H E L G O ~ E R MEERESUNTERSUCHUNGEN Helgol~inder Meeresunters. 36, 323-335 (1983)

Observations on the reproductive and larval b io logy of Blenn ius p a v o (Pisces: Teleostei)

H . v . Westernhagen

Biologische Anstal t Helgoland (Zentrale); NotkestraBe 31, 2000 Hamburg 52, Federal Republ ic o f Germany

ABSTRACT: Social behaviour and spawning of adult Blennius pavo kept in the laboratory are described. Eggs are deposited in batches on the walls of artificial spawning places (PVC pipes). One male guards and tends the eggs of different females in one spawning place. Larval hatching occurs in groups according to oviposition. Minimum incubation temperature is around 14-15 °C. Larval survival in 1-1 rearing jars is not related to larval total length but to density of larval stock. An experimental population of laboratory reared juvenile and adolescent B. pavo displays a male to female ratio of 1:1.4. Factors possibly influencing the sex ratio of this littoral fish are discussed in view of the situation in its natural environment.

INTRODUCTION

Blermius pavo is a small bo t tom l iv ing fish exhib i t ing a total l eng th (TL) of about 120 ram. It is common in the sha l low littoral zone of the Medi t e r ranean Sea. There, it

abounds in the upper one me te r of the water preferably among stands of g reen a lgae such as Ulva or Enteromorpha or under bolders and in crevices. The species is charac-

te r ized by a dist inct sexual dimorphism, descr ibed by Zander (1975) and Papaconstan- t inou (1979). The larger male (maximum length around 130 ram) can be dis t inguished

from the smal ler female (maximum leng th around 110 ram) by a typical hood or crest on

its neu roc ran ium and its br ighter coloration especia l ly dur ing the season of reproduct ive

activity. The mature ma le displays a l ight b lue des ign in the face (white in Fig. 1) and

the total body has a b lue i sh -g reen hue with br ight b lue vert ical l ines and dots (Fishel- son, 1963a). The female lacks the crest, and its colorat ion is domina ted by brown

patterns. In the nupt ia l phase, it too shows l ight b lue vert ical l ines and dots on the lateral

sides. The ma le guards the eggs wh ich are deposi ted by the females in crevices and holes

of the l i t toral rocks. Breed ing season in the Med i t e r r anean is said to last from May unti l

Ju ly (Eggert, 1932) or Augus t (Fishelson, 1963a). Whi le observat ions on feed ing and

reproduct ive behav iour of B. pavo are ava i lab le (Fishelson, 1963a; Zander, 1975), l i t t le is known about la rval survival and larval or juven i le deve lopment . H e n c e this study tries to

close this gap in our k n o w l e d g e on the b io logy of B. pavo, one of the very common fish in

the l i t toral zone of the Medi te r ranean .

© Biologische Anstalt Helgoland, Hamburg

324 H . v . Wes te rnhagen

MATERIAL AND METHODS

Juven i l e Blermius pavo (TL: 50 to 75 mm) were caught dur ing Ju ly in Ulva and Enteromorpha stands at Fiascher ino (La Spezia, Italy) us ing a hand net. 23 specimens,

the sexes of which were not yet d is t inguishable by secondary sex characters, were

transferred into a bas in (90 x 90 x 20 cm) conta ining 120 1 of seawater , connec ted to a c i rculated sea water system run at a salinity of roughly 34 %0 at 15 °C. The fish were kept under natural dayl igh t condit ions and the fish tank was addi tonal ly i l l umina ted by a

f luorescent l ight in a 12 : 12 h l igh t /da rk rhy thm in order to provide e n o u g h l ight for the growth of algae. The bas in was p rov ided with PVC-pipes about 15 cm long wi th

different d iameters (20 to 40 ram) to serve as shel ter and spawning places. The fish w e r e fed commerc ia l fish feeds (TetraMin} and occasional ly Myti lus edulis meat was offered,

which was t aken readily.

After 9 months in ' cap t iv i ty the spec imens d isp layed clear sexual d imorph ism and

after 12 more months spawn ing could be observed. As soon as a ma le B. pavo had its

PVC-pipe l ined with spawn, p ipe and fish were m o v e d to a separa te basin. The male

cont inued tak ing care of the eggs. Ha tch ing larvae w e r e counted. 30 to 50 indiv iduals

were anes the t ized (MS 222) and their total l ength de termined.

Table 1. Blermius pavo. Data on rearing conditions (± standard deviation of mean)

Type Temperature Salinity Light Water (°C) (%o) conditions volume (1)

Larvae 21 ° ± 2.7 30-32 natural 1 daylight

Juveniles 21 ° ± 0.7 34 ± 1.2 LD 12 : 12 200 in group recircntated

Juveniles 2t ° ± 1.8 34 ± 1.8 LD 12 : 12 100 in isolation recirculated

Adults 15 ° ± 0.8 34 ± 1.5 natural 120 daylight recirculated LD 12 : 12

Larval rear ing was conducted in 1-1 aera ted glass jars, the outer wai ls of which were l ined wi th dark cardboard pape r to p reven t the larvae from b u m p in g agains t the wails.

Larvae were fed with Brachionus and, once large enough, wi th Ar temia naupli i . Water

was exchanged every second day (water qual i ty parameters see Tab le 1). Benthic s tages w e r e transferred to a large rea r ing bas in (90 × 150 × 20 cm) in a rec i rcu la ted sea wate r

system. Rearing t empera tu re was 21°C at about 34 %0 S. Once the popula t ion was 6

months old a group of 80 ind iv idua ls was r emoved and t ransferred to ind iv idua l rea r ing

chambers 5 × 5 cm in d imens ions in a reci rcula ted sea water system (21 °C, S = 34 %°). The exper imenta l condit ions for all exper iments are dep ic ted in Tab le 1. The total l eng th

of the fish was measu red every one to two months to the neares t mm.

On this occasion the fish were sexed from their outer appea rance according to

whe the r or not the ma le crest (Eggert, 1932; Papaconstant inou, 1979) was present .

Observat ions on B l e n n i u s p a v o 325

RESULTS

O b s e r v a t i o n s o n s p a w n i n g b e h a v i o u r

As soon as the males showed secondary sex characteristics (hood and dark hue), they moved into one of the PVC-pipes which they kept as hous ing for the whole per iod of observation. The females cont inued roaming a round in the basin. Al though there were four mature males in a relat ively l imi ted area (0.8 m2), f ight ing b e t w e e n the males for b reed ing places or females was neve r observed. Dur ing the total per iod of observat ion (more t han three years) all parenta l fish survived.

Without any not iceable external s t imulus such as rise in temperature, al terat ion of i l lumina t ion or salinity, females became gravid and started to spawn in the pipes inhab i ted by the males. Pemales spawned several t imes and with different males. Thus a single male fertil ized and guarded eggs from different females. At times, all four males of the laboratory stock were guard ing eggs. Eggs were deposi ted ins ide the pipes in a s ingle layer and subsequen t egg deposi t ion occurred only at places free from previously spawned eggs. At t imes males "owned" two adjacent spawning places and females deposi ted eggs in two different nests.

C a r e of e g g s

Males guarded the eggs throughout incubat ion. The males constant ly performed u n d u l a t i n g movements with their body, thus p u m p i n g water through the pipes. At the same time, the bottom of the pipes was b rushed or wiped with the pectoral fins in order to remove feces or accumula ted debris. Larvae hatched dead were brushed out. In addition, the males performed f requent revolutions a long their long i tud ina l body axis. Dur ing this procedure, the males probably appl ied mucus to the eggs from g landula r areas on the dorsal parts of their lateral sides. Dur ing the care of eggs, these parts

Fig. 1. Blennius pavo. Male (a) guarding nest with eggs, and (b) after a longitudinal revolution

326 H .v . Wes te rnhagen

appeared conspicuously covered with grey thick mucus. Figure 1 shows a male B. pavo (a) dur ing the care of eggs and (b) after the complet ion of a revolution.

Whenever males were t ak ing care of two spawnings they a l te rnated b e t w e e n these two in fairly regular intervals. Observat ions over a per iod of 20 days revea led no signif icant differences (t-test; P < 0.01) be tween the t imes al lot ted to the care of two different spawns. After a total observat ion period of 480 rain the male had spent 246 mi n in Nest A and 234 min in Nest B. Even over long periods, the care was fairly equal ly divided. The fish never "forgot" that it had two nests, and always left one nes t to take care of the second. Only after all the larvae in one pipe had ha tched did the male stop a l te rnat ing b e t w e e n the two nests.

H a t c h i n g a n d l a r v a l s u r v i v a l

Ha tching

Incubat ion t ime var ied from 8 days at 23.7 °C to 16 days at 18-19 °C, 20 to 21 days at 16 °C to 41 days at 15 °C. 15 °C seemed to be the lowest possible incuba t ion tempera ture since frequently not all the eggs hatched at this temperature. As in other fish (Alderdice & Velsen, 1971), larvae hatched pr imari ly dur ing the n igh t and larval size var ied with rear ing temperature and origin. Al though only data from different spawnings are avai lable and size var ia t ion in larvae ha tching in one nest are considerable (Figs. 2, 3), m e a n size (TL) of newly ha tched larvae was found to be clearly tempera ture dependent . At 15 °C TL was 4.88 ± 0.11 mm~ at 18-19 °C larval total l ength fell to 4.85 ___ 0.13, and at 23.7 °C larvae measured 4.65 ± 0.16 m m only. The first larvae of one spawn ing batch were small; then, dur ing m a x i m u m hatching, larval size was largest f inal ly dropping at the end of the ha tch ing period. M i n i m u m total l eng th for newly ha tched larvae was 4.24 rnm (at 23.7 °C) whi le the largest ind iv iduals measured 5.2 mm (at 15 °C),

Due to the different sizes of the ha tching larvae a nd due to the ha tch ing peaks, the n u m b e r of egg deposits guarded by one male could be determined. Thus Figures 2 a nd 3 reveal that we were dea l ing with three (23,7 °C) and 4 (15 °, 18-19 °C) egg clusters per male. The different sizes of the larvae per brood a nd rear ing tempera ture indica te the existence of at least three different mothers for the egg clusters of Figure 2. For the egg clusters at 15 °C (Fig. 3), at least two different females can be de te rmined as mother individuals .

Larvae from the different egg deposit ions could be best separa ted at 15 °C (Fig. 3). Four batches of ha tch ing larvae could be recognized wi th dist inct ha tch ing peaks. Larval size increased from the b e g i n n i n g of ha tch ing towards the p e a k a nd decreased versus the end. At h igher temperatures , separat ion of larvae ha tch ing from different batches became difficult, s ince peaks were obscured due to shor tening of the ha tch ing period. Yet on the basis of larval length, even at 23.7 °C, three batches could be dis t inguished.

At temperatures be low 15 °C ha tch ing of larvae from one nest was spread over a long period of more than 40 days. No typical peaks for m a x i m u m ha tch ing could be d is t inguished and larval ha tch ing was dis tr ibuted more or less i r regular ly over the whole period of hatching.

Survival

Newly hatched larvae swam actively and had wel l developed pectoral fins. The mouth funct ioned and one day old larvae took Brachionus readily. After 4 days at 21 °C,

O b s e r v a t i o n s o n Blennius pavo 3 2 7

A r t e m i a naupl i i were devoured. It took 18 days at 21 °C for the first larvae of one batch to deve lope far enough to take to the bot tom (Fig. 4).

Al though gene ra l ly more than 80 % of the la rvae in i t ia ted feeding, larval survival

5.0-

4~9~

4.8-

-5 ~_ 4,7-

4.6-

4.5- t 38

I I I I I 1200 - I ,

1000

800

600

Z

400

200

18-19°C

I I I I I I I I I 1

5.0-

&9-

4.8-

4.7-

4.6-

/*.5-

I

23.7 °C

54

I I I - T - 1 I I I I

n= 5956

F ' " ' I I I I I I t t J ~ I May I June

1 1

Days

160

400

n=9406

September

Fig. 2. B l e n n i u s pavo . Hatching distribution and total length of newly hatched larvae from two different spawnings at different incubation temperatures. Double bars: adjoining hatching groups

3 2 8 H . v . W e s t e r n h a g e n

( l a r v a e w h i c h s u c c e s s f u l l y a s s u m e d b e n t h i c l i v i n g ) v a r i e d f r o m 3 .3 to 5 8 % (F ig . 5).

S u r v i v a l w a s n o t d e p e n d e n t o n t o t a l l e n g t h of n e w l y h a t c h e d l a r v a e b u t o n l a r v a l d e n s i t y

i n t h e r e a r i n g j a r s , a l t h o u g h f o o d (Brachionus) w a s g i v e n a d l i b i t u m . I n c r e a s e i n

! 5.1

5 . 0 -

I

I I

#

-5 ~o 4 . 8 -

4,7-

4.6-

600

15oC

I I I I I I I I I

• 48

l ~ - - I l I t l l

4O

T

I

l

I l t l t ~ l

b29

4,00 -

E - Z

200 -

0 I 1 I t t I l t I l - I t I i I I { I 1 I T I T ~ " ~ i ~ - - June l Ju ly

Fig. 3 . Blennius pavo. Hatching distribution and total length of newly hatched larvae from one spawning, Broken line: no data

Fig. 4. Blennius pavo. Late p l a n k t o n i c l a rvae r e a d y to a s s u m e b e n t h i c m o d e of l iv ing, TL: 13.1 m m

Observat ions on B l e n n i u s p a v o 329

number s of surviving larvae wi th dec l in ing densi ty followed a hyperbola.rs2 = - 0.55), approaching a m a x i m u m survival of 50 % at a densi ty of about 15 larvae per litre. At h igh stocking densit ies, mortali ty of larvae was expecial ly h igh dur ing the t ransi t ion from the larval phase to the ben th ic stage. While late pelagic larvae did not show any external signs of mut i la t ion (Fig. 4), the deve loping territorial behav iour of the young bot tom stages was accompanied by bit ing, and young bottom stages showed severe b i t ing marks on fins and tails, which were f requent ly fatal.

G r o u p r e a r i n g of j u v e n i l e s

After transfer from the 1-1 jars to larger rear ing tanks, the fishes grew considerably, and at an age of 6 months (Fig. 6 [6]) at 21 °C the m e a n total l eng th of the popula t ion was 34.4 + 7.2 mm (n = 128). Larger ind iv idua ls (starting at TL 35 mm) were a l ready recognizeable as males by their deve lop ing hoods. As the popula t ion grew older, the n u m b e r of clearly recognizeable male and female indiv iduals increased (Fig. 6A), r each ing a male to female ratio of 1:2.3 at an age of 13 months. At this stage, about 24 % of the popula t ion were male, 55 % were female and 2 1 % were either too small to be

1" 50 ,9 o o

q~o • o ~ o

o

30 - °

10 -

o B o

% \ • • t o

• . - ' % . . - , . ' - . . . - . . . ; - • • • • • A A • & •

' 1 ' - - 7 - - 1 ' ' ' 4..5 4.6 &7 4..8 4.9 5.0 t

100 200 300 4.00

n

- L 0

- 2 0

- 0

TL mm

n-1 -1

Fig. 5. Btennius pavo. Larval survival in 1-1 rearing jars in relation to total length (A) and stocking density (o). Left ordinate: survival (%); right ordinate: absolute numbers of survivors (O) in relation

to stocking density

sexed by external characters or kept an i n -be tween posi t ion as shown in Figure 7b. The m e a n total l eng th of the males was 50.0 ± 7.I mm and the females measured only 43.0 i 5.6 mm. Both males and females then showed an approximated normal distribution. With the popu la t ion growing, the male group developed a distinct two-peak distr ibution (Fig. 6 [16, 18]), whi le the females cont inued to display a normal distribution. The larger size group (more than 65 mm TL) consisted almost exclusively of males except for one female and one non-sexab le individual .

3 3 0

35-

H. v. W e s t e r n h a g e n

30-

25-

20-

15-

:I 0 / ....... I

20 3~0

A

n=t28 December (6)

February (8)

March (9)

May (11)

n=63

_17

B

~rz- 7

n=91

I M ii

n:90

,n r~- July n=90 I I~ I I (13)

September {15)

L _ :

n=97 October n=69 ~J _ [4 _

Dec pr

40 50 50 70 80 Total length (mini

I ~ mate nonsexabIe f e ~ i e

r-~130 401 ~ 5~0 ~ 60T--

Fig. 6. Blenniuspavo, Growth of juven i les r ea red (A) in groups (s tocking dens i ty = 84-128) a n d (B) in isola t ion in a c losed s e a wa te r sys t em (21 oC; S = 34 x 3 10- ) (Figures in p a r e n t h e s e s ind ica te age

of an imal s in months)

Observat ions on B l e n n i u s p a v o 331

During the last 4 months of the experiment , the sex ratio r ema ined around 1:1.4; with the males compris ing about 36 %, the females 48 to 50 % and the non-sexables be tween 13 and 14 % of the total populat ion.

R e a r i n g of j u v e n i l e s i n i s o l a t i o n

Development and growth of the ind iv idua ls held in isolat ion ini t ia l ly proceeded accord- ing to the pat tern a l ready observed in the former group. Males first became not iceable at a total l ength of 40 m_m~ both sexes' total l eng th d isplayed about normal distribution. At an age of 13 months the females ' m e a n total length was 44.0 ± 3.4 ram; similarly males

!i:

5 c m I'

f

t

Fig. 7. Blennius pavo. Ju.veniles reared in the laboratory, a, c: male; d-f: female; b: non sexable

measured 44.3 ± 4.1 m m (Fig. 6B). After three more months (16) sizes of males and females were still ident ical (TL: 44.6 mm); besides, the animals did not show any length increment . Male to female ratio in the isolated specimens was 1:3.5, and 14.5 % of the animals were non-sexables .

DISCUSSION

The observations on colour pat terns and behaviour made dur ing the study of B l e n n i u s p a v o are in genera l ag reemen t with those reported by Fishelson (1963a) for in situ observations. Yet the aggressive behaviour of adult males dur ing the reproductive season as descr ibed by Eggert (1932) and Fishelson (1963a) has never b e e n observed by us. Al though the popula t ion densi ty was fairly high (23 mature individuals , 10 males, in a bas in of 80 × 150 cm) compared to a natural ly occurring densi ty of 10 f i sh .m -2

332 H . v . W e s t e r n h a g e n

(Fishelson, 1963a), and desp i t e the fact that there were a lways severa l ma les d i sp l a y ing nup t ia l coloration, no f ight ing for nes t ing p laces or terr i tories , excep t for occas iona l th rea ten ing , was observed .

In fact it s e e m e d as if no e s t ab l i shed terr i tor ies ex i s ted at al l , s ince ma le s g u a r d i n g eggs occurred s ide b y side, the i r nes ts only some 10 cm apart . It was also not so tha t the l a rges t m a l e (TL: 123 mm) d o m i n a t e d al l o ther males to the ex ten t tha t t hey were en t i re ly p r e v e n t e d from pa r t i c ipa t i ng in the b r e e d i n g act ivi t ies , as obse rved b y F i she l son (1963a). At t imes, up to 4 different males were obse rved g u a r d i n g eggs, and the nes t a lways consis ted of at l eas t three egg clusters which, d e t e r m i n e d on the bas is of the different larval l eng ths at ha tching, h a d b e e n depos i t ed b y up to th ree di f ferent f emales (Fig. 2). One reason for the a p p a r e n t absence of agg res s ive b e h a v i o u r migh t have b e e n the fact that the b a s i n was p r o v i d e d wi th a surplus of shel ters for the ma le s in the group. Thus, s ince the ma le s we re not vagrant , once each ma le h a d m o v e d into i ts own she l te r no f ight ing oppor tun i ty deve loped .

In the exper iment , the reproduc t ive season p roved qui te long. The first s p a w n i n g could be observed b y the end of March, and the last s p a w n i n g occurred in December , la te r than in the f ie ld w h e r e the s p a w n i n g season is sa id to last unt i l Augus t (Fishelson, 1963a).

Rear ing t e m p e r a t u r e was 15 °C throughout , a n d s ince the fish we re e x p o s e d to na tu ra l dayl ight , bu t not na tu ra l t e m p e r a t u r e condi t ions, the l igh t r e g i m e mus t have b e e n the s t imulus for oviposi t ion, as found for B1ennius pho l l s by Shack ley & King (1977}. Some of the males , however , a p p e a r e d to b e a lways in reproduc t ive mood, s ince different males d i s p l a y e d nup t ia l colora t ion th roughout the year . The ex tens ion of the s p a w n i n g pe r iod migh t have b e e n due to the re la t ive ly low wa te r t e m p e r a t u r e of 15 °C, which is p robab ly b e l o w the op t imum rea r ing t empe ra tu r e for the species , but it p rov ides the inves t iga tor wi th e g g and la rva l ma te r i a l a lmost con t inuous ly th roughout the year .

At a l l t empera tures , l a rva l l eng th at ha tch ing was g rea te r t han de sc r ibed by F i she l son (1963b) who r epor t ed newly ha t ched l a rvae of B. p a v o 3.0-3.2 m m long, or Cipr ia (1936} who m e a s u r e d 4.2 nun (TL). As obse rved in other fish spec ies (Forrester & Alderd ice , 1966}, the dura t ion of ha tch ing dec reased wi th inc reas ing incuba t ion t emper - ature, las t ing from 4 to 5 days at 23.7 °C to 8 days at 15 °C for a c luster of eggs from one s ingle deposi t ion.

The incuba t ion p e r i o d i nc r ea sed cons ide rab ly wi th fa l l ing incuba t ion t empera tu re , 15 °C b e i n g p r o b a b l y the lowes t poss ib le t empe ra tu r e for successful i ncuba t ion as fo l lowed from the i r r egu la r d i s t r ibu t ion of ha tch ing b e l o w this t e m p e r a t u r e and from the fact that at and b e l o w 15 °C not a l l the eggs hatched, and the ma le fish left the nes t wi th d e v e l o p e d but non -ha t ch ing embryos stil l in their egg cases.

The larval l eng th d is t r ibu t ion fo l lowed that obse rved in other mar ine fishes. Wi th inc reas ing incuba t ion t empera tu re , la rva l l ength dec rea sed (Forrester & Alderd ice , 1966; Alde rd ice & Velsen, 1971; Ryland et al., 1975). A m o n g the l a rvae ha t ch ing from one s ing le oviposi t ion, l eng th va r i ed with t ime of ha tching. The ea r ly ha t c he d la rvae we re small , i nc reas ing in s ize once a p p r o a c h i n g the m e d i a n ha t ch ing t ime a n d dec reas - ing in length towards the end of the ha t ch ing period. This p h e n o m e n o n is commonly obse rved in the l eng th d i s t r ibu t ion of n e w l y ha tched l a rvae and has b e e n de sc r ibed in de ta i l by Alderd ice & Ve l sen (1971} for Pacific herr ing.

Observat ions on Blennius pavo 333

L a r v a l s u r v i v a l

Larval survival var ied from more than 50 % to 3 %, va lues which are commonly ob ta ined w h e n rear ing pelagic mar ine fish larvae unde r s imilar condit ions (Engraulis mordax, Lasker et al., 1970; Anchoa mitchilli, Achirus lineatus, Saksena & Houde, 1972}. The p h e n o m e n o n that larval survival was inversely related to stocking densi ty has also b e e n documented by Houde (1975, 1977) w h e n rear ing Archosaurus rhomboidalis, Anchoa mitchilli and Achirus lineatus larva in 1-t containers. In A. mitchilli larvae this effect occurred at relat ively low-level densi t ies b e t w e e n 1 a nd 32 indiv iduals per lit.re, and in A. rhomoidales opt imum survival was ach ieved at stocking densi t ies of 8 indiv iduals per litre. As in Houde ' s experiments , effects of ini t ia l s tocking densi ty on larval survival of B. pave were not wel l def ined but existent, a nd opt imum s tocking densi ty of 15 larvae per litre are in good agreement with the f indings of other authors work ing with various mar ine fish species (Okamoto, 1969; Olesen & Minck, 1983; Lasker et. al., 1970; Thei lacker & McMaster, 1971; Saksena & Houde, 1972}. In older larvae, the bot tom stages of B. pavo might have deve loped some sort of territorial behaviour which might have caused casualt ies through bi t ing; this is of part icular interest whi le it would occur only in demersal fish species with pelagic larvae, s ince the relat ive stocking densi ty of the larvae had gone up whi le the larvae were chang ing from the three- d imens iona l space of the total water volume to a two-d imens iona l distr ibution in the bottom of the rear ing jars. Houde (1975) postulated a competi t ive interact ion among larvae, which leads to reduced growth and survival. This p h e n o m e n o n could not be wel l def ined in pe lagic species such as in anchovies, bu t i n demersa l forms the above men t ioned behav iour might contr ibute to increased mortal i ty dur ing and after the t ransi t ion stage.

D e v e l o p m e n t of j u v e n i l e s

One of the observed p h e n o m e n a dur ing the deve lopment of the juveni les was the early occurrence of "crested males" in the experiments. At a total l ength of 40 ram, a size at which they were supposed to be still juveni le , Blennius pavo males developed a typical crest on the neurocran ium, a s ign for in i t ia t ion of maturat ion. In situ, the hoods develop much later and Eggert (1932} descr ibed a male spec imen of 68 mm that had not yet deve loped his hood a l though practically mature. Our own observat ion in the field confirm that crested males are usua l ly at least 70 m m in length. Differentiation be t w e e n male and female on the basis of crest formation, especial ly in small individuals , is difficult, s ince females, too, bear traces of a hood (Eggert, 1932). Figure 7 demonstrates that external sex discr iminat ion in adolescent /3 , pavo is somewhat critical. Spec imen a can be de te rmined clearly as male, spec imen c is considered male too. Spec imen b is

non-sexab le and spec imens d-g are considered females. The ques t ion whether the observed unequa l sex ratio is an artifact due to failure in

sexing because of the late or non -appea rance of the crest (see E. ggert, 1932}, or it is an effect of social control of matura t ion as described for Xiphophorus variatus (Yamagishi et al., 1974) which delays the matura t ion of the males, or whether it is social control of sex, as descr ibed by Fishelson (1970} for the protogynous fish Anthias squarnipinnis a nd the c leaner wrass Labroides dimidiatus (Robertson, 1972), is at this stage of the invest iga- t ions imposs ib le to answer; i n par t icular since, for the t ime be ing , deta i led histological

334 H . v . W e s t e r n h a g e n

i n f o r m a t i o n is w a n t i n g . T h e r e is, h o w e v e r , a n i n d i c a t i o n for t h e p r o b a b i l i t y of t h e

e x i s t e n c e of soc ia l s e x c o n t r o l i n B. pavo s i n c e H e y m e r (1980} r e p o r t e d s e x r e v e r s a l i n a

f e m a l e B. basiliscus, a s p e c i e s c l o s e l y r e l a t e d to B. pavo, a f t e r t h e d o m i n a n t m a l e of t h e

g r o u p h a d d ied , E x a c t i n f o r m a t i o n o n t h e s u b j e c t w i l l b e p r o v i d e d a f t e r t h e e v a l u a t i o n of

h i s t o l o g i c a l s e c t i o n s of d i f f e r e n t s t a g e s of d e v e l o p m e n t of B. pavo, the r e s u l t s of w h i c h

a r e g o i n g to b e p u b l i s h e d i n d u e course .

C o m p a r i n g t h e d e v e l o p m e n t of t h e t w o p o p u l a t i o n s , t h e i s o l a t e d a n d t h e g r o u p

r e a r e d one , w e n o t i c e d i s t i n c t d i f f e r e n c e s b e t w e e n t h e t w o g r o u p s . T h e d e v e l o p m e n t of

t h e g r o u p r e a r e d p o p u l a t i o n is t y p i c a l for p o p u l a t i o n g r o w t h i n f i sh w i t h t e r r i t o r i a l

b e h a v i o u r or s i ze h i e r a r c h y . T h e p h e n o m e n o n t h a t t h e d i f f e r e n c e i n s i ze b e t w e e n t h e

s m a l l e s t a n d t h e l a r g e s t i n d i v i d u a l i n c r e a s e d w i t h s i ze is k n o w n as g r o w t h d e p e n s a t i o n

( J o n e s & T h o m p s o n , 1980), a n d is w e l l k n o w n to a q u a c u l t u r i s t s . B e s i d e s , i t h a s b e e n

d o c u m e n t e d b y B r o w n (1946} for b r o w n t r o u t Salmo trutta, b y A l l e e e t al. (1948) for t h e

g r e e n s u n f i s h Lepornis cyanellus, a n d Y a m a g i s h i e t al. (1974) for Odontobutis obscurus r e a r e d i n cap t iv i ty . T h e p h e n o m e n o n h a s a l so b e e n o b s e r v e d i n t h e f i e l d b y D o h e r t y

(1982) i n t h e t e r r i t o r i a l d a m s e l f i s h Pomacentrus wardi. In t h e p o p u l a t i o n of i s o l a t e d B.

pavo s p e c i m e n s , t h i s p h e n o m e n o n c o u l d n o t o c c u r (Fig. 6). Ye t t h e i n s i t u s i t u a t i o n w i l l

d i f f e r f r o m t h e l a b o r a t o r y e x p e r i m e n t i n t h e s e n s e t h a t g e n e r a l l y b o t t o m s t a g e s a n d

j u v e n i l e s a r e n o t g o i n g to f i n d h a b i t a t s d e v o i d of o t h e r B. pavo, b u t t h e y w i l l s e t t l e i n

a r e a s w i t h a n e x i s t i n g B. pavo p o p u l a t i o n a n d t h u s t h e n e w c o m e r s for t h e t i m e b e i n g w i l l

h a v e to r e m a i n f e m a l e s if s o c i a l l y c o n t r o l l e d s e x d e t e r m i n a t i o n e x i s t s i n t h e s p e c i e s

u n d e r c o n s i d e r a t i o n .

T h e q u e s t i o n as to h o w fa r t h e e x p e r i m e n t a l l y o b t a i n e d r e s u l t s a r e a p p l i c a b l e to t h e

f i e l d s i t u a t i o n c a n n o t b e a n s w e r e d yet , s i n c e n o i n f o r m a t i o n is a v a i l a b l e o n s e x r a t i o a n d

s i ze d i s t r i b u t i o n of B. pavo i n n a t u r a l p o p u l a t i o n s . It m i g h t b e c o n c e i v a b l e , h o w e v e r , t h a t

i n s i t u s e x rat io , too, is d e t e r m i n e d b y o n e of t h e f ac to r s m e n t i o n e d a b o v e . G r o w t h

d e p e n s a t i o n a n d t h e n u m b e r of n e s t i n g h o l e s a v a i l a b l e m i g h t b e f u r t h e r f ac to r s i n f l u -

e n c i n g t h e m e c h a n i s m of s e x d e t e r m i n a t i o n i n B. pavo p o p u l a t i o n s .

Acknowledgement. I am indeb ted to G. Fi i rs tenberg for taking care of the exper imenta l fish and expert technical assistance.

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