27

Senckenbergiana maritima 25 I rrzrl I 21,-28 I Frankfurt am Main, 20. 17. 7994

Tiefenwasser- und Tiefseefische aus dem Roten Meer.X\/II. Oligopws robwstws, a New Record for the Red Sea,

with Comparative Studies on Specimens from the Gulf of Aden

(Pisces: Ophidiiformes: Bythitidae)

rVith 5 Text-Figures and 6 Tables

§florrceNc Kreuspwrtz & Fnexz f]rsrrrN

Assrnect

[Kre.usr,vrrz, §ü. & UrnrBrN, F. (1,99a): Deep-water and deep-sea fishes of the Red Sea. XVII.Oligopus robustus, a new record for the Red Sea, with comparative studies on specimens from theGulf of Aden (Pisces: Ophidiiformes: Bythitidae). - Senckenbergiana marit., 25 (1./3):21-28,5figs., 6 tabs.; Frankfurt a. M.l

During the MESEDA Expeditions 1,977-7981 and the METEOR Expedition 1987 to the RedSea and the Gulf of Aden 34 specimens of Oligopus robustus (Sr,uru & R.c.ocurnr 1913) were col-lected. This species is the first representative of the family Bythitidae found in the Red Sea. TheRed Sea specimens occupy larger depth ranges (363-L258 m) and inhabit much greater depthsthan the specimens found in the Gulf of Aden and other regions of the Indopacific (50-5a9 m).A thorough comparison o{ 12 morphometric and 4 meristic characters showed that the specimensof the Red Sea can be distinguished from those of the Gulf of Aden by univariate and multivariatestatistical methods: the Red Sea specimens have larger heads and larger mouths, deeper bodies, a

lower number of vertebrae, and mostly a higher number of dorsal and pectoral fin rays, as wellas a lighter coloration of the body and fins. The differences in meristic characters and colorationindicate a partial genetic isolation of the Red Sea forms, but do not justify the splitting into twosubspecies. The morphometric differences between the populations from the Red Sea and the Gulfof Aden, and the high variability among Red Sea specimens from different depths probabiy reflectstill ongoing processes of plastic dif{erentiation in ecologically separable regions.

Kurzfassung

fKreusrvrrz, ]ü. & UrnruN, F. Q99a): Tiefenwasser- und Tiefseefische aus dem Roten Meer. XV[.Oligopus robustus, ein Neunachweis für das Rote Meer, mit vergleichenden Studien an Exemplarenaus dem Golf von Aden (Pisces: Ophidiiformes: Bythitidae). - Senckenbergiana marit., 25 (1,/3):21,-28, 5 Abb., 6 Tab.; Frankfurt a. M.l

'Während der MESEDA-Expeditionen 1977-1981, und der METEOR-Expedition 1987 zumRoten Meer und dem Golf von Aden wurden insgesamt 34 Exemplare von Oligopus robustus (Swnru& Reocurrr 1913) gesammelt. Diese Art ist der erste Vertreter der Familie Bythitidae im RotenMeer. Die Exemplare aus dem Roten Meer zeigen eine breitere Tiefenverteilung (363-1258 m) undbewohnen wesentlich größere Tiefen als die im Golf von Aden und anderen Regionen des In-dopazifik gefundenen Exemplare (50-549 m). Beim Vergleich von 12 morphometrischen und 4

meristischen Merkmalen zeigte sich, daß die Exemplare des Roten Meeres von ienen des Golfs vonAden mittels univariater und multivariater statistischer Methoden unterschieden werden können.Die Rotmeer-Exemplare haben einen größeren Kopf und eine größere Mundöffnung, einen

höheren Körper, eine geringere '§flirbeizahl und meist eine größere Zthl an Strahlen der Rücken-und Brustflossen, sowie eine deutlich hellere Körper- und Flossenfärbung. Die Unterschiede in

Authors' Addresses: Prof. Dr. W. Kre.usrvrtz, Dr. F. lJrrruN, Forschungsinstitut Senckenberg, Senckenberganlage 25, D-60325Frankfurt a. M., Germany. - Present address of Dr. F. UrnrsrN: Konrad Lorenz-Institut für Vergleichende Verhaltensfprschung,Savoyenstraße 1a, A-1160 W'ien, Austria.

22

meristischen Merkmalen und Färbung weisen auf eine teilweise genetische Isolation der Formendes Roten Meeres hin, geben aber keinen Grund nt einer Trennuig in zwei Unterarten. Die mor-phometrischen Unterschiede zwischen den Populationen aus dem i.oten Meer und dem Golf vonAden und die ausgeprägte Variabilität zwischen Exemplaren aus unterschiedlichen Tiefen des RotenMeeres spiegeln möglicherweise noch ablaufende Prozisse plastischer Differenzierung in ökologischtrennbaren Regionen wider.

During the MESEDA-Expeditions 1977-t981 to theRed Sea and the METEOR-Expedition 1987 to the Red Seaand the Gulf of Aden several series of an ophidiiformspecies were collected consisting of 18 specimens from363-1258 m depth in the Red Sea, and 16 specimens from472-479 m depth in the Gulf of Aden. The position ofthe anterior nosrril, the joined vertical fins, thi single rayin the ventral fin, and the absence of palatine teeth showeäthat, according ro CoHEN (96a) and ConrN & NrprsBN(1978), the material belongs to Oligopus robustus (Srurrn6c Raocrrnnr in RaocrrErt 1913). This species, as well asthe genus and rhe family Bythitidae have nor been reportedfrom the Red Sea by earlier aurhors (e.g., Don t9S+).

Introduction

Oligopus robwstws (Srvrrru & Reocunrl l9l3)Figs. 1-2

In the current study, we provide a descriprion ofOligopus robustus from the Red Sea and the Gulf of Adenbased on the 34 specimens belonging to the fish collection(Ichthyologie I) of the Research institute and NaturalHistory Museum Senckenberg, Frankfurt a. M., Germany.In addition, we provide a srarisrical comparison of 1.2

^or-phometric and 4 meristic characrers. The scope of this lat-ter comparison was to examine the extent of morphologicaldifferentiation of the Red Sea specimens from those of theGulf of Aden.

1913 Gramrnonus robustus Srvrrru & Reocrrnnr, - in Reocurrr,Proc. Nat. Mus., 44 (1948): 168, p1. 13 fig. 4 (between Cebuand Leyte).

7964 Oligopws robustus, - CourN, Proc. U.S. Nat. Mus., 116:10- 11.

Material examined (34 specimens, 33.3-1,22mm SL): SMF26452 (1.; 41.8 mm); Red Sea (21.25.2,N, 37" 45.2'E), SONNU 56TA; closing trawl; 1043-1135 m; 23 Ocr 1977. - SMF 26453 (1;114.3 mm); Red Sea (21"28.5'N, 37"aa3'E); SONNE 68 TA; clos_ing trawl; 1,051-1134m; 24 Oct 1977. - SMF 26449 (2;37.9-56.2mm); Red Sea (21'19'N, 38"15.9'E); VALDIVIA j,O6

TA; closing trawl; 1085-1127m;11. Apr 1979. - SMF 26448 (3;

TA; closing trawl; 779-80L m; 15 Apr 1979. - SMF 26a50 (5;53.4- 8 1.7 mm); Red Sea (21.' 22'N, 39' Oa'E) ; VA LD I VIA 122 T A;closing trawl; 363-383m; 17 Apr 7979. - SMF 26a47 (3;56.3-1,22 mm); Red Sea (21'19.7'N, 38.17.4'E); VALDIVIA 712TA; closing trawl; 1130-1175m; 3-4 Mar 1981. - SMF 26451(1; 100.amm); Red Sea (24.46.8'N,36"26.2,8); VALDIVIA 739TA; closing trawl; 1108-1128 m;7-8 Mar 1981. - SMF 26446 (2,48.5-51.1 mm); Red Sea (26'38.14'N, 35'05.99'E); VALDIVIA753 TA; closing trawl; 1216-1258 m; 9-10 Mar 1981. - SMF2645a $6;51..7-1,1,1.7 mm); Gulf of Aden (12"16'N, 44.09.5'E);METEOR Me5-287; beam trawl; 472-479 m; 1.6 Mar 1987 .

Methods of measurements: Morphometric measure-ments of 12 characters were obtained using an electroniccaliper; 5 meristic characters were counted using radio-graphs. Most of the characters were measured according tothe definitions given by ConEN (1964) using the ter-minology of Hurrs & Lecrrn (1958). Body depth is theminimum distance berween the basis of the first dorsalfinray and the ventral conrour of the body. Head depth isthe distance along a yertical line beginning immediatelyanterior of the orbit and ending ar the ventral conrour of

the head. Orbit length is the horizontal diameter of the or-bit. Upper jaw width is the minimum distance between theposterior tips of upper jaws.

. Diagnosis (Figs. l-2): The meristic values givenhere are a combination of our measurements and the väluesobtained by earTier aurhors: D 75(?) 84-94, A 50-64, C 8,P 23-28 (Gulf of Aden D 84-88, P 23-26; Red Sea D84-94, P 25-28); 3 developed rakers on first gili arch;vertebrae a3-a6 (Gult of Aden and Red Sea), 46 (ConrN1964); vertrcal scale rows appr.75-85, upper row of lateralline organs appr. 21, lower row 25-30 (as rhe scale rowsar_e yery irregular, exact comparative studies are not possi-ble); no pore in lateral head caral system. Head large andbroad, mouth large, rear maxillary expanded; scales presenron head, opercle spine rather small, pointed; anteriornostril close to upper lip, eyes well developed. Dorsal andanal fins continuous with caudal fin; venrral fins present,in level of preopercle, with a single filamentous rayln each.Premaxillary and dentary with bands of granular orvilliform teeth, palatine reeth absent.

. Morphomerric characteristics in percentage of SL givenby ConrN (1964) compared with the SMF-specimens ]romthe Red Sea (RS) and the Gulf of Aden (GA): Head 31.3(CounN), 24.1-33.9 (RS), 27.3-32.1 (GA); body depth25.2 (ConrN), 20.6-29.2 (RS), 22.2-27.1 (GA); predoisal32.7 (CorrrN), 32.3-47.1 (RS), 34.9-39.1 (GA); preanal55.1 (CourN), 54.0-65.8 (RS), 53.6-61.8 (GA); snour 5.0(Conrrv), 6.5-10.0 (RS), 6.4-8.8 (GA); orbit 5.0(CourN), 3.3-5.9 (RS), 4.6-6.1 (GA).

The coloration of the specimens from the Indo-\üestpacific is reddish brown, darker on head, the dorsal finblackish brown, pectorals and ventrals dusky (according toKalraonane 195a). §flhile the specimens from the Gulf of

23

Fig. 1.. Oligopus robwstws, new record for the Red Sea. SMF 26450. - Drawing: R. NrsrssN.

Aden have the same colour, the coloration of the Red Sea

specimens is much lighter: head and body yellowish to san-dy brown, dorsal, anal and caudal fins yellowish, darker onthe base, pectorals and pelvics iight yellowish.

400 and 700 m by the same catching methods and duringthe same cruises (NrErsEN Ea UTBLETN 1993). Therefore, thedepth distribution suggests that there exist bathymetricaliyseparated populations of O. robustus in the Red Sea.

Fig. 2. Radiograph of Oligopus robustus from the Red Sea. SMF26451. - Foto: H. Ztrzscny.

Distribution: Oligopus robustus is known fromJapan (Prov. Tosa and Suruga bay, KauonaRA 1954,Masuoa et al. 1984), the Philippines between Cebu andLeyte (Surrn & Ra»cuEEr in Ra»crrrrr 1913), the Gulfof Aden (NonueN 1939), and from the Red Sea (Fig. :).The Indopacific specimens were found in depths of50-350m (Japan), 3a6m (Philippines), and 457-549m(Gulf of Aden, Norman and SMF 26454), while the RedSea specimens occupy a much larger depth range between363-1258 meters. tWith respect to their vertical distribu-tion, the Red Sea specimens belong much more to thegroup of the bathybenthic inhabitants (700-1500 mdepth) than to the archibenthic fishes (200-1000 m depth;Kreusrvrtz 1986, 1989), as from 8 series 7 were collectedin a depth of more than 700 meters and only one wasfound in 363-383 meter depth. This cannot be explainedas än artificial sampiing gap, because severai specimensof another ophidiiform species, Neobythites stefanooiNrErsrN & [JrsrprN 1993, were caught in depths between Fig. 3. Distribution of the material studied.

Statistical Comparisons

Methods: Following a

(UrnrErN & §TrNrrrn 1994),

variate comparative methods

recent quantitative approachdirect, univariate, and multi-were used to determine the

extent of morphological differentiation between thespecimens of the Red Sea (n: 18) and the Gulf of Aden(n: 16). For the purpose of direct comparison of the data,

24

Table 1. Means and standard deviations of the absolute values including measurements of SL and 12

morphometric characters in two geographical groups of Oligopus robustas from the Red Sea and the Gulfof Aden. - In mm; n = number of specimens examined.

Red Sea Gulf of AdenMean SD n Mean SD n

Standard leneth 66.21 + 27.06 8I 89.08 + 17.37 t 6lBody depth 15.89 + 7.19 t 7l 21.39 + 4.49 t 6lPredorsal length 23.54 + 10.05 t 7l 32.47 + 6.29 t 6lPreanal length 36.86 * 15.80 71 51.78 r 10,43 t 6lPectoral length 11.42 + 5.06 t 6I 18.60 + 3.47 t 6lHead length 19.30 + 8.15 I 71 26.37 + 5.30 I 6tPostorbital length 11.40 + 4.68 7l 14.85 + 2.81 t 6lSnout length 5.40 + 2.05 71 6.77 + 1.56 t 6lUpper.iaw leneth 11.46 + 4.68 7l 14.85 + 2.88 t 6lHead depth 13.67 + 5.66 t 7l 16.70 + 3.44 t 6lOrbit leneth 3.09 + 1.31 8l 4.68 + 0.98 t 6lInterorbital lensth 3.55 * 1.57 7l 5.20 + 1.07 t 6lUpper jaw width 9.',73 + 4.85 t 7l 12.46 + 3.08 I 6t

Table 2. Ranges and means of SL and 12

geographical groups of Oligopws robustas frommorphometric characters expressed in o/o SL in two

the Red Sea and the Gulf of Aden.

Red Sea Gulf of AdenMin Mean Max Min Mean Max

Standard length (mm) 33.1 (66.2) t22.0 51.7 (8e.1) 111.7

in%SLBody depth 20.6 (24.7) 29.2 22.2 (24.0) 27,1

Predorsal length 32.3 (36.e) 41.1 34.9 (36.s) 39.1

Preanal length 54,0 (57.8) 65.8 53.6 (58.1 ) 61.8

Pectoral length 13.9 (r 8.4) 21.s i 8.3 (21.0) 23.5

Head length 24.1 (30.3) 33.9 27.3 (29.6) 32.1

Postorbital leneth 15.5 (18.0) 20.2 ts.4 (16.7) 17.8

Snout length 6,5 (8.6) 10.0 6.4 (7.6) 8.8

Upper jaw length 16.4 (18.1) 19.5 16.0 (16.7) 17.8

Head depth 18.4 (21.s) 22.9 16.6 ( 18.7) 20.6

Orbit leneth (4.6) '5.9 4.6 (s.2) 6.1

Interorbital length 4.8 (s.6) 6.5 5.1 (s.8) 6.7

Upper iaw width 10.3 (14.e) l8. t 1 1.3 (13.e) 16.3

Table 3. Rangesandmeansof 4meristiccharactersintwogeographicalgroups of Oligopusrobustttsfromthe Red Sea and the Guif of Aden. - §flith G-values of G-tests. 't = p<0.05; n : number of specimens

examined.

Red Sea Gulf of AdenMin Mean Max n Min Mean Max n G-value

Dorsal frn 84 (89.9) 94 tet 84 (85.8) 88 t 6l 17.7 n.s.

Anal fin 58 (61.1) 64 l1 57 (60.3) 64 t 5t 3.8 n.s.

Pectoral fin 25 (26.3) 28 lt0l 23 (24.s) 26 t 3l 13.6 *

Vertebrae 42 (42.e) 44 U] 43 (44.4) 46 t 6l 10.6 {(

ZVZ Red seo! cutt of Aden

25

6

5

4

3

2

t-CI

_o

EfZ

Dorsol roys Pecto rq I roys Vertebroe7

84 85 86 87 88 89

Fig. 4. Frequency distribution for certain characters in Oligopus robustus indicating geographic variation. - Gulf of Aden andRed Sea.

239493929190 24 4645++43+228272625

Table 4. The results of allometric regressions of 12 mor-phometric characters on standard length. Intercepts (a), slopes (b)with standard errors, and correlations (r). - All regressions arehighly significant (p < 0.00001).

a b rBody depth -0.13 1.06 + 0.032 0.99

Predorsal length -0.50 t.03 + 0.025 0.99

Preanal length -0.34 I .06 + 0.021 0.99

Pectoral length 1.07 l.l9 + 0.057 0.97

Head length -0.60 1.04 + 0.030 0.99

Postorbital lensth -0.69 0.96 + 0.035 0.98

Snout length -0.92 0.90 + 0.061 0.94

Upperiaw length -0.68 0.96 + 0.028 0.99

Head depth -0.61 0.95 + 0.043 0.97

Orbit leneth -1.61 1.16 + 0.057 0.96

Interorbital length t.34 1.05 + 0.047 0.97

Upper jaw width 1.13 1.15 + 0.068 0.95

the morphometric values were expressed in o/o SL and theranges of these new values as well as the ranges of themeristic characters of the two groups were compared.

Before the uni- and multivariate comparisons were car-rieä out, the data were size-adjusted by running log-logregressions between SL and all the continuous variables.The adjusted measurement of each variable was expressedas the individual deviation from the regression functionthat described the size relationship of each variable.(Rlrsr1985). Meristic characters did not show size dependence.

One-way analysis of variance was used to examine thetwo groups for significant variations in each of the 12 mor-phometric characters. G-test was employed to examine 4

meristic characters. Principal component analysis (PCA)was carried out including all 72 size-adj,tsted mor-phometric variables. One-way ANOVA was run to ex-

amine the two groups for significant differences in the com-ponent scores of the first five principal components.

Results: The statistics of the absolute values of SLand 12 morphomerric characters are given in Tab. 1. Thereis a clear overlap between the groups of the Red Sea andthe Gulf of Aden in all morphometric characters expressedin o/o SL (Tab. 2).

The ranges of the four meristic characters show a wideoverlap, too (Tab. 3). G-test revealed significant differencesin the number of pectoral rays and verrebrae (Tab. 3). Nosignificant differences were found in the number of dorsalrays which is mainly caused by one individual of the RedSea group which showed only 84 rays in the dorsal fin(Fig. +).

Table 5. The means and F-values of ANOVA for transormedvalues (multiplied by 103) of 12 morphometric charace.rs in twogeographical groups of Oligopus robustus from the Red Sea and theGulf of Aden. - " : p<0.05; 'tx = p<0.01.

RedSea

Gulf ofAden

F-value

Body depth 11.49 12.21 7.01 :r'(

Predorsal length s.28 -5.61 2.12 n.s.

Preanal length 3.55 -3.78 1.36 n.s.

Pectoral length -11.57 11.57 1.88 n.s.

Head length 7.70 -8.1 8 3.34 n.s.

Postorbital length 12.0s t2.80 6.35 '<

Snout length 18.82 -20.00 4.89 *

Upper jaw length 13.41 t4.24 15.94 x*

Head depth 24.66 -26.20 27.14 **Orbit leneth - 15.69 t7.65 3.91 n.s.

lnterorbital lensth -6.02 6.40 0.75 n.s.

Upper jaw width 2s.28 -26.86 7.78 **

26

Tab. 4 shows the results of the allometric regressionsbetween the morphometric variables and SL. Tab. 5 showsthe means of the size-adjusted, log-transformed values o{the morphometric characters and the results of univariateF-tests. Highly significant differences (p <0.01) are foundin upper jaw length, head depth, and upper jaw width. Fur-ther differences concern body depth, postorbital length,and snout length. Accordingly, the Red Sea specimens havelarger heads, larger mouths, and deeper bodies than theGulf of Aden specimens.

Plots of the character loadings of PC1 and PC3 areshown in Fig. 5, the statistics in Tab. 6. The character

o Red Seo o Gu lf of Aden

5 4 2 101

PC1

Fig. 5. Scatterplot of the first and the third component scores andoutlines from the PCA in two geographical groups of Oligopwsrobwstus from the Red Sea and the Gulf of Aden. - The Red Sea

group is separated into two bathymetrical subgroups each markedby a dotted outline. All O. robustus {rom depths below 1OOO mbelong to the subgroup with exclusively negative PC3 scores. -See text for further explanations.

loadings of PC2 covered 74.1,9Yo of the total variance and

- similar to PC4 and PC5 - did not reveal any additionalgroupings. The Gulf of Aden group separares well frommost of the Red Sea specimens along PC1. The highestIoadings of PC1 are located in head depth, upper jawwidth, upper jaw length, head length, and body depth.Along the axis of PC3 the separarion between the Red Seaand the Gulf of Aden group is still significant (Tab. 6).However, there is also a clear separation among Red Seaspecimens belonging to two subgroups, one from less thanca. 800 m depth with mainly positive componenr scoresand the other subgroup from more rhan 1OO0 m depth withexclusively negative component scores along the PC3 axis(Fig. 5). The highest loadings of PC3 are located in orbitlength and pectoral length (Tab. 6).

Ta b I e 6. Character loadings of PCA for Eigenvectors 1 and 3.Loadings with magnitudes greater than 0.35 are shown in bold let-ters. Eigenvalues, contribution to total variance, and F-values o{univariate comparisons between the two geographical groups arealso given. - See text for further explanations.

EigenvectorsI J

Body depth 0.3562 0.t202Predorsal lencth 0.1 164 -0.2629Preanal lensth 0.2804 -0.0913Pectoral length 0.0449 0.5098Head lensth 0.3583 0.1018Postorbital lencth 0.3012 -0.2188Snout lensth 0.279r 0.1074Upper iaw lensth 0.3873 -0.1189Head deoth 0.4200 -0.1698Orbit lenelh 0.0590 0.7121Interorbital lensth 0.0094 -0.0118Upper iaw width 0.3896 0.t677

Eisenvalue 4.23 1.56o/o of Variance 35.26 13.02F-value 15.79 ** 5.09 *

lr)

On

Discussion

Until now, Oligopus robustushas been reported from phenomenon also found in the tongu e-sole Cynoglossus

Japan, the Philippines, and the Gulf of Aden. Thus, this acutirostris (Knunn 1982).species inhabits the western Pacific, the Indian Ocean, and, Although the Red Sea and the Gulf of Aden groups canas a new record, the Red Sea. All specimens examined have be distinguished statistically in several morphometricthe general criteria of the species O, robustus (Surrrr Er characters, a high individual variability exists that causes aReocrrrrr 1913). However, the Red Sea specimens can be broad overlap between the two groups in the ranges of size-distinguished by statistical methods from the Guif of Aden adjusted values. This *ppr."rrt-rr".äbility of bädy shapespecimens in meristic characters which indicates a partial may reflect still ongoing processes of plastic differentiationgenetic isolation of the Red Sea forms at the population in response to distinct environmental conditions ratherlevel. The same conclusion Tay be drawn from the devia- than genetically fixed pamerns (UrnruN & \TrNrrrn 1994).tion in body coloration of the Red Sea specimens, a The larger mouths in combination with larger heads found

V

Y

j

i......9

in the Red Sea specimens of O. robustus may be the resultof a specialization on large sized prey rypes (Gxz 1979)and the adoption of a size-selective, energy saving foragingstyle (\Wrurrncrr. 1991; Meuraqursr et al. 7992).

The specimens of the Gulf of Aden were collected inthe northwestern part (Fig. 3) near the ourflow of the warmRed Sea water into the deep water of the Gulf, an areawhich has been found to be strongly affected by theecological conditions of the southern Red Sea (Tünrev1987). Therefore, the morphometric variations betweenthe two populations cannot be simply explained as a resulrof temperature variations between the Red Sea and theGulf of Aden and may, for instance, reflect regional dif-ferences in food supply (see also BrcrlraNN 198a). Addi-tional examination of material from the sourhern Red Seaand the central and eastern Gulf of Aden will be necessaryto understand the relationship between morphological dif-ferentiation in O. robwstus and ecologically separableregions within the Red Sea and the Gulf of Aden.

In the Red Sea, O. robustus occurs along the samedegree of latitude at only a few kilometers distance inalmost lightless zones below 1000 m as well as in illumi-nated zones of less than 400 m depth. O, robustus isviviparous (ConrN 8c NrprseN 1978) and the wide depth

27

range found in the Red Sea specimens may be the conse-quence of a migration activity of juveniles or adults (e.g.,pregnant females, see also Lyorano 1993) between alter-native depth zones rather than passive drift of pelagic eggsor larvae like it may happen in oviparous fishes. The in-dividuals living in depths beiow 1OO0 m could be distin-guished by the principal componenr analysis from theother specimens in having particularly small eyes and smallsized pectoral fins. This indicates a shift towards a lessvisually oriented lifestyle (UrrrnrN 8c \ürNKrsn 1994) pro-bably associated with a less size-selective foraging mode as

indicated by the high variabiliry of the PC1 scores amongthis subgroup. A more detailed investigation of the depthdistribution patterns in the Red Sea specimens of O.robustus in comparison with the ophidiid Neobyrbitesstefanooi is currently in preparation.

Acknowledgements'W'e thank Dr. JancrN Nrrrsrr, Zoological Museum, Copen-

hagen, and Dr. Mrcrlerl Tünr.rv, Research Institute Senckenberg,Frankfurt/M., for discussions and many useful comments onformer versions of the mansucript.

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