Kenyan coral reef-associated gastropod fauna: a comparison between protected and unprotected reefs

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Kenyan coral reef-associated gastropod fauna:

a comparison betw een protected and unprotected

reefs

T. R. McClanahan*

C oral R eef C o n servation Project, PO B ox 99470, M om basa, Kenya and F rien ds W orld C o lle g e , East A frican C en tre, PO Box 526, M ach akos, Kenya

ABSTRACT: A com parative survey of prosobranch gastropods w as u n d e rta k e n w ithin 3 m arine parks and 3 unpro tected reefs to com pare fauna] differences attrib u tab le to m an ag e m en t an d n o n m a n a g e ­ m ent policies. The total gastropod fauna w as typified by low densities an d high diversity an d variability in species composition. P rotected reefs h ad a higher diversity for com bined reef locations an d h ig h er species richness on reef ed g es and lagoons but not reef flats. D ifferences in reef e d g es are attrib u tab le to physical differences in reef aspect and w ave energy but lagoon differences are attrib u tab le to hum an influences. Total gastropod densities w ere sim ilar for com parisons b etw een reef flat an d ed g es b u t unprotected reef lagoons had higher (p < 0.001) densities than protected lagoons. Overall, most com m ercially collected species w ere not significantly d en ser in protected sites than u n p ro tected sites. Some species such as Lam bis truncata and L. chiragra w ere clearly more dense w ithin p ro tected sites, w hich may be attributable to shell collection in unp ro tected reefs. M any im portant com m ercial species including Charonia tritonis, Cassis cornuta, Turbo m armarotus, C hicoreus ram osus an d C ypraecassis rufa w ere found at very low densities reg ard less of location an d m ay naturally occur at such low densities. An experim ental study of p redation on the gastropod Morula granulata su g g e ste d th at predation is low b u t m ay affect gastropod distribution p attern s as the total gastropod density w as well predicted (r= —0,96, p < 0.01) by p redation rates on M. granulata. U nprotected reef lagoons h ad low er predation rates [p < 0.06) th an protected lagoons. R educed pred atio n rates an d in creased densities m ay be caused by finfish reductions due to hum an fishing activities. Shell collecting ap p ears to be affecting a few but not most populations. Less direct ch an g es in th e gastropod fauna a p p e a r to be caused by overfishing of finfish an d a reduction in their predators.

INTRODUCTION

Ecological r e s e a r c h s u g g e s ts t h a t h u m a n utilization of coral reefs n o t only affects e x p lo ite d sp e cie s p o p u l a ­ tions b u t m a y indire ctly affect u n e x p l o ite d species, c o m m u n ity stru ctu re a n d ec ologic al p rocesses, s u c h as p roductivity a n d th e ca lciu m c a r b o n a t e b a l a n c e (Sam- m arco 1980, 1982, H a y 1984, H a y & T aylor 1985, Lewis 1986, H u g h e s et al. 1987, M c C l a n a h a n & M u th i g a 1988, 1989). M ost w o rk h a s fo c u se d on a l g a e -c o ra l- s e a urchin-fish in te ra ctio n s a n d less w o rk h a s b e e n u n d e r ­ ta k e n on o th e r ta x o n o m ic groups. This stu d y p r e s e n ts findings from a co m p a ra ti v e s u rv ey of p ro te c te d a n d u n p r o t e c t e d reefs to d e t e r m i n e p ossible effects of shell ‘ Present address: C en ter for W etlands, U niversity of Florida,

Gainesville, Florida 32611, USA

collecting a n d fishing on p r o s o b r a n c h g a s t r o p o d p o p u ­ lations a n d th e ir c o m m u n ity stru ctu re .

K e n y a n ree fs offer a u n i q u e o p p o r tu n ity to stu d y diffe re n c e s b e t w e e n e x p lo ite d a n d u n e x p l o i t e d reefs b e c a u s e of th e close proxim ity of 3 m a r i n e p a r k s a n d ree fs t h a t e x p e r i e n c e vario u s levels of fishing a n d shell collecting. Shell collection h a s r e c e n tly b e e n a n e n v i ro n m e n ta l c o n c e r n (Knowles 1970, Y a n i n e k 1976, E v an s et al. 1977, Wells 1981, Wells e t al. 1983). S c i e n ­ tific r e s e a r c h on p o p u la tio n c h a n g e s d u e to sh e ll c o llec­ tion h a s re c e iv e d s o m e p r e li m in a r y a t te n t io n (Brownell & S tev e ly 1981, C a tte ra ll & P o in er 1987, P o in e r & C at- terall 1988) b u t th e effects t h a t r e m o v a l m a y h a v e on m u ltip le sp e c ie s a n d c o m m u n ity s tr u c t u re r e m a i n poorly stu d ied . R ecently, M c C l a n a h a n & M u t h i g a (1988) s u g g e s t e d th a t th e r e m o v a l of finfish w ith in u n p r o t e c t e d K e n y a n reefs h a s c a u s e d s e a u r c h in p o p u

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latron i n c re a s e s a n d p a rtic u la rly i n c re a s e s in th e rock- b o r in g s e a u rc h in E c h in o m e tra m a th a e i. T h e rem o v a l of finfish a n d s e a u r c h in p o p u la tio n i n c re a s e s m a y also h a v e o t h e r in d ire c t effects on g a s tro p o d p o p u la tio n s (Ayling 1981, H a y & T ay lo r 1985). This s u rv e y w as i n t e n d e d , not to d e finitively d e t e r m i n e th e effect tha t th e s e facto rs h a v e o n th e ir p o p u la tio n s , d istribution a n d diversity, b u t ra th e r, as a p r e li m in a r y s u rv ey to s u g g e s t po ssib le rela tio n s h i p s r e s u lt in g from h u m a n utilization a n d o th e r biological a n d e n v i r o n m e n t a l factors.

STUDY SITES

K e n y a 's c oast so u th of M a lin d i is b o r d e r e d b y a n ea rly co n t in u o u s f n n g i n g r e e f w h ic h lies b e t w e e n 100 m a n d 3 k m from sh o re (Fig. 1). T h e 3 u n p r o t e c t e d sites c h o s e n w ith in this stu dy, Diani, B a m b u r i a n d K an am ai, lie a lo n g this frin g in g re e f a n d h a v e a ESE aspect. T h e 3

K E N Y A Watamu 3° 30'-K anam ai Mombasa D ia n i 4°30'-. ' K is ite

Fig. 1. K enya's so u th ern coastline a n d site locations

M a rin e Parks, Kisite, W a t a m u a n d M alindi, also lie alo n g this reef b u t Kisite a n d M a l i n d i ’s N o rth Reef are s e p a r a t e d a n d iso la te d from th e m a jo r fringing reef. Kisite h a s a SE a s p e c t a n d M a lin d i's N o rth Reef a n d frin g in g r e e f h a v e N E as p ects. W a t a m u is p a r t of th e m a jo r frin g in g ree f b u t t h e reef flat is low (0.3 m a b o v e datu m ) a n d rarely e x p o s e d . N e v e rt h e le s s , m ost reefs h a v e a ree f e d g e , ree f flat a n d reef la goon. E xce ptions are M a l i n d i ’s frin g in g ree f w h ic h la cks a well- d e v e l o p e d r e e f la g o o n a n d W a t a m u w h ich h a s a sh a llo w a n d r o u g h re e f flat a n d e d g e , n o t c o n d u c iv e to s u r v e y ­

ing. T herefore, s a m p lin g w a s only u n d e r t a k e n in W a t a m u ’s reef la g o o n a n d M a lin d i's fringing ree f flat a n d ed g e . M a lin d i’s Sm all Reef a n d T e w a Reef w e r e also s a m p le d a n d in c lu d e d in th e N o rth Reef la g o o n d a t a d u e to th e ir similarity in ree f structure. This res u lt e d in 3 r e p lica te sites for e a c h reef location a n d m a n a g e m e n t ty p e (p ro te cted vs u n p ro tec te d ). T h e only co n siste nt p h y sical d ifferen ces b e t w e e n p r o te c te d a n d u n p r o ­ te c te d sites is th a t u n p r o t e c t e d sites m a y receiv e g r e a te r w a v e e n e r g y on reef ed g e s. T h e ir a s p e c t faces almost directly into th e p r e v a ilin g sw ells d u r in g the s o u th e a s t m o n so o n s w h e n w a v e e n e r g y is g r e a t e s t ( M c C la n a h a n 1988). A priori, site dif ferences w o u ld b e s u s p e c te d b e t w e e n p ro te c te d a n d u n p r o t e c t e d reef ed g e s, less for reef flats a n d n o n e for reef lagoons. Strict controls for u n p r o t e c t e d sites w e r e not a v a ila b le since P ark s e l e c ­ tion w a s i n d e p e n d e n t of scientific considerations.

METHODS

S am p lin g w a s u n d e r t a k e n d u r in g d a y l ig h t low tides: w a lk i n g on ree f flat a n d e d g e s a n d sn o rk e li n g w ithin lagoons. Reef e d g e sites co n s isted of th e s e a w a r d e d g e of th e flat b e t w e e n M LWN a n d MLWS. Reef e d g e s w e r e s a m p le d exclu sively d u r in g sp rin g low tides. O nly sha llow (< 2.5 m) reef la g o o n s w e r e sa m p le d . S a m p lin g w a s u n d e r t a k e n by a 1 h r a n d o m w a lk or sw im w ith in e a c h site (Kohn 1968). D u rin g this s a m p lin g tim e p r o ­ s o b r a n c h g a s tro p o d s w e r e id entified, th e tim e that th e first in d iv id u a l of a sp e cie s w a s e n c o u n t e r e d w as r e c o rd e d a n d s u b s e q u e n t in d iv id u a ls of th e give n s p e c ie s tallied. S pry (1968) is th e single m ost useful r e f e r e n c e for id en tify in g East A frican g as tro p o d s an d th e re fo re his n o m e n c la tu r e is used, a l th o u g h it differs s o m e w h a t from m o re s t a n d a r d re f e r e n c e s (Abbott & D a n c e 1986). Initial a t te m p ts to u se spatial tran se cts in d ic a t e d a low d en s ity a n d an in efficient u se of tim e w ith in the in te rtid al z o n e a n d th e re fo re tim e sa m p lin g w a s u se d. Initial studies w e r e ta k e n at 3 d ifferent sites to d e t e r m i n e th e optim al s a m p le tim e a n d to d e t e r m in e if sp e cie s ric h n e ss le v e le d b e y o n d a g iv e n time. S p e cie s ric h n e ss did not level; th ere fore, a 1 h s a m p le w a s ch o s e n as tidal p a t te r n s a llo w ed ca 3 h of reef e m e rs io n p e r day, w h ic h a llo w ed e a c h ree f location to b e s a m p le d p e r day. S am p lin g w a s u n d e r t a k e n at different tim es for e a c h ree f b e t w e e n N o v e m b e r 1986 a n d M a rc h 1988. With in s a m p lin g tim es m o v e m e n t s w e r e slow b u t c o n tin u o u s in o rd e r to avoid s a m p lin g restricted areas. B e tw e e n s a m p lin g tim es different s e c ­ tions of the ree f w e r e s a m p le d in o rd er to o b ta in the b r o a d e s t r e p r e s e n ta t iv e sam p le s. D u rin g s a m p lin g tim es loose b o u ld e rs w e r e o v e r tu r n e d a n d e x a m in e d b u t only as th e y w e re e n c o u n t e r e d d u r in g th e walk. E ac h in d iv id u a l site w a s s a m p l e d a m in i m u m of 3

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times. A total of 82 h sa m p lin g , 41 h in ea c h of the p r o te c te d a n d u n p r o te c te d reefs, w e r e u n d e r ta k e n . S a m p le tim es are g iv e n in T a b l e 2.

D ensity (no. h - 1 ) a n d diversity (Sim pson's Index, D =

1 - (n,/N,)2) w e r e c a lc u la ted for e a c h site. Sim p so n 's Index w as u s e d in s t e a d of th e S h a n n o n W e av e r Index b e c a u s e it is less se nsitive to s a m p le size (Kohn & N y b a k k e n 1975, R o u tled g e 1979) a n d th ere fo re m ore re p r e s e n ta t iv e of div ersity w h e n de n s itie s a re low. S p ecies rich n e ss w a s d e t e r m i n e d by spe c ie s-tim e a n d s p e c ie s-in d iv id u a l curves. R a n d o m ly s e lec ted 1 h sa m p le s w e r e u s e d a n d in th e case of spe cie s-tim e curve s a v e r a g e n u m b e r of sp e c ie s found p e r time in t e r ­ val w e r e ca lcu lated . D a ta w e r e r e g r e s s e d to fit the e q u a tio n S = C x z to o b ta in the C a n d z co n s ta n ts w h e r e

X is tim e in h (f) or in d iv id u als (ƒ), a n d S is the n u m b e r of species.

D ata from p r o te c te d a n d u n p r o t e c t e d reef sites w ere p o o le d for co m p a riso n s of sp e c ie s richness, family c o m ­ positio n a n d p o p u la tio n den s itie s of co m m ercial a n d n o n c o m m e rc i a l specie s. C o m p a ris o n s (M a n n W h itn ey ¡7-test) of co m m ercia l a n d n o n c o m m e rc i a l species in c lu d e d 41 sp e cie s of g a s tro p o d s s e e n in shell stalls a n d this survey. O nly 15 of th e 41 sp e cie s w e r e suffi­ ciently d e n s e to w a r r a n t co m p a riso n a n d th e re m a in in g 26 sp e cie s w e r e p o o le d into o n e sam ple. Fifteen c o m ­ m o n n o n c o m m e rc i a l sp e c ie s w e r e ran d o m ly se le c te d to act as controls to d e t e r m in e if dif fe rences w e r e a t tr ib u t­ able to collection r a th e r th a n intrinsic site differences.

To d e t e r m in e th e effect th a t o b s e rv e r bia s m ig h t play in th e d a t a collection, 2 o b se rv e rs w o rk e d i n d e p e n d ­ ently for 17 h w ith in the s a m e locations. D u rin g r a n d o m intervals, the o b s e rv e r's p a c e s p e r 30 s w e r e rec o rd e d . T h e l e n g th of th e o b s e rv e r's step w a s m e a s u r e d an d field of vision es tim a ted . T h e n u m b e r of individuals a n d sp e cie s e n c o u n t e r e d a n d the o b s e rv e r's rate s of m o v e m e n t w e r e co m p a red .

To d e t e r m i n e th e role of p re d a tio n on g as tro p o d d istribution a n d density, a p r e d a tio n e x p e r i m e n t w as u n d e r t a k e n . M orula gra n u la ta w e r e a t t a c h e d to m o n o ­ filam en t line (0.4 mm) w r a p p e d a r o u n d th e ir shells a n d a d h e r e d w ith 's u p e r glue'. Shells w e r e d ried a n d s a n d e d befo re application. M . g ra n u la ta w as c h o s e n as it is u b iq u ito u s on K e n y a n reefs a n d its shell s h a p e allo w e d for go o d a d h e r e n c e to th e m o nofila m ent. Within th e M a lin d i N orth, W a ta m u , K a n a m a i a n d Diani reefs 10 e x p e r im e n t a l in d iv id u a ls w e r e a t ta c h e d to n ylo n tra n s e c t lines at 2 m in terv als alo n g th e line. T h re e tra n s e c t lines w e r e p la c e d w ith in e a c h ree f flat a n d lagoon site, e x c e p t W a ta m u , w h e r e only the reef la g o o n w a s studied. T ra n s e c ts w e r e v isited every 24 h for 3 d. M issing in d iv id u als w e r e r e c o rd e d an d a s s u m e d to r e p r e s e n t p r e d a tio n or a t t e m p t e d p r e d a ­ tion. Control in d iv id u a ls (n = 15) w e r e p la c e d in sea w a te r ta n k s for th e s a m e tim e p erio d to d e t e r m i n e if

in d iv id u a ls m ig h t b e c o m e d e t a c h e d from th e m o n o fila ­ m e n t d u e to s e a w a t e r im m ersio n . C o m p a r is o n s of s u r ­ vival b e t w e e n ree f flats a n d re e f la g o o n s, p r o t e c t e d a n d u n p r o t e c t e d reefs, a n d th e effect of p r e d a tio n on g a s ­ tropod d en s ity a n d diversity w e r e u n d e r t a k e n .

RESULTS

A lth o u g h o b s e rv e rs m o v e d a t signific antly d ifferen t rates, c o m p aris o n s b e t w e e n o b s e rv e rs in d ic a t e a close a g r e e m e n t in the n u m b e r s of s p e c ie s a n d in d iv id u als o b s e rv e d (Table 1). T h e 2 o b s e rv e rs u s e d d ifferen t strategie s. T h e a u t h o r m o v e d a t a p p r o x im a te ly tw ice T able 1. C om parison of n u m b er of species a n d individuals (x

± SD) found during 1 h sam pling periods (n = 17) by 2 in d e p e n d e n t observers w orking w ithin the sam e location, and their rates of m ovem ent (n = 15). M ann W hitney ¡7-test c om panson b etw een observers. NS: not significant, ‘ ‘ '

p < 0 .0 0 1

Species h 1 Ind. h 1 Paces 30s 1 O bserver 1 13.06 ± 4.96 56.82 ± 43.31 21.93 ± 4.64 O bserver 2 13.53 ± 6.77 66.11 ± 49.59 11.73 ± 6.42 Significance NS NS th e s p e e d of th e s e c o n d o b s e rv e r b u t m a i n t a i n e d a n a r r o w 1 m field of vision c o m p a r e d to th e s e c o n d o b s e rv e r's 2 m field of vision. C o n s e q u e n t ly , th e a u t h o r c o v e red 1450 m 2 h -1 c o m p a r e d to 1270 m 2 h -1 for the s e co n d o b se rv e r. As s o m e in d iv id u a ls w e r e p ro b a b ly m is sed, a n a r e a of 1000 m 2 h _1 c a n p r o b a b ly b e t a k e n a s a n a p p r o x i m a t e tim e -to -s p a ti a l conversion.

D u rin g th e total s a m p l e 127 s p e c ie s from 25 families w e r e r e c o rd e d (species list a n d d e n s itie s a v a ila b le on r e q u e s t from th e author). T h e i r distrib u tio n w a s h ig h ly v ariable, C o m p a r is o n s b e t w e e n ree f locations in d ic a tes low a v e r a g e sim ilarity in d ic e s (S o ren so n 's Index) for t h e 10 m o s t c o m m o n s p e c ie s w ith in re e f e d g e s (0.33 ± 0.11, x ± SD), r e e f flats (0.41 ± 0.13), ree f la g o o n s (0.44 ± 0.15) a n d p a rtic u la rly for all c o m p a ris o n s (0.25 ± 0.16). A nalysis of g a s t r o p o d p e r c e n t family co m p o sitio n (Fig. 2) in d ic a t e s a sim ilarity b e t w e e n e d g e s a n d flats, th e d o m i n a n c e of th e T h a i d id a e , a n d th e i m p o r ta n c e of th e C o n id a e , V a s i d a e (= T u rb in ellid a e ) a n d C y p ­ r a e id a e . A Wilcoxon r a n k s u m te st fo u n d n o significant diffe re n c e s in family co m p o sitio n for lo cations or m a n ­ a g e m e n t ca te g o rie s. In te rm s of tro p h ic ra n k , this an aly sis sh o w s th a t m o s t g a s tro p o d s on reef flats a n d e d g e s a re p re d a to rs . T h a i d i d a e a p p e a r to d o m in a te th e u n p r o t e c t e d reefs w h e r e a s p r o t e c t e d sites a p p e a r to h a v e a g r e a t e r rela tiv e a b u n d a n c e of cy p ra eid s. Reef la g o o n s h a v e a m o re e v e n p ar ti tio n in g of th e relative

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UNPROTECTED PROTECTED Conidae 164 Cypraeidae F a s c i o l a r i d a e ' V C ^ / / S t r o m b id a e n W / / O t h e r s 3.1 C o llu m b e llid a e 2.1 \ Conidae Thaididae 70 3 - F a s c io la n d a e -T ro c h id a e 1.0 - B u r s i d a e 2.0 - O t h e r s 1.3 T h aid id ae 1.7 Cypraeidae 12.6 S tro m b id a e ' ^ 12.0 - y - O t h e f s 2 9 /-F a s c io la n d a e N a ssarid ae 21 Conidae C o ra llio p h ilid a e Ovulidae S tro m b id a e 2 2 7 -C erith n d ae Conidae 10.5 / T haididae 36.9 Cypraeidae 22.3 Vasidae 11.9 O th e rs 3.0 Conidae 12.1 T h aidid ae 18.2 REEF LAGOON C y p raeidae O th e rs 2 5 1A , . . Trochidae F a sc io la n d ae T h aididae 21 Fasciolaridae M itridae 2.6 Buccinidae 2 5 Stro m b id ae 1.1 B u c c in id a e 2 9 - C o llu m b e llid a e S t r o m b id a e 3 .6 ^ O t h e r s 2.C N e r itid a e ^ C e rith rid a e

Fig. 2. Family com position of the g a s­ tropod fauna on protected and u n p ro ­ tected reefs en co u n tered w ithin the various reef locations. (T haididae = M urcidae, V asidae = Turbinellidae)

a b u n d a n c e of families a n d c o n s e q u e n t l y a g r e a t e r tro p h ic diversity. Soft a n d h a r d corallivorous g a s ­ tropods, O vu la o v u m a n d C oraliophila violacea, a n d t h e d e t ri tiv o r o u s /h e r b iv o r o u s S tro m b id ae , a p p e a r r e l a ­ tively m o re i m p o r t a n t w ith in lag o o n s. W ithin u n p r o ­ te c t e d ree f la goons, p a rtic u la rly Diani, a l a rg e p o p u l a ­ tion of C oraliophila violacea d o m in a te s a n d i n c re a s e s th e re la tiv e i m p o r ta n c e of th e C o rallio p h ilid ae. No O v u lid a e a n d f e w e r S t ro m b id a e w e r e fo u n d w ith in u n p r o t e c t e d reefs.

D en sity a n d div ersity m e a s u r e m e n t s i n d ic a t e low d e n s ity a n d h i g h div ersity for m o s t sites (Tables 2 a n d 3). D en sitie s on ree f e d g e s a n d flats w ere , in g e n e ra l, of m a g n i t u d e 1 g a s tro p o d p e r 10 m 2. Reef flats a n d e d g e s h a d th e h ig h e s t d e n s itie s a n d c o m p aris o n s b e t w e e n p r o t e c t e d a n d u n p r o t e c t e d reefs are n o t s i g ­ n ific an t (Table 2). P ro te c ted re e f la g o o n s h a d lo w er d e n s itie s o n th e o rd e r of 1 p e r 100 m 2 b u t g a s tro p o d

d en s itie s w e r e sig nificantly h ig h e r in u n p r o te c te d reef la g o o n s (Table 2). Diversity, as m e a s u r e d by Sim p so n 's Index, is h ig h for m ost sites a n d th e re a re no clear distin ctio ns b e t w e e n e d g e , flat a n d la g o o n sites. C o m ­ p a ris o n s b e t w e e n p r o te c te d a n d u n p r o te c te d sites in d ic ate th a t th e total diversity of p ro te c te d a r e a s (D = 0.88 ± 0.14, X ± SEM) is greater, a l th o u g h ju s t short of sig nificantly so ( M a n n -W h itn e y D-test, p = 0.056), th a n u n p r o t e c t e d sites (D = 0.78 ± 0.05). This may, in part, b e a t tr ib u te d to th e h ig h relative den s itie s of ce rtain sp ecie s such as C oraliophila violacea within D ia n i’s ree f la goon, D rupa m o ru m on K a n a m a i a n d B a m b u r i’s reef e d g e s a n d C ypraea a n n u lu s a n d

M orula g ranulata on reef flats.

C o m p a ris o n s of in d iv id u a l sp e cie s p o p u la tio n den s itie s (Table 4) in d ic a te low d en sities a n d high variability for most specie s. M a n y of the c o m m o n c o m ­ m ercial sp e cie s such as C haronia tritonis, C yp ra eca ssis

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T ab le 2. To tal d e n si ti e s (n o . h 1 ) , x ± SEM , (n ) oi p ro so b ra n c h s fo un d w it h in th e 3 pro te c te d an d u n p ro te c te d re ef s w it h in th e 3 loc at io n s an d to tal d e n si ti e s fo r ea ch lo c a ti o n A st a ti st ic a l co m p ar iso n (M an n W hi tn ey U -te st ) of th e si te to ta ls is included at th e en d of ea ch row. N S : n o t si g n if ic a n t, ' ' ' p< 0.0 01 tT>

rufa, T urbo m a rm a ro tu s, C h ico re u s ra m o su s a n d C assis cornuta w e r e found at very low d e n s itie s in b o th p r o ­ te c te d a n d u n p r o t e c t e d sites a n d th e re fo re n o t a m e n ­ able to statistical com parison. Results of a c o m p aris o n b e t w e e n p ro te c te d a n d u n p r o t e c t e d reefs, co m b in in g t h e s e a n d 21 o th e r co m m ercia l species, w e r e in sig n ifi­ cant. C o m p a ris o n s of d en s itie s of th e 15 m o re c o m m o n co m m ercial sp e cie s in d ic a t e s th a t a v e r a g e d en s itie s w e r e often h ig h e r w ith in p r o te c te d reefs b u t only 4 sp e c ie s w e r e statistically (p < 0.05) d e n s e r. S e v e n sp ecie s w e r e d e n s e r at th e p < 0.10 level. O n e c o m m e r ­ cial species, C o n u s leo p a rd u s, w a s d e n s e r w ith in u n p r o ­ te c te d reefs. C o m p a r i n g n o n c o m m e r c i a l s p e c ie s i n d i ­ ca te s th a t 10 c o m p aris o n s w e r e not significant, 2 sp e c ie s w e r e d e n s e r a n d 3 less d e n s e in u n p r o t e c t e d th a n in p ro te c te d reefs. U sin g th e n o n c o m m e r c i a l s p e c ie s as e x p e c t e d f re q u e n c ie s in a G -test (Sokal & Rohlf 1981) in d ic a t e s th a t co m m ercia l s p e c ie s a re fre q u e n tl y d e n s e r (p < 0.02) in p r o t e c t e d t h a n in u n p r o t e c t e d reefs.

S p ec ies-tim e c u rv e s (Fig. 3) in d ic a t e th a t p r o t e c t e d ree f la g o o n s a n d e d g e s h a v e m o re s p e c ie s p e r area, as la g o o n s h a d signific antly h i g h e r (i-test, p < 0.01) z v a lu es a n d ree f e d g e s significantly h i g h e r (p < 0.01) C values. Reef flats w e r e sim ilar b y this com parison. Sp ecies as a function of in d iv id u a ls (Fig. 4) also h a d h i g h e r (p < 0.01) z v a l u e s for ree f la g o o n s a n d e d g e s b u t lo w e r (p < 0.01) z v a l u e s for ree f flats. It is clea r th a t a b s o lu te sp e cie s r ic h n e s s will d e p e n d on th e to tal a r e a of th e reef a n d g a s tro p o d density. A lth o u g h p r o t e c t e d reef la g o o n s h a v e h ig h e r z v a l u e s very la rg e a r e a s m a y b e n e c e s s a r y b efo re th e a b s o lu te n u m b e r of sp e c ie s is g r e a t e r t h a n w ith in u n p r o t e c t e d reefs.

Results of th e p r e d a tio n e x p e r i m e n t (Table 5) i n d i­ ca te th a t rem o v a l rate s w e r e low. C o m p a r is o n s b e t w e e n reef flat a n d la g o o n s in d ic a t e t h a t la g o o n s h a d h i g h e r p r e d a tio n rate s, a l th o u th the d iffe ren c e is short of significant ( M a n n W h i tn e y U-test, p = 0.07). Survival in p r o te c te d la g o o n s (2.51 ± 1.02 d, x ± SD) w a s also ju s t short of b e i n g signific antly lo w er (p = 0.55) t h a n in u n p r o t e c t e d re e f la g o o n s (2.92 ± 0.38 d). T h e e x p e r i ­ m e n t m ig h t h a v e b e e n im p r o v e d by i n c re a s i n g its d u r a ­ tion. N e v e rth e le s s , g a s tro p o d d e n s ity a t th e s tu d i e d sites w a s w ell p r e d i c t e d by th e p r e d a t i o n r a te ( p r e d a ­ tion ra te = to tal l e n g t h of e x p e r i m e n t (3 d) — survival (d)) a n d fit w ell to a n e g a t i v e e x p o n e n tia l c u rv e (Fig. 5). S tra ig h t-lin e c o rrela tions of p r e d a tio n v e rs u s S im p s o n 's In d e x a n d z w e r e positive (D, r = 0.52; z, r = 0.62) b u t short of significant at th e g iv e n d e g r e e s of freedom .

D ISC U SSIO N

T h e s a m p lin g m e t h o d u s e d w ith in this stu d y h a s the 2 w e a k n e s s e s of (1) h a v i n g no a b s o lu te m e a s u r e of d e n s ity a n d (2) th e possibility of o b s e rv e r bia s w h e n

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T able 3. Diversity (Sim pson's Index) of the prosobranch fauna at each reef site and location » M alindi north reef P rotected reefs M alindi fringing reefa Kisite Diani U nprotected reefs Kanam ai Bamburi Reef ed g e 0.90 0.83 0.87 0.90 0.66 0.77 Reef flat 0.92 0.90 0.79 0.84 0.75 0.89

Reef lagoon 0.91 0.90a 0.88 0.44 0.85 0.92

a W atam u reef lagoon

T able 4. C om parison of density no. h x ± SEM (n) of com m ercial and noncom m ercial species found w ithin m arine p arks and u n p ro tected reefs. NS: not significant; + p < 0 .1 0 , 'p < 0 .0 5 , " p c O - O l, pcO.OOl

Com m ercial species N oncom m ercial species

Protected U nprotected Protected U nprotected

Lam bis chiragra 0.94 + 0.36(16) 0.0(16) Nerita albicilla 2.00 + 1.06(26) 1.39 ± 0.86(27) NS

Lam bis lam bis 1.31 + 0.42(16) 0.83 ± 0.31(16) NS Cerithinum caeruleum 8.40± 5.09(10) 0.0(11)

Lam bis truncata 1.13 + 0.68(16) 0.0(16) Cerithrium piperitum 0.12 + 0.12(26) 1.84 + 1.02(27) +

Strom bus lentiginosus 0.25 + 0.10(16) 0.39 ± 0.20(16) NS Strom bus gibberulus 1.06 + 0.41(39) 1.43±0.46(36) NS

Cypraea annulus 12.81 ± 2.87(39) 6.79 ± 1.96(36) + Strom bus m utabilis 0.95 + 0.47(39) 0.70 + 0.44(36) NS

Cypraea ly n x 3.13 ± 1.08(23) 1.70 ± 0.92(20) NS Coralliophila violacea 2.28 + 0.87(16) 19.13± 11.6(16) NS

Cypraea m oneta 0.77± 0.28(23) 0.21 ± 0.17(20) + Morula graunlata 2.21 + 1.33(23) 13.48± 2.66(20)

Cypraea tigris 1.38 ±0.47(39) 0.61 ± 0.18(36) NS Morula squam osa 2.00 + 0.94(23) 5.05 ± 1.50(20) NS

O vula ovum 1.31± 0.76(16) 0.0(16) + Thais distinguenda 2.43 + 0.96(23) 0.10± 0.10(20) NS

Pleuraploca trapezium 0.81 t_ 0.26(16) 1.23 ± 0.27(16) NS Thais tuberosa 4.87 + 1.39(23) 4.87 ± 1.07(20) NS Vasum ceram icum 1.52 + 0.41(36) 0.08 ± 0.06(36) * * * Engina mendicaria 1.26 + 0.47(23) 1.25± 0.57(20) NS

C onus chaldeus 0.60 + 0.16(23) 0.10 ± 0.07(20) * Vasum rhinocerus 1.07 ± 0.28(39) 3.77 ±0.53(36) +

C onus ebraeus 1.60 + 0.50(23) 1.11 ± 0.49(20) NS Vasum turbinellus 4.49 ± 1.05(39) 0.76± 0.23(36) . . .

C onus leopardus 0.26 ±0.13(16) 1.34 ± 0.36(16) * C onus ceylensis 0.53 + 0.13(39) 0.24± 0.12(36) NS

Conus m iles 0.38± 0.15(39) 0.25 ± 0.08(36) NS C onus lividus 1.28 + 0.37(39) 1.44 ± 0.41(36) NS O thers (26 species) 1.31 r 0.32(39) 1.05 ± 0.23(36) NS

c o m p a r in g results. R esults s u g g e s t th a t o b s e rv e r b ia s is sm all b u t this m a y b e b e c a u s e b o th o b s e rv e rs w e r e e x p e r i e n c e d in coral re e f s u rv ey m e th o d s . I s u g g e s t t h a t fu tu re r e s e a r c h e r s u sin g this m e t h o d familiarize th e m s e l v e s w ith th e m e t h o d a n d fa u n a b efo re collect­ in g d ata, t h a t th e y q u a n tify th e ir ra te s of m o v e m e n t a n d fields of vision, a n d c o m p a r e th e m s e lv e s w ith a n o t h e r o b s e rv e r w h e n possible. D esp ite th e d r a w ­ b a c k s, th e m e t h o d h as th e distin ct a d v a n t a g e of tim e efficiency in th e in te rtid a l zone. This allo ws o b se rv e rs to c o v e r l a rg e a r e a s w ith in reefs a n d to c o m p le t e la rg e s u rv ey s w ith in r e a s o n a b l e tim e limits. A dditionally, m a n y eco lo g ic al m e a s u r e s su c h as div ersity a n d s p e c ie s r ic h n e s s ca n b e c a lc u la t e d from th e results.

Results s u g g e s t th a t K e n y a 's coral r e e f-as s o c ia ted g a s tro p o d fau n a, lik e o t h e r tropical Indo-Pacific r e g ­ ions, is ty pified b y low d e n s ity a n d h ig h variability a n d diversity (Kohn 1959, 1968, Kohn & N y b a k k e n 1975, R eic helts 1982). T h e s e 3 factors m a k e g e n e ra liz a t io n s difficult b u t s o m e p a t t e r n s e m e r g e . S p ecies com position b e t w e e n sim ilar sites s h o w h ig h variability b u t th e re w e r e f re q u e n tl y u n q u a n t i f i e d d ifferen ces in reef height,

w a t e r d ep th , su b s t ra te type a n d w a v e e n e r g y th a t m ay h a v e a c c o u n t e d for this varia tion. T h e s e dif fe rence s m a y h a v e o v e r r id d e n divisions b a s e d on reef location, p articularly w h e n c o m p a r in g re e f e d g e a n d flat sites. Prior r e a s o n in g for d ividing t h e s e 2 locations w a s th a t reef e d g e sites a p p e a r e d to e x p e r i e n c e g r e a t e r w a v e e n e r g y th a n flats. Yet, this d ifferen ce w as not a lw ays co n s iste n t d u e to variations in a s p e c t a n d th e reef's p ro tec tio n from w ave s. N o n e th e le s s , som e sp ecies w e r e consistently fo u n d on reef e d g e s w ith h ig h e n e r g y a n d seld o m on ree f flats, ju s t as o th e rs w e r e g e n e ra lly restric ted to ree f flats. A dditionally, so m e sp e c ie s w e re u b iq u ito u s a n d not strongly as so c ia te d with an y site. I s u g g e s t th a t this g a s tro p o d fa u n a is not a g ro u p of hig hly in te ra ctin g sp ecies c re atin g a c o m m unity, b u t is affected by e x t e rn a l biological a n d physic al factors such as s u b stra te, w a t e r m o v e m en ts, d esicc atio n stress, r e c ru it­ m e n t a n d p r e d a t i o n b y o th e r ta x o n o m ic g ro u p s (i.e. finfish, crabs) th a t affect th e ir distribution a n d limit their p o p u la tio n densities. T h e s e ex t e rn a l factors may, co in ­ cidentally, affect o th e r sp e cie s distributions c re a tin g a c o m m u n ity of dive rse ta x o n o m ic a s s e m b l a g e s .

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REEF EDGE (13) (13) 30 REEF F L A T 20 10

Fig. 3. Species-tim e curves (no. of species found per time spent sam pling) and equations of the form y = Cx2 com paring species richness b e ­ tw een protected (•) an d unprotected reefs (o). Bars: ± 1 SD; sam ple sizes given in brackets

REEF LAGOON 2 0 15 10 (1 2) 5 4 5 3 2 T IM E (H O U R S )

Prior to reports by L eviten & K ohn (1980) a n d R eichelt (1982), a g r e a t d e a l of r e s e a r c h on e o n e shells fo cu sed on possible reso u rce p artitio n in g m e c h a n i s m s (Kohn 1959, 1968, 1971, Kohn & N y b a k k e n 1975, Leviten 1978). T h e la tte r r e s e a r c h s u g g e s t e d th a t eo n e shells h a v e diverse diet, d e p t h a n d s u b s t ra te r e q u i r e m e n t s th a t could allow for species coexistence. H o w ev er, Reichelt (1982) c o n ­ c l u d e d th a t sp a c e is n o t a lim iting re s o u rc e a n d Leviten & Kohn (1980) c o n c lu d e d th a t n o n e q u i lib r iu m factors such as in te n s e storm s m a y k e e p th e s e sp ecies at d en sities b elo w levels th a t re q u ire interspecific co ex ist­ e n c e m e ch a n ism s. E ast African coral reefs rarely e x p e r i ­ e n c e la rg e storms a n d w a v e s a p a r t from s e a s o n a l m o n ­ soons of low in te nsity ( M c C la n a h a n 1988). Additionally, reef lagoons, w h ich h a v e the low est g a s tro p o d densities, w o u ld not b e s u b je c t e d to their in fluence. Yet, C o n u s

d e n s itie s r e p o r t e d h e r e a r e as lo w or lo w e r t h a n o th e r reg io n s (Kohn 1959, 1968, K ohn & N y b a k k e n 1975). T h e p r e d a t i o n d a t a p r e s e n t e d h e r e s u g g e s t s th a t p r e d a t i o n on ad u lts is low, b u t m a y in f lu e n c e g a s tr o p o d d en s itie s a n d d istribution p a t te rn s . O u r s tu d y su p p o r ts th e h y p o th e s is th a t g a s tro p o d s , lik e o th e r coral ree f p o p u l a ­ tions, a re r e g u l a t e d by d e n s i t y - i n d e p e n d e n t factors ( S h u lm an & O g d e n 1987) b u t n o t t h a t th e y are n o n ­ e q u ilib riu m a s s e m b l a g e s ; th e y m a y b e in e q u i lib riu m w ith th e ir p re d a to rs . Early p o s t - s e t t l e m e n t m orta lity (Perron 1983, Victor 1986, S h u l m a n & O g d e n 1987) m a y r e s u lt in low a d u l t p o p u la tio n densities. T o p o g ra p h ic com plexity , w h ic h h a s b e e n s h o w n to in c r e a s e p o p u l a ­ tion d e n s itie s a n d d iv ersity (Kohn & L ev iten 1976), cre a te s re fu g e from p r e d a t o r s w h ic h allow s for p o p u l a ­ tion persis te n c e .

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C / i UJ o LU CL C/5 6 0 4 0 20 0 6 0 4 0 20 0 6 0 4 0 20 □ P R O T E C T E D □ + U N P R O T E C T E D R E E F E D G E □ s = 1 . 0 6 1 ° e 1 r = 0 . 9 9 + s = 1 . 5 7 I 0 -5 4 r = 0 . 9 7 U U L M / ' U ' □ - X / + +a/ f > □ / R E E F FL A T & / □ s = 2 . 4 8 I 0 -5 8 r = 0 . 8 8

/

+ s = 0 . 2 3 I 0 ' 8 3 r = 0 . 9 8 R E E F L A G O O N + á 9 □ s = 1 . 9 9 1 ° 5 2 r = 0 . 9 8 + S = 5 . 4 7 ] 0 -3 2 r =■ 0 . 9 9 2 0 0 4 0 0 6 0 0 8 0 0 INDIVIDUALS 1000

Fig. 4. Species-individual curves for the various reef locations C o n s id e r in g th a t m o st g a s t r o p o d sp e c ie s h a v e low d en s itie s, h ig h variability a n d low re s p ira tio n ra te s s u g g e s t s th a t th e y a r e n o t i m p o r t a n t in d e t e r m in in g coral ree f c o m m u n ity s tr u c t u re or in flu e n c in g e n e r g e ­ tics. L ab o ra to ry a n d field e x p e r i m e n t s on o n e g a s ­ tropod, th e s e a u rc h in p r e d a t o r C y p ra e c a ssis rufa,

( M c C l a n a h a n & M u t h i g a 1989) in d ic a t e d low p r e d a tio n r a te s o n s e a u rc h in s c o m p a r e d to p r e d a t i o n by finfish. C o n s e q u e n t l y , th eir re m o v a l t h r o u g h shell collec ting is

120 r = - 0 96 p < 0.01 80 40 0.2 0.4 0 0.6 PREDATION RATE

Fig. 5. Total gastropod density as a function of pred atio n rates on Morula granulata within the 7 studied locations lik ely to h a v e less effect on r e d u c in g s e a u rch in s w ith th e ir d es tru ctiv e effects on coral reefs (M c C la n a h a n & M u t h i g a 1988) t h a n is finfish rem oval. E n v iro n m en ta l c o n c e rn s a b o u t shell collecting m a y b e o v e r s ta te d a n d m ig h t b e b e t te r fo cu sed on overfishing of finfish.

C o m p a r in g p ro te c te d a n d u n p r o te c te d reefs s u g g e s ts s o m e differences, b u t te a s i n g a p a r t factors affecting th e s e differences, su ch as i n h e r e n t site differences, shell collecting a n d fishing, rem a in s difficult. In g e n ­ eral, p r o te c te d reefs h a v e a h ig h e r div ersity a n d a h ig h e r d en s ity of so m e collected species. Differences a re h o w e v e r not g r e a t an d so m e d iffe ren ces m a y b e c a u s e d by factors o th e r t h a n shell collecting. U n p ro ­ te c te d ree f e d g e s h a d f ew er sp e cie s p e r a r e a th a n p r o t e c t e d reefs a n d a g r e a t e r d o m in a n c e of the T h aid id ae . It is likely tha t this d iffe re n ce is d u e to g r e a t e r w a v e e n e r g y on u n p r o t e c t e d re e f e dge s, a l th o u g h shell collecting m a y h a v e c o n t ri b u te d to the lo w er d en sitie s of cy p ra eid s w ith in th e s e a re as. Within reef lagoons, site dif ferences a re less likely to b e i m p o r ­ ta n t a n d low er d en sities of L a m b is truncata, L. chiragra

a n d O vula o v u m ca n b e a t tr ib u te d to shell collecting in u n p r o t e c t e d areas. Yet, it is also n o ta b le th a t densities of m a n y collected sp e cies a re not lo w er in u n p r o te c te d areas.

H ig h e r g a s tro p o d den s itie s within u n p r o t e c t e d v e r ­ sus p r o te c te d la g o o n s is a co n s iste n t b u t less intuitively clea r p a ttern . M c C l a n a h a n & M u th i g a (1988, 1989) s h o w th a t se a u rch in den s itie s a re h ig h e r a n d fish den s itie s low er on u n p r o te c te d reefs. Ayling (1981) fo u n d a positive correla tion b e t w e e n se a urch in a n d g a s tro p o d d en sities a n d s u g g e s t e d th a t s e a urchins T able 5. M orula granulata. Survival rate in days (x ± SD (n)) for a 3 d period w ithin reef flats an d lagoons at 4 sites. Control

survival = 3.0 ± 0.0 (15)

Protected

M alindi W atam u

U nprotected

K anam ai Diani Totals

Reef flats Reef lagoons 2.90 ± 0.55 (30) 2.64 ± 0.99 (25) 2.40 ± 1.04 (30) 3.0 ± 0 .0 (30) 3.0 ± 0 .0 (30) 2.8 ± 0 .6 1 (30) 3.0 ± 0.0 (30) 2.97 ± 0.32 (90) 2.72 ± 0.78 (115)

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m odify th e s u b s t ra te e n h a n c i n g conditions for g raz in g g as tro p o d s . Results of our g as tro p o d a n d se a urchin p r e d a tio n s tu d ies ( M c C la n a h a n & M u th i g a 1989) i n d i­ ca te th a t both ta x o n o m ic g r o u p s h a v e r e s p o n d e d to red u c tio n s in finfish pred a to rs. I s u g g e s t th a t p r e d a tio n is a d o m in a n t controlling force for m a n y i n v e rt e b r a te sp e cie s within coral reef la g o o n s a n d th a t finfish rem o v a l th r o u g h fishing activities ca n g rea tly affect i n v e r t e b r a t e c o m m u n ity composition.

T h e low de n s itie s of m a n y i m p o r ta n t com m ercial shell species, re g a rd le s s of m a n a g e m e n t strategy, is a n o ta b le result of this survey. Factors likely to ex p lain this in c lu d e (1) sa m p lin g at tim es or p la ce s not coinci­ d e n t w ith their distribution a n d d iu rn al activity p a t ­ terns, (2) p o a c h in g w ith in p r o te c te d reefs, (3) natu ra lly low den s itie s a n d (4) p o p u la tio n s w ith in p ro te c te d reefs no t h a v i n g r e c o v e r e d since th e P a rk 's inception. T h e first factor (samplin g) m ig h t w ell ex p la in so m e differ­ en c e s as m a n y sp e cie s a re n o ctu rn a lly active a n d cry p ­ tic d u r in g daylig ht. This is p articula rly tru e of C y p ­ ra e ca ssis rufa w h ic h h ides in coral s a n d d u r in g d a y ­ light. N o c tu rn a l s a m p lin g w o u ld likely result in g r e a t e r den s itie s (Yaninek 1978). S p e cies su c h as C haronia tritonis m a y natu ra lly occ ur at low den s itie s (O rm a n d & C a m p b e ll 1971). L arge sp ecies m a y also h a v e slow g ro w th rate s (H o n m a 1988) a n d reco v ery after collec­ tion m a y re q u ire a long time. P o a c h in g is p ro b ab ly erratic a n d a less im p o rta n t factor. It is likely th a t the de n s itie s of th e s e sp e cie s are n a tu ra lly low b u t all of the a b o v e factors m a y h a v e in flu e n c e d this result.

A ckn o w led g em en ts. I am grateful for the assistance of N. A. M uthiga, J. M ariara, M. Carey, M. F euerstein and the M alindi-W atam u and Kisite M arine N ational Park Staff for field assistance, the Office of the P resident for research clear­ ance and the Kenya M arine and Fisheries Research Institute and C enter for W etlands for logistical support. This study received financial support from IUCN and the East African Wildlife Society.

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M anuscript first received: Ju ly 1, 1988 R evised version accepted: January 27, 1989

Figure

Fig.  1.  K enya's  so u th ern   coastline  a n d   site  locations
Fig. 1. K enya's so u th ern coastline a n d site locations p.2
Fig.  2.  Family  com position  of  the  g a s­
Fig. 2. Family com position of the g a s­ p.4
Fig.  3.  Species-tim e  curves  (no.  of species  found  per  time  spent  sam pling)  and  equations  of  the  form  y  =  Cx2  com paring  species  richness  b e ­ tw een  protected  (•)  an d   unprotected  reefs  (o).
Fig. 3. Species-tim e curves (no. of species found per time spent sam pling) and equations of the form y = Cx2 com paring species richness b e ­ tw een protected (•) an d unprotected reefs (o). p.7
Fig.  4.  Species-individual  curves  for  the  various  reef locations
Fig. 4. Species-individual curves for the various reef locations p.8
Fig.  5.  Total  gastropod  density  as  a  function of  pred atio n  rates  on  Morula  granulata  within  the  7  studied  locations
Fig. 5. Total gastropod density as a function of pred atio n rates on Morula granulata within the 7 studied locations p.8

References

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