pacifie were reported reef fish larvae (see below), it is probably most (E LDIN, 1983). The South Equatorial Counter- realistic to regard the estimates of size and duration current is a weak (less than 10 cm/sec) surface of the pelagic stages (Table 1) as guides only. current located between 7 and 140 S. The South The list of 55 transpacific bony reef llshes (Table 1) Subsurface Countercurrent is a somewhat stronger was drawn from ROSENULATT et al. (1972) with (to 20 cm/sec) subsurface current (deeper than modifications from BERRY and BALDWIN (1966), 150 m) located between 5 and 100 S. Both are RANDALL and KAY (1974), MCCOSKER and ROSEN- seasonally variable. Although these currents may BLATT (1975), RANDALL and IVICCOSKER (1975), pass through the Marquesas Islands, their low FRITZSCHE (1976, 1978), RANDALL et al. (1978), velocities and the low temperature ( < 15 OC) of the DOOLEY (1978), THOMSON et aZ.(1979) and DAWSON Subsurface Current make them less likely candidates Océanogr. trop. 19 [2): 181-192 (1984). UISPERSAL .\Nr) FI~;. 1. - The distribution of pelagic stages of coral recf fishes in thc h:asl I’acific Barrier (which extcnds from IIawaii, I.ine, .MarquC- sas, and East,er Islands to the American mainland and Revillagigcdo, Clippcrton, and Galapogos Islandsj. The locations of capture of Pelagic stages of reef fMes are (0) Thalassoma(?), (A) Lacforia (diaphana.?), and (wj Diodon hgslrix. Thc area covcred by EASTROPAC is shown by diagonal lines, and t.hat by DOMES hy stipplcs. Thc major cquatorial surface currcnls arr: XEC-Sorth Equatorial Current, EU:-Equalorial Counler Currenl, and SEC-Soulh Equalorial Currcnt. Occurrcnrcs Of pclasic stages Of XV~ fishes lrss than 800 km from land in t,hc Easl I’acific are omitted Distribution des sfades larvaires pélagiques des poissons coralliens dans la (I Barriere de l’Est Paciffque d (qui s’tlend des Iles IIavJali, de la Ligne, Marquises et de Pâques à la côte américaine ef auz Iles I~evillagigedo, Clipperfon et Galapagos). Les sites de cupfure des stades pélagiques des poissons coralliens sont marqués (0) pour Thalassoma (?), (A) pour 1,actoria (diaphana?; e1 (B) pour Notion hystrix. Les régions couvertes par EASTAOPALC ef par DOMES sont, respecfivemenl, slri8es de lignes obliques ei pointilites. Les principaux courants de surface son1 : NEC-Couranl Nord dquatorial, ECC-Contre Courant Equaforial et SEC-Couranf Sud Equalorfal. Les occurrences de stades larvaires de poissons coralliens sifuées dans 1’Esl Pacifique ù moins de SO0 km d’une ferre ne son1 pas reporféss than the Equatorial Countercurrent or Equatorial ?‘halassorna), while others are elaborately, often Undercurrent for transport of larvae across the bizarrely, specialized (e.g. flcanthrwus, l?lyripristis, Barrier. Priacanfhusj (LEIS and RENNIS, 1983). The maximum size of the pelagic stage also varies tremendously from over 1 m for some adults that SlOHPHOI,OCICAt, EVIDESCE FOR I)ISPE:IX- are facultatively pelagic to less than 10 mm (Table 1). SAL ABILTTIES Large size at aettlement implies a long pelagic period, but small size does not necessarily imply a Pelagic eggs cari favour pelagic dispersa1 parli- short pelagic period. For example, larvae of some cularly if adults spawn into ocean-ward currcnts. marine invertebrates remain competent. to seMe for However, because pelagic eggs of most reef M~es extended periods without furt,her growth ( KEMPF, have short incubation periods (usually 48 hours or 1981), and it is possible the same applies in fish less, WATSON and LEIS, 19741, a pela@ egg secrus larvae, although this bas nofr been rstabliahed. of small importance in dispersa1 across the East Pelagic phase duration estimates for t.he trans- Pacifie Barrier. Whet,her importanf; or nol. this pacifie taxa are few (Table 1), but range from one potential advantage is available to the large to t,en months with most, eslimates between majority of transpacific species, because at least l-2.5 months. The estimates of pelagic phase 42 transpacilic speciea bave pelagic eggs (Table 1). duration in Table 1 should be treated wkh partkular Al1 transpacific species about which something is caution, because most are based not on the trans- known have a pelagic larval, juvenile or adult stage pacifie species, but on related taxa, and such (Table 1). However, there is great variat,ion in the estirnates cari vary by over 100 O/” between some degree of ruorphological specialization t.o pelagic life. confamilials (BROTHER~ ei al., 1983). UhNh (1973) Some larvae lack obvious specializat,ions (e.g. Scarrrs. est,itnates 125 days are required to cross the Barrier TABLE 1 Characteristics of potcnlial dispersa1 stages of the transpaciflc reef fishes. (‘) based on congener, (‘*) bascd on confamilial (U) unpublished observations, (L & R) Leis and Rennis (1983) and refcrenccs therein Caracftrisfiques de dispersion potentielle des stades larvaires des poissons coralliens iranspacifiques. (‘) sur une base conghnérique, (“) sur une base confamiliale. (U) non publié, (L. d; R) Leis et Hennis (1983) et références citées FAMILY/Species Pelagic Pelagic Duration of Maximum References egg stage pelagic stage pelagic (monthsl size (mm) MURAENIDAE ~-Echidna zebra Yes’” Leptocephalus 6-s**, s-10** 45 E. nebulosa YeSx* 'Leptocephalus ,, II 42 Uropterygius tigrinus Yes** Leptocephalus* 1( 8, 441 Enchelynassa canina YeS** Leptocephalus ,t II 62 Enchelycore lichenosa yes** Le~t.ocephalus** ” II 60-70** Gymnothorax buroensis YCS” Leptocephalus’ 6-8*, ‘1 82" -~c. eurostus YeS’ Lcptocephalus* ‘1 II a2* G. flavimarglnatus Yes” Leptacephalus* ” t1 82* G. panamensis YeS* LcptccephaLus* ” II 82* G. pictus Yesr L,eptncephalus* ” 31 82" G. undulatus YeS* Lcptocephalus” ” ” 82X HOLOCENTRIDAE Holotrachys lima ? Rhynchichthys** 48"' L&R Myripristis murdjan 7 Rhynchichthys*Y ? 4a** L 6: R AUL,OSTOMIDAE Aulostomus chinenls ? Yes -ca.90 u FISTULARIIDAE Fistularia commersonii xes ? 145 Watson and Leis 1974, L&R SYNGNATHIDAE Doryrhamphus exc.isus NCI Yes 13 Dawson 1981 PRIACANTHIDAE Priacanthus cruentatus !Ces”* YeS ? 0. 80 L k R, U, Suzuki --et al. 1980 Cookeolus boops YesxY Yes ? 226 Fritzsche 1978, L & R MALACANTHIDAE Malacanthus brevirostris Yes Dikrllorbynchus 70 Colin, pers. com.,Hubbs 1958 LUTJANIDAE _~Aphareus furcatus YlZS+* yes** 0.75-7.51" 16-43X* U, L k R, Coldman --et a1 1983, Brothers -et -Ia1 1983 CHAETODONTIDAE Forcipiger flavissimus YCS’ Tholichthys 1-2** 60 Suzuki --et al. 1980, Burgess 1978, Ralston 1975, Sale 1980 LABRIDAE Xyrichthys pauonius yes* Yes’ o.a7-l.2** 11.x Thresher ( 1984), L & R, Brothers -et -.*a1 1983 X. taeniourus YeS* YesX 0.87-1.2** 11” Thresher ( 1%4), L k R, Brothers, --et al. 1983 Stethojulis bandancnsis Yes” Yes" 0.87" 7" Mito 1962, 0, Brothers e&g., 1953 Thalassoma lutescens Yes” YeS* 1.5-2.5* Il-143 Vatson and Leis 1974, Victor 1982, Leis 1983, Brothers -et -.a1 9 1983 SCARIDAE Calotomus spinidens Yes” Yes 7-2x* 15 L & R, Brothers -ft --al., 1983 Scarus rubroviolaceus Yiesx Yes’ l-2** 8” L & R, Brothers --et al., 1983 2. ghobban YesX Yes% l-2** 8* L k Fi, Brothers _et --9a1 19â3 KUHLIIDAE -~Kuhlia taeniura YeS* 2* 3. 25s U, Tester and Tanaba 1953 TABLE 1 (suite) FAMILY/Species Pelagic et?!3 CIRRHITIDAE yes** Oxycirrhites typus 3a** Suïuki --et a1.1980,L. & ii Cirrhitichthys oxycephal,ls Yes 381’ Thrasher ( 1984), L 8s Fi -~c. serrat.us YeS* 3a** Thresher (19841, L & K CARANGIDAE caranx melam?ÿgus Adul t James 1976, U ~-Gnathanodon _--sp~tiosus Yes Adul t Watson and Leis 1974 ~~Alactis ctliaris YSS Adul t Gelsman 1926 KYPHOSIDAE ~~sectator ocyurus YeS*” Adult ACANTHURIDAE Acanthurus triostegus Yes 2.5 29 Randall 1956, 1961a -~A. achilles YeS* 1-2.5x GO Randall 1956, 19613, Sale 1980 -A. glaucopareius Yes* l-2.5’ 60 Randall 1956, 1961a, Sale 7980 A. xanthopterus yes* 1-2.2” 60” Randall 1956, 1961a, Sale 1980 -Ctenochaetus cyano&utt,at”s yes* 1-2.5x 32* Randall 1955, 1961b, Sale 1980 ZANCL,IDAE ~~-zanc1us canescens ? lb Stîasburg 1962 BALISTIDAE Baiistes polylepis No ? 108 Threshes (1984), Berry and Baldwin 1966 Xanthichthys --ment,, Yes”? ? > 55 Thresher 119S4), Berry and Baldwin 1966 Melichthys -- niger No** 7 1 4 4 Thrrsher 11984), Berry and Baldwin 1966 MONOCANTHIGAE Ali!tera scripta No 7 150 Clark 1950, Fedoryako 1981 IETRACDONIIDAE -Canthigaster amboinensis YCI’ 2-X.5’ 25* Stroud, pers. cçm., IJ ~~Arothrun hispidus No** 2-3.5** M. 15 t u R, Stroud, pers. com., 3 ~-Arothron -.~meleagris No** -ca. 15 L h 9, Stroud, pers. ccm., u DIODONTIDAE Diodon holocanthus YiS i.eis 1978 -~Diodon hystrlx Yes Leis 1978 Chilomycterus affinis Yes Mu~er, pers. cor!., u Yes .--c3. 1 ‘2 Ostracion --meieagr:s Yes YeS 3 $7 ANïENNARIIDAE -Antennarius --drombus (varies with 7 3 % sperlrs)* Kanazawaichthys’ ? 6, !iito 1964, Pietsch snd Grûbeckcr 1980, H~r!,s 1958 186 J.-RI. LEIS in the Equatorial Countercurrent. DANA'S (1975) Gobiid larvae arc relatively unspecialized morpho- estimate is for average conditions for the Equatorial logically, scarid larvae are littlc to moderately Countercurrent (GO cm/sec), but during El Nino specialized (LEIS and RENNIS, 1983), and bothid events, eastward velocities may more than double larvae are usually considered highly specialized. (FIRING and LUKAS, 1983), which would reduce the Why more Indo-Paciflc species do not bridge the time for a crossing to about two months. Average Barrier is unknown, but the answers may lie with crossing time is within the estimated pelagic phase ecological requirements of post-settlement juveniles duration of muraenid eels and perhaps Canthigaster, and adults (WALKER, 1966) or with historical but about twice duration estimates for other taxa, accidents such as geological or climatological events while crossing time under El Nino conditions is (e.g. DANA, 1973), more than with pelagic dispersa1 within the estimated pelagic phase duration for abilities (ROSENBLATT et al., 1972). In other words, nearly a11species for which an estimate is available. it is possible the pelagic stages of these non-trans- BARLOW (1981) reviews data which indicale some pacillc fishes do cross the Barrier but do not settle fish cari prolong their pelagic phase if conditions for or survive long enough to be detected on the reefs. settlement are not suitable, and otolith data indicate It is known that pelagic dispersa1 abilities may that some Indo-Paciflc reef fishes may be able to dlow Young Lo travel outside areas where adult prolong their pelagic phase for more than 20 days population maintenance is possible (SCHELTEMA, (BROTHER~ et al., .1983). This could place a crossing 1971, 1975). For example, Young of tropical fishes within the pelagic duration of a11transpaciflc species. may be transported into temperate areas where Morphological evidence indicates larvae or juve- adults are not normally found (HOBSON, 1969, niles of the transpacific species should be capable of MARKLE et al., 1980). The Young may not survive, surviving in the open ocean. The limited evidence or may survive for an indeflnite period, but be from the duration of the pelagic stage also indicates unable to establish a self-sustaining population that the pelagic stages of many transpacific species (HOBSON, 1972). The occasional appearance of one have a capacity for long-distance dispersa1 Lhat is to a few individuals of Central Paciflc fish species in sufficient for crossing the Barrier at least under some Hawaii (RANDALL, 1981) may represent an instance conditions. The above analysis and discussion could of this phenomenon within the tropics. not have conflrrned cross-Barrier pelagic dispersal, Evidence summarized above merely indicates a but could have ruled it out for species obviously not potentisl to cross the East Pacifîc Barrier. Al1 the adapted for pelagic dispersa1 (e.g. those without a transpacifîc species seem to have this potential, but pelagic stage) or those with a very short pelagic none has been observed to make such a crossing. stage duration. No species could be ruled out on this Observation of such a crossing, either through basis. discovery in the East Paciflc of an Tndo-Pacifie A11 transpacific flshes belong to families whose species previously known not to be present in the larvae are often found in abundance several to East (as opposed to possibly overlooked), or through many kilometres from shore (MILLER, 1974; Lms following dispersa1 stages across the Barrier is and MILLER, 1976; BOURRET ei ul., 1979). This extremely unlikely. However, distributional studies indicates an ability to survive in oceanic conditions, of larvae could provide evidence for cross-Barrier and is additional indirect evidence in favour of dispersai. dispersa1 ability, but cannot confirm cross-Barrier pelagic dispersal. DISTRIBUTIOKAL EVIDENCE FOR DISPER- Although morphological evidence indicates that SAL ABILITIES the transpacific species should bave good dispersa1 abilities, the same evidence indicates that many Distributional information on pelagic stages of non-transpaciflc Indo-Pacifie reef fishes should bave shorefishes within the East Pacifie Barrier cari help similar dispersa1 abilities (e.g. Chaetodon, Poma- cstablish if these stages are presently making canthidae, Mullidae, Synodontidae, Scorpaenidae: crossings of the Barrier. This would indicate that LEIS and RENNIS, 1983). This supports the obser- Indo-Paciflc populations currently have a genetic vation that morphological specializations of larvae input into East Pacifie populations. Also, if such are not a reliable indicator of dispersa1 potential crossings are possible today, they may have been (LE~S, 1983). Shorefish larvac which bave travelled possible in the past, and if SO, this would strengthen far from their source are not necessarily highly the circumstantial evidence that East Paciiîc specialized morphologically as was also shown by populations of transpacific fishes were established LOEB'S (1979) study of larval fishes approximately via pelagic dispersal. However, a failure to llnd such 870 km from the nearest shallow waler (Hawaii). direct evidence of such crossings today does not Only six shorefish larvae were present in LOER’S necessarily mean they do not take place (or have samples: four gobiids, one scarid and one bothid. not Laken place in the past), and for this reason Océanogr. Pop. 10 (2): lôl-192 (13Sd). I)ISPEHSAI. >\NI) THE Ei\ST PACIFIC RARRIER IN distributional information on pelagic stages cannot PERRIN and &VA~N, 1966). sa pelagic stages of falsify the hypothesis of t,ranspacific pelagic dispersai. sharefishes were reparted to be present in the Larvae of many shallow-wat,er invertebrates are “Coriolis” samples (thc Apogonids were of t,he commonly found many hundreds of kilometres from masopelagic genul: EEo~oeffa),but, as the study was the nearest shallow water (SCHELTEMA, 1968; one of mesapelagic Mies, pelagic stages of shore- SCHELTEMA and WILLIAMS, 1983), thus implying Iishes may bave been ignored. genetic connections between widely separated popu- The only ather record 1 could iind of a trans- lations. Unforlunately, similar studies on invertebrate pacifie rcef fis11 more than 800 km from shore was larvae have not been made in the East Pacifie of a juvenile (lS0 mm) Diodo/l hystrix (Diodontidaej Barrier (except on lobster larvae - see below and from approsirm&ly 1 100 km east of the Galapagos JOHNSON, 1971, 1974), and the available information (I,HIs, 1978j, probably in the saul,hern edge of the on the pelagic stages of sharefishes cornes from a few Equatorial Countercurrcnt (Fig. 1). This may have studies which were not designed t.o examine the been an Indo-Pacifie individu:11 which was nearly question of transpacific pelagic dispersal. Few pelagic across t,he Barrier or an East Pacifie individual stages of reef fishes were recorded from within the which was jusl. starting across. The exchange of Barrier by these siudies (summarized below). water between the currents of t.k~e Equatorial The major study of fish larvae within t,he system maltes it difficult. t-o pinpoint, t,he aripin of East Pacifie Barrier, the EASTROPAC, cruises any individual animal. (AHLSTROM, 1971, 1972; H. G. MOSER, pers. com.), ri number of éggs, larvae and one large individual extended from tbe central American toast to 1270 W (ca. 100 mm) tentat,ively identified as the reeî- (about midway across the Barrier, Fig. 1) and dwelling Lacloria diaplza/za (Ostraciidae) were collec- included 837 l-me& net plankton samples (ca. ted durinq both DONES and EASTROPXC up t,o 2 x 105 Iarvae). These were oblique taws from an 1 200 km from shallaw water (Fig. 1). Xewly- average depth of 200 m both day and night. IXe1 spawned cgps werc: prescnt in areas ca. 5 000 m larvae were not considered. While many shorefish deep. I could not determine if these individuals larvae were taken within 500 km of shore, fewer represent a wholly pelagic, species closely related ta than 50 were captured further from shore, and with 1,. diapharza or a lifa hietory sl;age of L. diaphnna the exception of 16 Lacioria diaphana (see below), which spawns pelagically (L. diapharla in the none more than 800 km from shore. Thus, little Western Pacifie cari remain pelagic to at least movement of shorefish larvae away from shore was 100 mm standard length, J. T. ~TOYEH, pers. com.). indicated, and no movement from the Indo-Pacifie Bccause of thia confusion, these osLraciids are not could be documented. The observation that many considered further herc, but if L. diapharla does shorefish larvae were captured within 500 km of mature and spawn before set,Lling, an extreme shore and very few further from shore might, indicate specializatian for pelagic dispersa1 would be repre- that reefs separated by less than 500 km regularly sented in a reef lish. exchange fish larvae, while reefs separated by greater I)istribut,ional stuclies of pelagic st.ages provide distances do SOless regularly, if at all. at best. anly weak support for Cross-Barrier dispersa1 A less extensive sampling program (ca. 390 7C)-cm by pelagic stages of shorefishes. Unequivocal crossing bongo net plankton samples from a11 depths to of a major porlion of the East Pacilic Barrier by t.lle 1 000 m day and night) captured 1Oj larvae in thc pclagic stage of any reel’ lîsh species bas net been western porlion of t,he Barrier (DOMES: see reported. Septivi: eviclencc is always suspect, anti BISCHOFF and PIPER, 1979, for a review of some the scarcity of shorelish larv-ae within t.he Easl. aspects of the DOMES programj, but, taok very few Pacifie Ijarrier dacs not necessaril? rnean tliat shorefish larvae (Fig. 1). Four labrid larvae (probably larvae do net nlo\~! across il. ‘l’his 1s particularly Thalassoma) were taken in the Equatorial Counter- t,rue in this case because the available studies werr current 600-l 200 km east of the Line Islands in nat. desipned t.0 address the question of cross- three different samples during twa seasans (LEIS, Barrier disperaal. 1lovements of larvae, if they do 1983) on the western edge of Lhe Barrier (Fig. l), but occur. are probably episoclic (for example correlated no other sharefish larvae were found (except, with El Sino event,s) and shoreiîsh larvae should be Lactoria diaplzana, see below). LEIS (1983) considered scarccL sa far Irom shore. Further, a larva well there may be a relatively constant leakage of reef across Lht: Barrier would presumably be alder and fish larvae eastward from the Line Islands. but (for many species) larger fhan ;J Iarva jus1 ent,ering could not explain why only the labrids were captured the Barrier, and if larger. il woulcl be more difficult in DOMES samples. to capture hy a Lowed net. Howe\er. even for The research vesse1 “Coriolie” taok 21 oblique relatil-ely common lobst.er larvae, JOHNSOK (1971. mid-water trawl samples at night from 300 m alang 1974) concluded there was no evidence from disL.ri- the equator within the East Pacifie Barrier (GRAXD- butional studies for cross-Barrier pelagic dispersa1 188 J.-M. LEIS by the transpacific species. Possibly there already (NUR and BEN-AVRAHAM, 1977), and could have exist in various collections unreported specimens of transported an Indo-Pacifie reef fauna along its pelagic stages of transpacific fishes from within the edges. However, for the distribution of the trans- East Pacifîc Barrier. It is hoped this review Will pacifie fîshes to have arisen from this, speciation stimulate the publication of reports on such by these fishes would have ceased for 65 million specimens. years. This seems unlikely in view of the extent of speciation in the ichthyofauna that has taken place following the emergence of the Tsthmus of Panama THE VICARIANCE ALTERNATIVE 2-5 million years ago (VAWTER et al., 1980). Further, The alternative to dispersa1 via pelagic stages SPRINGER (1982) estimates that modern flsh species requires a hypothesized continuous distribution and genera probably date little, if any, earlier than which has been interrupted. ESCHMEYER and Poss Miocene (25 million years ago). On a more recent (1976) proposed that during cool periods, currently scale, islands may have originated on the East separate populations of the temperate scorpaenid Paciflc Rise (Fig. 1) and moved eastward thus Maxillicosta in Australia, the Kermadec Islands providing a means of transporting an Tndo-Paciflc and Islas Juan Fernande2 could have been linked fauna into the East Pacifie (not too different from through a series of islands and sea mounts located the process proposed by ROTONDO ef al., 1981). between 200 and 300. Possibly, during warm periods, However, apart from the Nazca Ridge/Sala-y-Gomez populations of Indo-Pacifie reef fishes could have Ridge system, there is no evidence for this. The been connected across the same series of seamounts literature on tectonic movements in the Pacifie and islands, thus allowing access to the East Pacifie. Basin and its relevance to biogeography were This would have involved the Nazca Ridge (Fig. 1) reviewed by SPRINGER (1982). and the Sala-y-Gomez Ridge (which is located DANA (1975) summarizes evidence indicating the between Easter Island and the Nazca Ridge). The East Pacifie mainland coral reefs and fauna were present deepwater fish fauna of the Nazca and “greatly reduced, if not eliminated” (p. 371) during Sala-y-Gomez Ridges has a representation of Indo- the Pleistocene. If DANA is correct, recolonization Pacillc taxa (PARIN et al., 1981) which supports the of this fauna must have occurred since this idea that these ridges could have served as a route Pleistocene elimination, and older evidence of for movement of a shallow water Indo-Pacifie fauna distributional pathways is irrelevant. This applies into the tropical East Pacifie. Fossil Indo-Pacifie to many of the transpacific flshes because they are corals of Miocene to Pliocene age (DANA, 1975) obligate coral reef species. However, it is possible have been found on the Nazca Ridge, and Pleistocene that refugia existed either on the Central American corals have been dredged on the Sala-y-Gomez Ridge toast or on oceanic islands (e.g. CLIPPERTON) during (NEWMAN and FOSTER, 1983), giving support to the the reduction process that DANA (1975) postulates, notion of a continuous Indo-Paciflc fauna across thus providing centres for recolonization. Refugia the Barrier. Further, NEWMAN and FOSTER (1983) would extend the continuity of the East Pacifie present evidence for the continuous presence of reef fauna back before the Pleistocene and would islands in the Nazca and Sala-y-Gomez Ridge area make older evidence of distributional pathways for the past 29 million years. These could have relevant. These are important considerations, because served as zoogeographic stepping stones. Larval if establishment of the East Pacifie coral reef fauna dispersa1 between these “stepping stones” would including the transpacific fîshes was a recent event still have been required, but the distances would (DANA, 1975, daims the East Pacifie coral reefs are have been smaller than the present Barrier (less less than 5,000 years old), the probability increases than 500 km), and perhaps more easily bridged. that this establishment was due to cross-Barrier Continental drift may have been involved, because pelagic dispersal, and that cross-Barrier movement the distance between South America and island is still occurring. This is because the geological groups on the Pacifie plate would have been less in events required by the vicariante alternatives the past. A problem with this hypothesis is that the probably occurred much more than 5,000 years ago. cold Humbolt Crurent might have been a thermal ‘In a somewhat similar case, GRIGG (1981) has barrier to eastward movement of tropical forms even shown that the coral genus Acropora was eliminated during warm periods. from the Hawaiian archipelago during the Pleistocene Rafting of shallow water faunas could have thus obviating vicariante explanations for the occurred by tectonic movements of continents, present occurrence of Acropora in the archipelago. oceanic islands or shallow sea mounts. The proposed This led GRIGG to conclude that Acropora was continent, PaciGca, is supposed to have moved re-established in the Hawaiian archipelago relatively from near Australia in the Mesozoic, fragmented, recently via pelagic larval dispersal. and collided with the Americas by the early Tertiary Of the vicariante alternatives to pelagic dispersal, Oce’anogr. Pop. 19 (2): 181-192 (1984). DISPERSAL AND THE EAST PACIFIC BARRIER 189 only movement across the Barrier involving thr Pacifie and East, Pacifie populations (M~ULES and Nazca Ridge/Sala-y-G6mez Ridge SysLem seems I~OSENDLATT, 1983j, and this supports ihe pelagic likely: However, the possibility that coral reef dispersa1 hypothesis for these species at least. ecosystems in the East Pacifie were eliminated during Studies of species which are net transpacific could the Pleistocene and not reestablished until about also aid in understanding the zoogeographic processes 5,000 years ago (DANA, 1975) must be investigated involving the East Pacifie Barrier. hpproximately further, because on this possibility hinges the entire 7 y0 of the East Pacifie reef fishes are endemic and vicariante case. are mosl. closely related to Indo-Pacifie species (estimate derived from literature survey). It could be determined via the molecular clock (VAWTER CONCLUSION et al., 1980) when thc ancestral population of such Opinion has previously, and often uncritically, speciea was split,. This would give an estimate of the favoured the hypothesis of larval drift across the time of closure of the route used by their ancestors East Pacifie Barrier to explain the Indo-Pacifie to cross the Barrier, which could help to determine element of the East. Pacifie ichthyofauna. This what the route was. Cladistic studies of fish groups view is based on assumptions about the dispersa1 with related species on either side of the Barrier abilities of the pelagic stages of the species involved. could also help establish which route was taken While indirect evidence supports the long distanc,e across t.he Barrier. For exarnple, if the closest dispersa1 potential of these pelagic stages (and t.hus relative of an East Pacifie endemic species was the above assumptions), Lhere is no direct, un- found to be restricted to Easter Island, a dif’ferent equivocal evidence from distributional studies that route into the East Pacifie would be suggested than if pelagic dispersa1 across the East Pac,ific, Barrier does the closest relative was restrict,ed to the Line Islands. take place. The vicariante alternative is also The above analysis emphasizes that knowledge of supported by some indirect evidence, but is the mechanisms of dispersa1 of reef fishes is very challenged by the view of DANA (1975) who considers poor. It. also shows how difficult it is to est,ablish the East Pacifie coral reef fauna Lo be Young that pelagic dispersa1 is responsible for a given (ca. 5,000 yr). It is not possible to reject or accept, distribution. Such oft-cit.ed evidence as the morpho- either alternative, and further evidence must, be 1ogicaI specializations of larvae is realiy not relevant, sought. except in a very limit,ed sense, because there seems Two lines of research which might shed some light to be little correlation between morphological on the subject are distributional st,udies of pelagic specialization and dispersa1 abilities. Even inform- stages, particularly in the Equatorial Counter ation on duration of the pelagic stage cari only Current, and electrophoretic studies comparing indicate that the potential for long-range dispersa1 conspecifics on either side of the Barrier. Future does exist,. 1 set out t,o examine some assumptions distributional studies should utilize both plankton nbout the dispersa1 abilities of the larvae of reef and nekton sarnplers (including predatory oceanic fishes. Few of them could be supported and some are fishes such as Alepisaurus whic.h commonly eat not relevant to the question of dispersal. Further research on a11 aspects of this problem is needed pelagic stages of reef fishes, e.g. FOURMANOIR, 1969) to ensure the larger pelagic stages of many reef before a ciear picture Will emerge. fishes are captured if they are present. Preservation of larvae in such a way that otolit,hs could be aged might prove illuminating. In addition, seasonality of h prrliminary version of this paprr \vas presented at thr reproduction and vertical distribution and migrations Second Inlernational Symposium on Marine Biogcopraphy of the larvae should be taken into account when and Evolution in the Pacitîc, hrld in Sydney, July, 1982: many planning the sampling. ‘If the distribut,ion of the participants at thal. symposium, G. J. STROVD, s. BULLOCK and an anonymous refcrce of thc present journal madc transpacific fishes is the result of vicariante events, hclpful criticisms. R UESHOSIKRES transkltcd the referce’s the popuIations on either side of the Barrier should rornmcnts. Ii. E. THR~SHEH providcd segments of his book bave been separated for some time (Pliocene to prior La publication, and 1’. L. COUS, G. J. STnozn and Pleistocene), and detectable genetic difl’erences J. T. Moue~ allowed me ta reporl thcir unpublished observ- should have emerged. If the distributions are t.he alions. II. G. MOSER and B. E-. Sc>~lnA-McCAI.L prnerously result of continuing pelagic dispersal, little or no provided EASTROPAC data net. included in the published genetic differences should be present. If the reports, and J. HIROTA made thr DOMES larvae availablr distributions resulted from pelagic dispersa1 which for study. D. 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