Population Densities of the Sea Urchin, Echinometra Mathaei, in Relation to Defined Habitat Types in Diani Lagoon, Kenya

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Population Densities of the Sea Urchin, Echinometra Mathaei, in Relation to Defined Habitat Types In Diani Lagoon, Kenya.

Item Type Report

Authors Gray, C.M.

Publisher Exeter University

Download date 28/09/2021 13:31:30

Link to Item http://hdl.handle.net/1834/8484 POPULATION DENSITIES OF THE SEA URCHIN, ECHINOMETRA MATHAEI,

IN RELATION TO DEFINED HABITAT TYPES IN DIANI LAGOON,

KENYA

CATHERINE M GRAY

EXETER UNIVERSITY

1990 POPULATION DENSITIES OF THE SEA URCHIN, ICHINQMETRA MATBAEI, IN RELATION TO DEFINED HABITAT TYPES IIIJ DIANI LAGOON, KENYA.

ABSTRACT Eight habitat types within the lagoon were defined including, sand; 50% seagrass cover; 80% Thalassodendron ciliatum cover; 100% Thalassodendroo ciliatum cover; sparse grass; rubble and debris; eroded coral with algae; and dissected reef platform. The populations of ~chinometra mathaei within each habitat were defined to show density, habitat preference and associated behavioral tendencies. It was found that rubble is the preferred habitat with an average of 24.S/E, rnathaei/m2. In addition, the extent of each habitat type was measured and an analysis of the lagoon was carried out. Furthermore, comparisons with previous work were done, showing that numbers do not seem to have increased in the last 20 years as is the common belief.

INTRODUCTION Echinometra mathaei has been described as an omnivorous, burrowing sea urchin that feeds mostly on fleshy benthic algae but also on other invertebrates, including corals inhabiting the coral reef benthos (Muthiga and McClanahan, 1987). It is a relatively small, ovoid shaped sea urchin which may vary considerably in colour from purple to greenish brown although it is generally black. ~. mathaei are easily identifiable however, as they have a white ring at the base of each spine (Oxford University Expedition, 1971). E. mathaei is by far the most common sea urchin found in the lagoon at Diani Beach, Kenya, although six other species are present in lower densities. E. mathaei distribution on a worldwide scale is from central Japan to Southern Australia; and from the Clarion Islands off Mexico to the Gulf of Suez (Khamala, 1971), and they may be found in a variety of habitats. There is considerable concern in many areas of the world, both temperate and tropical, over the consequences of sea urchin population increases. It has been suggested that in Diani Lagoon, high sea urchin densities may reduce coral recruitment and increase bioerosion of coral reefs, which could result in a loss of live coral, calcium carbonate accretion, associated faunal diversity and coral reef ecosystem services of fisheries productivity and coastal protection (McClanahan and Muthiga, 1988). Some say that high densities of the sea urchin E. rnathaei are indicative of the last stages of reef destruction (Oxford University Expedition, 1971).

- 1 - E, mathaei seems to playa significant role in the organizational process of lower intertidal or shallow subtidal communities, due to their high density and great activity in terms of bioerosion. However, as grazers of the benthic algae, they reduce algal cover and break down reef substratum, which creates topographical complexity and can enhance coral recruitment (Muthiga and McClanahan, 1988). Previous work done on E, mathaei in the Oiani Lagoon, Kenya, has mainly been in relation to the increase in numbers and the relation of this to over fishing (McClanahan and Muthiga, 1987), as well as to the destruction of the reef. populations of sea urchins may fluctuate in density with time and may often be aggregated (Andrews, 1989). Although studies have been carried out on the distribution, density and behaviour of E. mathaei (Khamala, 19711 McClanahan and Muthiga, 1987, 1988), little work has been done to assess the density of those sea urchins present in relation to the ir hab i tat. Oiani Beach is on the coast of Kenya at a latitude of 4 s. and is a typical tropical coastal shore. It is long and narrow and separated from the main body of the Indian Ocean by a fringing reef platform with the lagoon being about 0.5 0.7km wide. A map of the Kenya coast showin9 Diani Beach

KENYA

Ungwana Bay

Msambweni Shimoni ango

- 2 - The reef at Diani Beach consists of two distinct areas, the lagoon, inner reef or reef flat and the raised reef platform, outer reef or reef crest, which is usually exposed at low tides.

A section through the Lagoon fro. the shore to exposed reef platform

Beach

Sand Lagoon Reef Platform

Previous work on the subject seems to assume that Eo mathaei exist in a constant pattern across the whole area. However, this study examined the principle habitats which exist within the lagoon and reef platform areas, to elucidate which are the favoured habitats of the species. Estimates of population densities in relation to each of the defined habitats were made, and observations made on obvious behavioural tendencies such as burrowing, clumping etc. that may be unique to each of the habitats, so that a correlation could be made between habitat type and E. mathaei behav iour. In the examination the amount of coral growth was noted so that a suggestion can be made as to whether the sea urchin populations are indeed destroying the reef community. Further work was carried out to try to ascertain whether the densities of E. mathaei have increased in the last 20 years.

MATERIALS AND MEmOD The study was carried out during the months of June and July, when the S.E. monsoons prevail (March to October). S.E. monsoons are characterised by high cloud cover, rain, wind energy and decreased temperatures and light~ in contrast to N.E. monsoon when variables are reversed (McClanahan, 1988). These seasons are caused by the Inter Tropical Convergence Zone (ITCZ) which is the single most important climatic factor affecting the seasonality, with the two seasons experienced around the equator in East Africa being caused by its migration.

- 3 - A diagram of the site examined in this study. (AGA KHAN) (1) ------... ------S02m Hawkins (BOCK) 140m

(2) Diani - .... ------.... -- - -496m Sea Lodge 105m (3) ------"1)57. 5m (Moana) UNI Nairobi 90m (4 ) ------664m Sea Crest 180m 725m

Two Fishes (5) !------li12m

210m (Ba rker-Neuman)

(Mitton) (6 ) ------570.5m Africana Sea Lodge/Jadini The study site consisted of that area of the lagoon stretching 725m northwards from the Northern edge of the Africana Sea Lodge on Diani Beach. The seaward side of the study area was enclosed by the reef platform and varied from 400-700m depending on the width of the lagoon. With the exception of one deep, narrow channel, the lagoon at Diani Beach is generally shallow. All work on E. mathaei was carried out during the two to three hours over the low tide period during daylight hours, when the low water level was less than O.84m above datum, i.e. around Spring and Neap tides. This was necessary as turbulance of the water caused by the S.E. monsoon actions resulted in relatively poor visibility through deep water and hampered the taking of measurements. All measurements were carried out whilst - 4 - snorkelling and the outer edge of the reef platform was not eKamined as scuba diving equipment was necessary. Initially it was necessary to define the habitats which occ~r within the lagoon and although these are obviously not discreet, separate areas, they are distinct from one another with grading ecotones between. As soon as the habitat types were familiar, 50 random 1m 2 quadrats were taken using a metal quadrat within each habitat in the study area, to estimate the average number of E. mathaei which occur in 1m2 of each habitat; as well as to ascertain which habitat types were preferable to E. mathaei. Behavioural patterns, such as burrowing, aggregation etc. of E. mathaei were also noted within each habitat type. The width of the lagoon was measured in siK transects approKimately 100m apart within the study area, by moving over it with a tape measure. Whilst measuring the width of the lagoon, the habitats within the area were measured to an accuracy of 0.5m. The length of each habitat crossed was measured but deviations in eKtent or width on either side of the line were not eKamined. Without the use of beacons it was not easy to remain on a direct line out to the reef, so some distance error may have occured but this would not have been a large deviation and therefore not considered significant. The exact distances between the transects were measured. reef 100m ---7 I I 500m I Diagram to illustrate error caused by I 510m deviation of the transect ! (2% error) I beach These measurements of the lagoon were carried out to attempt to give a detailed description of the areas of habitats within the lagoon and to give a fairly accurate estimate of the number of E. mathaei that would be present in a 1m wide transect of the lagoon as well as the locations of highest densities of E. mathaei within that transect. In these studies, the reef platform itself was not measured although averages were found for it, as once the platform is reached there is little or no variability in habitat type and wave action would make measurement difficult and inaccurate. In addition to the above measurements, Khamala's transect of 1971 was repeated at the Northern edge of Africana Sea Lodge; taking l~ quadrats every 6m across the lagoon, rather than every lam as done by Khamalaj and eKamining the habitat type and number of E. mathaei found within each of

- 5 - the quadrats. This was carried out to investigate whether the number of E, mathaei found within the lagoon has changed dramatically over the last 20 years as well as to compare the sampling method to the one used in this study, Whilst the"number of E. mathaei in each habitat was counted, other points of interest were noted; such as other species of sea urchin, starfish etc., as well as corals as they appeared within the quadrats.

RESULTS Habitat Types: The habitat types that occur within the lagoon and reef platform ecosystems of Diani Beach, Kenya, may be grouped into one of the three following subsections:- 1. Sand: A large proportion of the lagoon, especially on the inner edge consists of bare sand. Sand patches and sand-bottomed channels of varying size, shape and extent occur throughout the lagoon. In fact, many of the rocky areas support on overlay of sand which may have considerable depth variations but is generally very thin. 2. Seagrass meadows: These are widespread throughout the lagoon but always restricted to sandy substrates wh ich they stabilize. Seagrass meadows are variable in character, with major differences which are due to species composition, % cover, depth of water, possible emersion at low water Neap and Spring tides, water flow and turbulence. For the purpose of this study, four possible habitat types have been described within seagrass meadows. The dominent species of seagrass at Diani is Thalassodendrgo ciliatum which may occur in pure, dense, robust stands giving 100% canopy cover. These stands often give a fairly level carpet of Th. ciliatum which is nearly emersed at Neap and Spring low tides, The substrate for these stands var ies from very soft and silty sand to fairly coarse shell gravel sand. A second habitat which consists solely of Th, ciliatum is that where approximately 80% cover is found with sand showing through, In these areas, which are frequent throughout the lagoon, the Th, ciliatum tends to be shorter than in the 100% stands and does not form the same waving 'carpets·.

Th. ciliatum is often found in association with ~madocea serrulata, areas of which frequently line the shore and have here been classed as the third habitat type. Cover ranges from 40% to 60% with a roughly equal proportion of Th, ciliatum and C. serrulatA on a substrate generally consisting of soft, silty sand, Small stands of a third seagrass, ~godium isoetifolium, may be found

- 6 - intermingled with Th. ciliatum and C. serrulata in the 40% to 60% seagrass habitat type but cover is not high. A fourth seagrass habitat type containing a small seagrass, Halodule species (probably uninervis), is known as 'sparse grass' areas. This seagrass may be seen occasionally in the 40% - 60% seagrass zones but generally in ecotone regions. A larger seagrass more common than Halodule spp, Thalassia hemprichii, is also present in this habitat. Extensive areas of sparse grass occur within the lagoon and they are frequently exposed at low water at Neap and Spring tides. 3. Hard substrate; The remaining habitats fall under the subsection of hard rocky substrates, of which there are three principal types:- (a) The Reef Platform encloses the lagoon and has an actively growing coral reef front on its seaward edge. At Oiani Beach, this raised platform is emersed at Neap and Spring low tides, but due to local variation in height, the extent and period of emersion is variable. The substrate consists of a more or less finely dissected but stabilized (by calcareous algae) platform which is exposed to violent wave action and thus to severe tidal turbulence. (b) Eroded Coral/Algae. On the landward side of the reef platform is a more or less extensive zone of extremely dissected rock, which has presumably been carved and fretted in the past by wave action. This area is characterised by the presence of several species of algae forming approximately 20% cover. The most common of these algae is ~gassum labifolium: with others such as Pad ina spp, Turburina spp (probably ~eD¥aensis), Sargassum binderi, and Gelidella acerosa being present. A large number of Porites heads occur in this zone, as well as scattered throughout the lagoon, and there is an extensive cover of often actively growing corals, with small coral gardens appearing in deeper areas. (c) Areas of Rocky Debris and Rubble, apparently derived from weathered elements of an old eroded reef, are present. These frequently support actively growing colonial corals of a number of species (Porites, Acrop.ora, Echinopora, Galaxea and others). The rubble and debris are very variable in size and shape, and areas are rarely encountered as 'bare' rock; the surface is generally covered with a superficial 'matt' of soft entrapped or consolidated sediment. These areas are often difficult to distinguish from the eroded coral/algal areas as coral growth is present in both.

(Appendix 1; A spec-ies list of seagrasses and algae found in Oiani Lagoon.)

- 7 - Rumbers of E. Mtbaei Numbeis of E. mathaei found were variable with averages from 50 1m quadrats of each habitat being as follows:-

Table 1: Average nuJDber s of E. _tbaei/al occurring in tbe various defined habitats

HABITAT TYPE TOTAL NOMBD AVBBAGE/M1 Bare Sand a 0.0 Seagrass 40%-60 % 709 14.2 Th, ciliatum 100% 3 0.06 Th. ciliatum 80% 135 2.7 Sparse grass 0 0.0 Rubble/debr is 1224 24.5 Eroded coral/algae 727 14.5 Reef Platform 98 2.0

No E. mathaei or any ather sea urchins were seen throughout the study an either bare sand or sparse grass. This is probably due to the shifting nature of the sand and to the fact that the sparse grass is frequently emersed and is thus nat preferable to sea urchins. However, sea cucumbers appear to be able to withstand emersion and occur occasionally in sparse grass areas. Another habitat which supports few or no sea urchins is the 100% Th. ciliatum. During this study only four E. mathaei were seen, three of which were in one quadrat, giving an average of a .06/m' for these stands. In fact, during the study the only ather benthic organisms which were recorded in this habitat were one primitive priQnocidaris baculos~ which appeared to have fallen intQ the stand, twa ~pneustes, Qne prQtoneusta and one sea cucumber. Small soft corals however, were frequently seen growing within the stands of 100% Th. ciliatum. In the 40% to 60% cover seagrass meadows, an average of 14.2 E. matha;i/m' was obtained with a range occurring from 2 to 37 in 1m quadrats. Aggregations of B. mathaej were cammon in this habitat which was frequently fQund close to the beach. More than half of the individuals were fQund in grQups of three Qr mQre with SQme grQups numbering up to 10 individuals. It is alsQ commQn to find B, mathaei aggregated under clumps or loops of seagrass. Other sea urchins were not common in this habitat, although SQme were recorded, especially DiQdema and ~neustes. (see Apendix 2 fQr a species list of other sea urchins and other organisms also present) The average number of B. mathaei/ml found on eroded coral/algae regions was similar to that above, being

- 8 - 14.5/m~. However, unlike the seagrass habitat, many other sea urchins of other species may be found in this area with Diodema and Echinothrix being the next most common. All the sea urchins in this area tended to inhabit crevices in the eroded coral, as well as in or under Porites heads which were fairly common within this habitat. In addition some aggregations occurred although not in large numbers but this appeared, from observation, to be due to crevice availability and was not necessarily preferential.

Obviously the habitat type preferred by E. mathaeia was that of rubble, where the ave1age number was 24.S/m , with a range from 0 to 59 in 1m. Although other species of sea urchin were observed here, they were not as common in this habitat as in the eroded coral/algae habitat. The E. mathaei in this habitat did not show any preference for aggregating or inhabiting crevices or burrowingi although some were found carrying out these actions, others were present alone on open rock. Almost all of the quadrats taken in this habitat contained soft and/or hard corals, with some showing more than 20% cover by new coral. In fact, some E. mathaei were found underneath patches of soft coral. One behavioural trait occasionally seen within this habitat was that of 'shading'. Some of the E. mathaei seemed to hold pieces of rubble and shell on top of their spines, although this was not done by all of the ind iv idua1s. The third rocky habitat, the reef platform was apparently not a preferred habitat for E. mathaei, with an average number of 2.0/m~ being found here with a relatively small range of variance. All the sea urchins, with E. mathaei being the only species sighted, existed within deep crevices, and appeared to be generally smaller in size than those in the lagoon, although no direct measurements were made. 80% Th. cilia tum cover does not appear to be highly preferable to E. mathaei with the average number per m2 being 2.7. However, another sea urchin, ~neustes seemed to have much higher numbers in this area than elsewhere in 2 the lagoon with an average of approxima te1y 1/m • The E, mathaei did not appear to show any aggregating tendencies, although they did seem to cover themselves in strands of seagrass. However, this tendency was not as strong as with the ~neustes who frequently totally covered their spines in seagrass strands. Tripneustes also tend to clump together and large clumps have been seen, which number up to 10 individuals.

- 9 - Transects of the Lagoon

Table 2: To show actual cover (in m) of the different habitats (without secondary platform) in six separate transects

BOCK N. DIANI S. DIANI SEA CREST TWO AFRICANA SEA LODGE SEA LODGE PISHES SEA LODGE

Sand 62.5 52 29 52 29.5 45 Sea- 121, 191.5 189.5 165.5 143.5 146 grass 80% 72 69 81.5 95.5 21 82 Th. c 100% 29 15 75.5 93.5 43 26 Th. c sparse 93 18 36 60 71 112.5 grass rubble 65 52.5 80 107.5 95 109 a1gae/59.5 98 91 35 101.5 50 rubble

TOTAL 502 496 582.5 609 612 510.5 Using these results for the six transects taken of the Diani Lagoon, the percentage cover of each habitat type within each transect was calculated (see Graph 1, % Cover of Defined Habitats in Several Transects).

Table 3: To show percentage cover of the different habitats in the transects

BOCK N. DIANI S. DIANI SEA CREST TWO AFRICANA SBA LODGE SEA LODGE PISHES SEA LODGE

Sand 12.45 10.48 4.98 8.54 4.82 7.89

Sea- 24.10 8.61 32.53 27.18 23.45 25.59 grass 80% 14.34 13.91 13.99 15.68 21.00 14.37 Th. c 100% 5.78 3.02 12.96 15.35 7.03 4.56 Th. c sparse 18.53 3.63 6.18 9.85 11. 60 19.72 grass rubble 12.95 10.59 13.74 17.65 15.52 19.11

algae/ 11.85 19.76 15.62 5.75 16.58 8.76 rubble

From the six transect figures, the estimated average actual and percentage cover of each habitat type within an 'average transect', was calculated (Table 4). Using these estimated

- 10 - " COVER OF DEFINED HAIl AlS IN SEVERAL lRANSEC S

1. BOCK 2. NORTH EDGE DIANI SEA LODGE 3. SOUTH EDGE DIANI SEA LODGE 4. SEA CREST 5. TWO FISHES 6. NORTH EDGE AFRICANA SEA LODGE 7. AVERAGE %

LEGEND

RUBBLE 10

S E G 70 100% Th. C

WIG «C> ~ Zao SEAGRASS W U 0::: SAND W.co (L

o 123 567 ~ :--L=:::. l ._, L_, I [" --, L_ -1 ~ J -~ [[ • t -"--L . _---rTI . [ I i L--: 1 I ~-' l- - -1 r:-~. -.:l c::::-- 1 ~

f _ f7JI~ '-__ I. _

r -- J ~-1 J;l r-. I ~ t-J ;:].: t_ ,.-.__.J- I I r J J L- o r---::.-:J I .. r I l r- J,

5~ I 1 I, ,_J ~ ~_J §

S'--3 L e--r::;=; t- ~rl f ., t:=.- j

clJ'I~

I I I ] ~ \J ~ ~ B 'T §~ -1 ~ ~ {O ~ .} ~ ~ ~ i ~ /' -,,-". A /' /,'-; \is8 /' ./ _. /' -" /' ...- '" ------I,

----"---iil-----" --"-""--"""- i I

------.

.-. : - -;._-'2-'-~ r -...,- : ~ • -';;r.- - -- • 'i~ i ! f §¥ ~ ill ._._._==:l a t.._ .:=:::=::====--] , L[_ .. -===: ;d (: .~

[=--n----l -

I~'f i ~ ~ Q .J r rr J 0( ..J 0( Z 0( % :.:

~ CXI z L.J ~-::J lIl: -0( 0( [1 . -%

-% ~ ~ -:II: [ l_-::J \j) L V ~ -L.J ell Z 0( -a:: L.J a:: or: Q. Z 2 Q V 2 Q-

HAatTAT~

luU;ll~, linnll~ IJC:IoV'WlAW"" figures in combination with the results found for the number of Eo mathaei within a 1m' area of each habitat (Table 1), an estimate was made of the number of E. mathaei that would be expected to be found in a 1m wide transect of the lagoon at any point.

Table 4: TO sbow estimated actual and percentage cover figures for an 'average' transect of tbe lagoon, and tbe number of E. watbaei tbat would be present Actual (m) Percentage E. _tbaei/m' NuJlber of distance E. watbaei Sand 45 8.01 o 0.0 Sea grass 159.5 28.38 14.2 2264.9 80% Th. c 88 15.66 2.7 237.6 100 % Th. c 47 8.36 0.06 2.82 sparse 65 11.57 o 0.0 grass rubble 85 15.12 24.5 2167.5 algae/ 72.5 12.90 14.5 1051.25 rubble TOTAL 562 5724.1

From Graph 1, it may be seen that in anyone transect the percentage cover of each habitat is fairly constant (platform not measured), although from Graph 2, which depicts the actual distances covered by each habitat within the transects, it may be seen that over a short distance the habitat types within a certain transect may be very var iable. It is possible to detect from Graph 3, showing the zonation of the lagoon, the approximate bands of each habitat. This shows us that starting at the beach, the lagoon goes from sand, through 40% to 60% seagrass cover, to 80% Th. ciliatum and 100% Th. ciliatum. In the middle region of the lagoon there occurs rubble, followed by sparse grass and eroded coral/algae. Between this and the reef platform, a band of 100% Th. ciliatum occurs. Some bands, however, are not distinct as habitats are spread throughout the lagoon.

Repeat of ltba..la· s Transect of tbe Inner reef/lagoon. (Appendix 3: Actual figures from Khamala's transect with quadrats taken every 6m) A total of 99 quadrats were taken in this transect of the inner reef/lagoon, with a total of 346 E. mathaei recorded.

- 15 - This gives an average of 3.5/m.2 Therefore, using Khamala's technique one would predict that there would be 2076.0 §. mathaei in this 1m transect of the lagoon, whereas using the techniqui from this study an estimate of 10.19 E. mathaei/m would be made from the combination of average transect values, and therefore one would expect an estimated total of 5724.1 E. mathaei in a 1m2 transect of the lagoon.

Table 5: To compare the esti~ted number of E. mathoei present in 1990 with those figures for 1971

Average number of E ...thoei/m2 1971 1990 Inner reef/lagoon 5.3 3.5 10.19* Northern outer reef/ Reef platform 1.7 2.0*

* Figures gained from this study method

Transect number 6 in this study was taken at the same place as the repeat of Khamala's 1971 transect. As we can see by comparison of transect 6 with the depiction of Khamala's transect in Graph 4, the technique of investigation of the lagoon used by Khamala in 1971 does not show the full extent of variation which may be seen in the lagoon. In the repeat of Khamala's transect there was less room for inaccuracy as the quadrats were taken every six metres rather than every 10m. From Graph 2 it is possible to see that within 10m the habitat type may change dramatically and this would be missed using this technique. Therefore, it is probable that the number of E. mathaei estimated as present by this method is inaccurate and possibly low.

DISCUSSION It has been suggested that wave action, which is strong during the S.E. monsoon, is the principle controlling factor in E. mathaei distribution (Khamala, 1971). However, it would appear from the results of this study that although wave action and water turbulence would certainly be influential, habitat type is the most important factor limiting the distribution of E. mathaei at Oiani Beach, Kenya. The prefered habitat type is obviously that of rubble and debris. Although this habitat type is probably favourably positioned within the lagoon, being generally fairly central, and therefore receiving a less than maximum turbulence effect, currents are still strong, and the E. mathaei do not seem to attempt to reduce the effect of this by burrowing. In fact, as seen in the results, many of - 16 - the individuals were observed on bare rock. Although all behavioural analysis was purely observational in this study, it would seem that particularly in their preferred habitat E. mathaei do not need to avoid turbulence and current flow. However, wave action is possibly more influential within the outer regions of the lagoon in the eroded coral/algal areas, where the sea urchins were seen to occupy crevices, although this appeared to be a passive action with E. mathaei occupying crevices opportunistically rather than actively creating burrows. The numbers in this habitat did not differ significantly from those in an area with least turbulence influence, the 40% to 60% seagrass meadows. As these areas are frequently close to the shore, they are generally shallow: although the deep channel seen in the inner edge of the lagoon is occupied by this habitat, albeit with very few E. mathaei present: and it would be expected that this would be ideal for avoiding wave action. We may assume therefore that wave action does not directly influence numbers but possibly behaviour may be modified to a certain degree. This behavioural modification of avoidance of wave action, is most clearly seen on the reef platform itself, where there are low numbers of E. mathaei which occupy deep burrows. Possibly, the combined effect of emersion as well as the high forces experienced due to wave action lead to this habitat type being less favourable, even inhospitable to E. mathaei. Although E. mathaei is described as a burrowing echinoid, the results reported here suggest that burrowing is related to unfavourable environmental conditions (.such as wave action, turbulence, emersion, etc.) and that the species does not expend energy on burrowing unless inhospitable conditions prevail. Clumping or aggregating behavior in inshore habitats is exhibited by a great number of species of echinoderms, either continuously or sporadically. E. mathaei were seen to aggregate noticeably only in the 40% to 60% seagrass habitat. The reason for this is not fully understood. It is possible that clumping under arching rhizomes of Th. ciliatum affords some protection from water turbulence as well as perhaps from predation. Young corals, both hard and soft were found growing throughout the lagoon and were not apparently affected by E. mathaei presence. Although many believe that the E. mathaei destroy the reef by boring, it would seem that far greater damage is done by human disturbance, intentional or non-intentional•. Fishermen scouring the reef for any remaining shells (Cowries) of an extremely depleted popUlation, and fishing for octopus, cause damage to the reef. Fishing within the lagoon is often carried out using the beach seine {juya) method which results in a great deal - 17 - of destruction as the nets are dragged across the reef. Unintentionally, the large number of tourists that visit the area break up the reef structure as they walk over the lagoon and reef platform. The suggestion that the population explosion of E. mathaei is due to overfishing (McClanahan and Muthiga, 1988) needs to be further investigated as it has also been found that the numbers at Malindi, where overfishing does not occur, are not significantly different from those at Diani, which is an overfished area (Samoilys, 1988). It is not possible to postulate whether in fact a population explosion has occurred from the results of this study as they are difficult to compare to previous research. In addition, in 1971, the highest population densities occurred on the inner reef where densities of E. mathaei as high as 49/m were found (Oxford University Expedition, 1971). In a transect from the reef to the shore, the following average densities were found:- 1 - 100m o 100 - 200m 13.4 200 - 300m 21.9 This central area of 200 - 300m probably contained a high density of rubble and coral debris, which can be seen from Graph 3 showing zonation indicating that rubble is generally centrally situated. Therefore, if compared with the results from this study; where the maximum number is 59 individuals in 1m , and the average for rubble areas is 24.5/m; one can see no reason to suppose that the maximum number in 1971 is significantly different from that found today. It was suggested that high densities of E. mathaei are indicative of the last stages of reef destruction (Oxford University Expedition, 1971), but as the numbers have not apparently altered dramatically over the last 20 years, and the reef is still present, with a great deal of young coral growth showing, it would appear that E. mathaei are not responsible for the destruction of the reef. It is possible that numbers of E. rnathael have increased in certain areas but as no one has done a similar study in the past it would be impossible to say. When results are compared with estimated figures found by Khamala in 1971, the results found using her method are so different from those found in this study that it is difficult to ascertain through lack of accuracy whether numbers have indeed increased. Although in this study it has been found that rubble areas have an extremely high density of E. mathaei there are other areas, such as the 100' Th. ciliatum, sparse grass and sand, that have very low densities which compensate for these high numbers.

- 18 - ACBNOWLEGEMENTS I thank The Kenya Marine and Fisheries Institute, The Office of the president, Dr E. Martens at the University of Nairobi and especially Dr K. Bock for their help with this project.

REFERENCES Andrew, N.L.: Contrasting ecological implications of food limitation in sea urchins and herbivorous gastropods. Mar. Ecol. Prog. Sere 51, 189-193 (1989). Downing, N., and El-Zahr, C. R.: Gut evacuation and filling rates in the rock-boring sea urchin, Echinometra mathaei. Bull. ma r. Sc i. 41 (2), 579- 584 , (1987). Hamilton, H. G. H., and Brakel, W. H.: structure and coral fauna of East African reefs. Bull. mar. Sci. 34(2), 248-266, (1988) • Keller, B. D.: Coexistence of sea urchins in seagrass meadows: an experimental analysis of competition and predation. Ecology, 64 (6), 1581-1598, ,(1983). Khamala, C. P. M.: Ecology of Echinometra mathaei (Echinoidea: Echinodermata) at Diani Beach, Kenya. Mar. Biol 11, 167-172 (1971). McClanahan, T. R.: Seasonality in East Africa's coastal wa ter s. Mar. Ecol. prog. Ser. 44, 191-199, (1988). McClanahan, T. R.: Kenyan coral reef-associated gastropod fauna: a comparison between protected and unprotected reefs. Mar. Ecol. Prog. Sere 53, 11-20, (1989). McClanahan, T. R. and Muthiga, N. A.: Changes in Kenyan coral reef community structure and function due to exploitation. Hydrobiologia 166, 269-276, (1988). Moore, H. B.: Ecology of echinoids. In: Physiology of Echinodermata, pp 73-83. Ed. by R. A. BOOLOOTIAN. New York: John Wiley & Sons 1966. Muthiga, N. A., and McClanahan, T. R.: Population changes of a sea urchin (Echinometra mathaei) on an exploited fringing reef. Afr. J. Ecol. 25,1-8, (1987). Oxford University Expedition to the coast of East Africa, 1971. Oxford University Exploration Club Bulletin 20 No 4, pp60-84. Ed. by Andrew Goudie. Pearse, J. S.: Reproductive periodicities of Indo-Pacific invertibrates in the Gulf of Suez. II. The Echinoid Echinometra mathaei (DE BLAINVILLE). Bull. mar. Sci. 19, 580-613, (1969),. - 19 - Reese, E. 5.: The complex behavior of echinoderms. In: Physiology of Echinodermata, pp 157-218. Ed. by R. A. BOOLOOTIAN. New York: John Wiley & Sons 1966. Samoilys, M.: A survey of the coral reef fish communities on the Kenyan coast, (1988, not published). WWF Project, 3797, Kenya Technical Report prepared for the Ministry of Tourism and Wildlife Tsuchiya, M., and Nishihira, M.: Re-colonization process of two types of the sea urchin, Echinometra roathae1 (Blainville), on the Okinawan reef flat. Galaxea 5, 283-294. (1986).

- 20 - Appendix 1 Species list of seagrass and algae found in Diani Lagoon, Kenya Thalassia hemprichii - is the commonest angiosperm on old reef platforms which have become sanded over, both inshore and more especially further out. It is easily distinguished by its very shaggy stem covered with many old leaf bases. Halodule uninervis/wrightii - two species found on inshore sand or sandy reef platforms. They are not always easy to distinguish either by the width of leaf or difference in leaf tips, as there are many specimens which appear intermediate between the two - possibly hybridization. Thalassodendron ciliatum - is the commonest angiosperm growing in deep water on a sandy bottom, in all channels and pools; it never grows where it becomes completely uncovered. The curved leaves grow in clusters of S-7cm above the sand on the ends of long (lO-30cm) stems which show conspicuous annular scars. The leaves are very variable in width (6 l4mm), but are commonly about lOmm wide with serrated rounded tips. The sheathing leaf bases are pink and triangular in shape. Leaves can be collected from the beaches, washed and used as a mulch or fertilizer. Cymadocea serrulata - is found in pools bordering the beach. Leaves are S-9mm wide with serrated tips and sheathing leaf bases which are triangular, white or pink, 2-3cm long. Leaves are sessile or on short erect stalks under the sand. syringodium isoetifoljum - (one Kenyan species) - is easy to identify as it is the only angiosperm with a completely solid cylindrical leaf. It is found in large quantities on the sand at the seaward edge of the reef platform. Sargassum labifolium - is the commonest species of Sargassum. Its 'leaves' are rounded, not usually crinkled and vary considerably in size and distribution on the main axis. The average leaf length is about 3cm. There may be one main axis with only short side branches or the side branches may be long. Sargassum binderi - has long narow 'leaves', which vary in size on the same plant and are conspicuously toothed. The lower 'leaves' are larger and there are no air bladders. The upper parts can have much smaller 'leaves' and bear bladders. Pad ins spp - forms erect brown fans (up to lOcm) marked with concentric rings. It is a genus of sandy pools and occurs on all shores. There are several species, distinguished by their fruiting bodies, but otherwise looking alike. They sometimes have a white deposition of lime on surfaces.

- 21 - Turbunaria (probably ken~ensis) - Inverted pyramid- shaped appendages and root-like hold fast. A number of species which occur at various depths. It is fairly common near the outer edge of reef platforms. Gelidiella acerosa - is stiff and almost prickly with branched thallus which bears smaller branchlets. It is usually found in pools on the reef platform in the shallow reefs, uncovered at most low tides.

- 22 - Appendix 2 Species list or other organisms pre.sent in the Lagoon Sea Urchins Tr ipneustes spp - round with short, whitish spines. Diodema savegny - long black spines with blue lines between. Diodema cetosum - long black spines with white spots between. Astopygo radiata - a beautiful orange, pink and blue, relatively large sea urchin. Toxopneustes spp. - poisonous with flower-like podiacae. Tends to cover itself with rubble and shells. Echinothrix diodema - a very large, ovoid, black, sea urchin. Prionocidaris baculosa - a primitive sea urchin with massive, greyish spines.

Other Organisms common in the Lagoon Synampia metulosa - a brownish/yellow, long, extensible, snake-like echinoderm. Protonuesta - a red and grey coloured star fish. Ophiroides - Brittle stars Cushion star fish Myrichthys maculosus - a brown and yellow spotted snake-like eel. Jardacne - clam Sea anemones Several species of sea cucumbers.

- 23 - Appendix 3 Khamala's transect of the Inner Reef/Lagoon - actual figures Quadrat Number of Details of Habitat Number E. mathaei 1 a Dissected reef platform REEF 2 a Pool of sand on egde of platform 3 a Algal area but only slightly dissected 4 a Algal area but only slightly dissected 5 1 Algal area, few crevices, 1 Protonuesta 6 a Sparse grass/tho ciliatum 7 a Sparse grass/The cilia tum 8 a Depression - 1/2 seagrass, 1/2 rubble 9 a 100% Tho ciliatum 10 a 100% Th. ciliatum, 1 Protonuesta 11 a 100% Tho ciliatum, 1 Tripneuste§ 12 25 Coral rubble/algae, 1 Diodema 13 20 Coral rubble/algae, 5 Diodema savegne 14 2 80% Th. ciliatum, 3 Tripneuste§ 15 5 80% Tho ciliatum, 2 ~p.neustes 16 2 80% Th. ciliatum 17 1 80% Th. ciliatum, 1 ~p.neu§tes 18 7 Coral rubble/algae. 19 9 80% Th. ciliatum, 2 Tripneustes 20 1 Sand/rubble/seagrass, 1 Toxopneuste§ 21 a 80% Th. ciliatum 22 a 80% Th. ciliatum, 1 Protonuesta 23 8 1/4 rubble, 3/4 seagrass, 1 sea cucumber 24 27 rubble, 1 Tripneustes 25 19 rubble garden, 4 Diodema, 2 Tripneustes 1 protoneusta 26 a Sand/seagrass 27 2 80% Tho c, 3 Tripneu§tes, 1 Protoneusta 28 12 rubble, 1 Link ia spp 29 1 80% Th. c, 3 Tripneustes, 1 Erotonuesta 30 a 80% Th. c, 2 Tripneustes 31 a 80% Th. c, 2 Tripneustes, 2 sea cucumber 32 1 80% Th. c, 1 Tripneustes 33 18 rubble 34 a seagrass/sparse grass 35 a 80% Tho c, 1 Tripneustes 36 a 80% The c, 5 Tripneustes 37 1 80% The c, 1 protonuesta, 1 sea cucumber 38 7 rubble, 1 Protonuesta, 1 Echinothrix 39 a rubble 40 a Seagrass 41 a 1/4 100% The c, 3/4 sand 42 1 80% Th. c, 1 Tripneustes 43 a 80% The c, 1 Tripneustes 44 5 80% Th. c, 1 Tripneustes 45 a 80% The c, 3 Tripneustes, 1 Synampia 46 a 80% Th. C, 3 Tripneustes - 24 - (Khama1a's transect continued ••• ) 47 1 80% Th. c, 3 Tripneustes 48 0 Seagrass/sparse grass, 1 sea cucumber 49 13 Rubble, 1 Protonuestg, 1 sea cucumber 50 0 Sparse grass, 1 sea cucumber 51 0 Seagrass/sparse grass, 1 Tripneustes 52 0 Sparse grass 53 0 1/2 sparse grass, 1/2 80% The c, 1 Protonuesta 54 1 Rubble 55 0 80% Th. c, 3 Tripneustes 56 0 Sand 57 0 Sand 58 0 Sparse grass 59 0 Sparse grass, 2 sea cucumber 60 0 80% Th. c, 1 Tripneustes 61 0 Sparse grass 62 0 Sparse grass 63 0 80% Th. c/sand, 1 Tripneustes 64 0 1/4 100% The c, 3/4 sparse grass 65 2 Sparse grass (sea urchins in crevice) 66 0 Sparse grass 67 2 80% Th. c 68 28 Rubble 69 7 80% Th. c, 1 cusion starfish, 1 sea cucumber 70 14 1/2 rubble, 1/2 sparse grass 71 25 Rubble 72 4 Rubble 73 0 Sparse grass 74 0 Sparse grass, 1 Protonuesta 75 0 80 % The C 76 0 100% The C 77 0 Sand 78 0 Sand 79 0 Sand 80 0 Sand 81 0 Sand 82 0 Sand 83 0 sand 84 0 Seagrass 85 3 1/2 seagrass, 1/2 sand 86 7 Sand/debris - old roots, etc. 87 4 sand/debris/seagrass 88 0 Sand 89 11 Seag rass/sand 90 0 80% The C 91 9 Seagrass, 1 Tripneuste§ 92 4 Seagrass 93 17 Seagrass 94 0 Sand 95 0 Seagrass 96 6 Seagrass 97 7 Seagrass 98 6 1/2 seagrass, 1/2 sand 99 0 Sand .BEACH Total of 346 - 25 -