Indian Journal of Marine Sciences Vol. 28, September 1999, pp. 325-328

Short Communications

Intraspecific regulation in the population densities of the intertidal gastropod (, ) in the Suez Canal

S ZMoharnmed Marine Science Department, Faculty of Science, Suez Canal University, Ismailia, Egypt Received 3 September 1997. revised 3 May 1999

Monthly monitoring of the intertidal gastropod (Planaxis sulcatus) population in the Suez Canal rocky shores revealed a decrease in the adult abundance during recruitment peaks. This produced a periodical imbalance on the adult densities and reflect the regulatory roles of recruits on Planaxis population. Results of experimental manipulations to investigate these roles indicated that an increase in the overall densities reduced the rate of individual growth and declined the adult abundance. The results agreed with the hypothesis that recruits and juveniles have a competitive advantage over adults a, they are small and require less energy to survive. It is postulated. therefore that. the rapid decline in the individual numbers during intense competition could be one of the regulatory agents in the Planaxis population dynamics in the Suez Canal.

The regulation of population densities has been the intertidal gastropod Planaxis sulcatus (Born investigated widely which assisted in understanding 1778) (Gastropoda: Planaxidae) to determine the l the dynamics of populations -'. The effect of intraspecific competition on the rate of fluctuations of the population densities in the recruitment and 011 the adult mortali ty. This enables -~ intertidal gastropods can be regulated within to assess the natural variations of the adult densitie:, ecological boundaries which on the other hand can't in relation to the recruitment timing and its intensity. be controlled or predicted regularll. Adult mortality, This study was conducted on the intertidal l-odv recruitment, reproductive output and growth are the shore of the west coast of the Suez Canal (Dcversiur. main biological criteria of density dependent lat.30025''N, long. 32°21 "E.) which is covered mainly regulations which affect the life cycle of the species by encrusting filamentous algae and ~ess j Ie- fauna. and could alter its abundance~.4. Marine gastropods Many patches of the gastropod, Plw!Ox i.1 sulculu.\", particularly, the intertidal ones, have intraspecific were found in clusters on rock surfaces, in crevices competition due to an increase in the local densities 2 5 and under small boulders. Other like oyster of species . . Underwood(' recorded decline in the Crassostrea calcutta, mussel Brachidonre.l' va riabilis, growth and increase in mortality at increased barnacle Balanus {ll11phil m ,-, and periwinke Lllloril1 (f densities in the Nerira atramentosa population. spp. were also present, in association with the Similar resu Its were also reported for intertidal 7 Planaxis clusters. The present inve:-; tigation wa~ limpets . Other investi gations recorded decrease in carried out on rock surfaces with dense Plw wxis growth but no increase in mortalities ~ . The negative population (> 1000 individual /m"), Fi ve quadrates influence of increased adult densities on the ) (0.25 m- each) were randomly chosen and marked tor recruitment cf young ones have been inferred from monthly sampling. In each quadrate, the number of vanatlOns. . among natura I popuI') atlOns" I ) or adults (> 7.0 mm shell height), juve niles (3 .7 mill) experimentally monitored under controlled 4 9 iO and recruits « 3.0 mm) were counted and the ir mean conditions . . . However, other studies revealed a numbers were recorded monthly. positive relationship between adult densities and the rate 0 f young recruitment. II . JO- . . Th ere f ore, the mode To investigate the intraspecific competition for and intensity of competitive interactions may vary Planaxis population, two experiments were amongst species and even between year classes of the conducted. In the first experiment, the manipulation same species. was carried out for all individuals (without In the present investigation, the density dependent segregation between adults, juveniles and recruits) regulations were monitored for adults and recruits of while in the second experiment, the influence of 326 INDIAN 1. MAR. SCI., VOL. 28, SEPTEMBER 1999 juvenile and recruits removal were assessed to Experiment results-In the first experiment there determine their role in the population density was a remarkable decline in the densities of the regulation. The present manipulation followed the original adult cohorts (first cohorts) over the 4 standard methods modelled for other gastropods ,6. monitoring time and particularly during May-July period (G test, P < 0.01). The decline patterns In individual manipulation: Five isolated plots 2 appeared to differ among the treatment. In the control (each with an area of 0.25· m ) were selected on the treatment the densities of the original cohorts intertidal flats and their borders were marked with declined rapidly following the appearance of recruits stainless steel rods. These plots were located away (second cohort) during April (Fig. 2A). This resulted from each other to inhibit the movement of Planaxis in the increase of the overall density in the among them. Two sets of experiments were subsequent months (Fig. 2B). For the reduced density conducted, each had four replicates. They were treatment 012 C) the densities of the first cohort also control (C); in which no manipulation was done and 2 declined (but less than in the control ones, G test, the animal density was about 60 / 0.25 m , and half P < 0.05). Recruitment of the second cohort in control 012 C): in which the density maintained at reduced treatment appeared to be relatively higher approximately half the control. The numbers of than in the control ones (Fig. 2B). Growth in shell Planaxis in replicates were counted monthly (from heights of both cohorts (Fig. 2C) was significantly January-July 1997) and its mean was calculated for gre.ater in the reduced treatments than that in the different treatments. Also, their shell heights were control ones (one way ANOVA, P < 0.05). In the measured monthly and the increments were estimated. second experiment the densities of the first cohort in The required alterations were conducted to maintain the control treatment declined rapidly during heavy the correct densities within each treatment. This recruitment which occurred in late spring (Fig. 3A). involved removing of some recruits and adults from In other treatments, such decline was less marked as some plots to maintain the reduced densities. compared to control ones. Growth in shell height in In juvenile recruit manipulation: Three treatments all reduced treatments (Fig. 3B) was significantly (each with two replicates) were conducted in three higher than in the control ones (P < 0.05). In the isolated plots (0.25 m2 area) during heavy recruitment treatment from which juveniles and recruits had been period (March-July 1979). They were: control (C): no removed (1. R. R), the animals recorded significant 2 manipulation, densities = 70 / 0.25m , J.R.R: all increase in growth (P < 0.05) than in treatment with Juveniles and recruits were removed, and V2 C J .RR: half control plus juvenile recruit removal 012 C J. R. densities maintained half the control in addition to lhe R). The total numbers of recruits (Fig. 3C) occurred removal of all juvenile and recruits. The recruits in during April-July period were significantly low in the each area were counted monthly and the size (shell control treatment (G test, P < 0.05) as compared to height) of all individuals was measured. Also, some reduced ones. Among these treatments, recruitment random individuals were removed to maintain the were slightly intense in J .RR than in V2 C J .R.R. correct densities in each treatment. 800 Recruitment and adult densities-Recruitment . - •. -Adluls - • - Juveniles took place throughout the year with a pronounced --..- Recruits peak from May to September (Fig. 1). Smaller peaks were recorded during January and February 1997. Juvenile and adult densities fluctuated widely (Fig. 1). Variations of densities between some monthly 2 samples were> 70/ m • These fluctuations appeared to be correlated with the months where there were

~ ~ ~ '2 ~ ~ ~ ~ ;;; high recruitment rates. The increases in adult "!! 0. ~ D D 1l ~ < i ~ ~ 0 ~ ~ ~ .i'" ~ ;;; E densities which occurred within 2-3 months after ~ 0 ~ ~ u. ~ ~ ~ i U) Z D "- recruitment peaks probably revealed the death of , Months ~ adults during the recruitment times. Veracity of these findings could be verified from the following Fig. I-Monthly variations in the density (no. / m2 ± S.D.) of experimental results adults, juveniles and recruits of Palanaxis sulcales. SHORT COMMUNICATION 327

The present results generally revealed the increase Patel/oida murifa and suggested that there was a of mortalities in Planaxis population and the decrease cyclical pattern in the densities of adults coinciding in its growth with the increase in densities. Adult with the recruit appearance which play a partial mortalities increased greatly subsequent to the regulatory role. The present results are in agreement appearance of recruits. These results reflect the with these findings and the changes in adult (or all) catastrophic decline in adult population when natural densities can be attributed to the changes in intensity densities and rate of recruitment increased above the of Planaxis recruitment. carrying capacities of Planaxis habitats which was Results of experimental manipulations indicated constrained by intraspecific competitions between that recruitments were relatively high in density ad u I ts an d recruits. g '13 . U nd erwoo d214. Investigate. . d t h e reduced treatments and suggested that recruitment interactions among age classes in the prosobranch intensities are influenced by individual numbers. The gastropods and recorded an increase in adult densities of Planaxis population therefore seem to be mortalities during great recruitments. Fletcher4 also regulated directly by recruitment intenSItIes or recorded similar results in the subtidal limpet, indirectly by adult mortalities. In contrast to the

..+.. eo --+--c ~ a- u; ce 70 A -. - J.R .R + 70 - .. - Control II> 60 ...... 112 C • J.R.R. N A :;;;SO E 60 an _____ Half control --: 40 'T' - .... - o ~ 50 ~30 ----J... Q ...... t· . -- .. , .. ... j .... l.~--· 40 , .~ 20 0 , ~ 10 ~ 30 -- ~ o~--~--~ ____~ __~ .~ 20 ' ...... en .... March April May June July c 10 4l Month. 0 --+-Control 16 120 ..••. Half control B 14 100 B I 12 .t: 10 ~ 80 .. Cl III C CI 6 --+--c 4l :c 'tI 60 ...... 6 -. -J.R.R 'ii jij J: 4 ..... ' 112 c· J.R.R. ... 40 (/) 4l . .- 2 > ...... 0 20 0 ~-----+------~------~--~ March April May June July •••. • Control 1 Months 14 - • - Half control _Y Fir s teo h 0 r t ---.-Control . } _ ..... 12 - .)f- • Half control Sec 0, nj.a:."Oo 0 r t 500 C , 4SO en 10 .t: I c , ~ ...... JIll: 400 ,'...... '" 2 3SO 8 &l ...... '" ... 300 ~ r--~ ...... "'.' X· '0 2SO 0 , /..0/ ...... ~ c 200 E 150 0 l- 100 SO :r:'H~/ 0 L-. C J ,R,R, 112 C J.R.R , ~ ~ ~ t >- ~ Treatment ~ ~ ~ « ~ ~ ~ ~ Months Fig. 3-Second experiment results . variations in (A) density (mean no. /0.25 m2 ± S.D.) of the tlrst cohorts, (B) increment of shell heights of first cohort and (C) total 110. of Palanaxis recruits Fig. 2-First experiment results-variations in (A) density (mean during April· July inclusive period. Treatments are C= Control, 2 no. / 0.25 m ± S.D.) of the first cohorts, (B) overall density and 1.R.R.= Juvenile and recruits removal and Y2 C J.R.R.= Half (C) increment in shell heights of the first and second cohorts. control + juvenile and recruits removal. 328 INDIAN 1. MAR. SCI., VOL. 28, SEPTEMBER 1999 present results, Fletcher4 recorded non significant suffer not only from competItIons, but also from relationships between the recruitment intensities and ultimate increase in the recruits and decline in adults. the overall number of individuals and postulated that From the results it is clear that the competitive process of recruitment was indirectly involved in the interactions between recruits and adults play an partial regulations of densities by affecting only the important role 111 the population dynamics of rate of mortality. P[anaxis. The strong competItIve regulations between Other factors (as space, food, predation ..... etc.) recruits, juveniles and adults are mainly attributed to cou Id a I so a f elect Its' popu Iatlon . regu I'at Ions <) ..14 I X an d the differences in the individual energy requirements should be investigated to draw a complete picture of different life stages 13-15. Recruits and juveniles about the density dependant dynamics of this species need less food than adults, so they are able to subsit in the Suez Canal. on less food than adults4. Moreover, their large numbers could impede the feeding activity of adults References by generating competitIve interactions. This Wiens, lA, in Ecological cOlIl/nll/litics, edited hy Simherloll phenomenon (of recruitments being able to adversely 0, Strong 0 , Abele L, Thi stle A R (Princeton Uni v. Press, affect adults) is however not uniform. In many littoral F rinceton, N J), 1984, 439 organisms, such as mussels 16 and barnacles 17, adults 2 Underwood A 1, Adv Mar lJiul, 16 ( 19 79) II I . outcompeted the recruits and juveniles and a decrease 3 Bowman R S & Lewis J R, .1 Mar Bioi Ass UK. 57 (1977) 793. in their survival. Therefore, it seems, that under 4 Fletcher W 1, Oecologia. 75 ( 198X) 272. certain circumstances recruitment intensity can either 1111