University of the Pacific Scholarly Commons
University of the Pacific Theses and Dissertations Graduate School
1979
Shore-level size gradients in Tegula funebralis (A. Adams) : seasonal changes influenced yb interaction of predator preference and prey behavior
Daniel Victor Markowitz University of the Pacific
Follow this and additional works at: https://scholarlycommons.pacific.edu/uop_etds
Part of the Life Sciences Commons
Recommended Citation Markowitz, Daniel Victor. (1979). Shore-level size gradients in Tegula funebralis (A. Adams) : seasonal changes influenced yb interaction of predator preference and prey behavior. University of the Pacific, Thesis. https://scholarlycommons.pacific.edu/uop_etds/2017
This Thesis is brought to you for free and open access by the Graduate School at Scholarly Commons. It has been accepted for inclusion in University of the Pacific Theses and Dissertations by an authorized administrator of Scholarly Commons. For more information, please contact [email protected]. SH OR E>LCVEL S I ZE GRADIENTS IN
T e9.~1la fu n ~ bra l is ( A . Ad a ms ) :
Sea sona l cha nges i n fluenced by inte r action o f predator prefer e nce a nd p rey beh a vior .
A 1'he3iS
Prese;;ted to the F aculty of the Gr ad~a t e School
Univer sity o f the Pac i fic
In P a rtia l Ful f il l ment
of the Requirement s for the Degr ee
Mas t e r o f Science
by
Da n iel v. ~ark ow i t z
~1 a y '19 79 This thesis, written and submitted by
Dan iel V. Markow i t z is approved for recommendation to the Committee on Graduate Studies, University of the Pacific.
Department Chairman or Dean:
Thesis Committee:
Chairman
Dated /1tJ..'y / Cf 7Cf --~~~:,~--~----~------ABSTRAC'l,
Aspects of the P isas t er -T eg~) c: inter acti0!1 are re- examined. Reproductive porti ons o f T. fun eb.:.·aJ:.~.£ pop-· u l ations arc shown to be immune to s eastar pre d ation through a combination of pre d ator preference for larger snail s and a withdra wal beha vior t hat favors the escape of smaller snail s after capture by a seast a r . Experi- mental addition of p; ochra~ in winter causes chan g~s in the intertidal d istribution of T . funebralis similar to those observe d during the summe r increase ii1 seastar numbers . :~ t i s suggested that these r esult s s u ppl a nt the hypothesi.s : tha t lowe r e d pre r e productive mortality influe nce3 formation a nd ma intena nce of v ertica l size gr a d ients in the lower intertidal . ACKNO vlLEDGEf·f: Er-JTS :
I would l ike to thank Dr . Steven Obrebski for the help a nd ~dvico he gave which grea t ly f urt h ered the develop ment of this thesis , and Dr . Jame s Blake for his h e l p particular ~y in exploration of my s tudy area . The advice and c r it~clsm of the other students and staff of the Marine
Station was i nva luable i n many d iffi cult situations . My parents deserve special recognition f or their mor.al a nd fina ncial support without. which I coul d not have even b egun this work . 1
IN'l'RODUCTION :
Many species of i n tertidal gastropods h a ~e a v e rtical size gradient in the range of their di.stribution. This distribution was originally thought to result from g rad- i ents in juvenile mortality in intertidal rcgj.ons (Vernei j
1972), hut Bertness ( 1977) suggested t hat gene raliza tions about specific causativ e factors cannot adequately e xplain the development of size gradients sinc e they might r e sult from evolutionary optimization due to several interacting factors .
Lu.rger Tcg_';:ll2:_ funebral is_ are distr ibutcd lo·.'ler in t h e intertidal than the more abundant smaller sna ils (Paine
1 969 ). For the la.cger snails, Paine ( 1969) suggests t.ha t the advant~ges of incr eased individual fecundity due to lowered dcHsi t y ar.d greater food a vailability out•tJeigh the disadvantages of habitat overlap with thepredatory seastar ,
Pisaster ochraceus (Brandt) in the lower intertidal .
Paine (1969 , 1971) demonstrates t he effects of physiol ogical str ess on larger snails in the upper intertida l but does n J t accoun t f o ~~ the maintenance o f h igh d ensitie s of smaller snails in the uppe r area , nor does he feel T . fune bra lis generally has adequate escape responses against P . och~~~-~ predati on .
Obrebski ( MS ) h as determined that lower intert i d~ l
T . funcbralis migrate up\·:a rds concurrently v1i th s c a :.; onal incrc
fluctua tions in s n ail size and a rgues that lower escape v elocities of smaller snails tend. to s ubstantia te
Vermeij' s (1972) hypothesis. The prese nt study shows
'I'. f une br.u.l is migra t ion i n a nothe r loca1. i ty . Ne\·J infor mation on seastar feeding b e havior and snail escape mech an ism is r eported v1h i ci1 sugges t-. s an a l t c rna t i ve explan ation to T. fu~e~E~!i~ s ize gradients . In p articular, smaller sna ils a r e shown to be imnrune from digestion by P . o c hracc u s . FI ELD SAMPLING METHODS :
Field sampling was done in the Point Hc yes Nc.ttional
Seashore at the Palomar in Beach acce ss ( 122°45 ' 30" vJest X
3 7°56 ' North) . The area was chosen because of the abur.-
the isolation from human disturhance , and the absence of
signifi.cant numbers of prey other them ~ for feeding
seastaxs.
A transect measuring 10XSO m with the long axis per
pendicular to the v1ater's edge was (!Stabl ished . The verticc.:l difference between the upper and lower intertidal leve ls of the tra~sect was l ess than 2 meters . The entire transect was exposed at a 0 . 0 tide excep~ for a few shallow pools .
Five 10X10 m squares were labe l e d X, A,B,C and D from the
seav,rard edge upward . The lowest square (X) was only spars •~ ly
populated with T. funebralis being below their limit of
distribution.
Sampl es were taken between March 1978 and January 1979
to determine P . ochraceus abundance and T . funebralis s ize
und abundance. All seastars were counted at each samplinc;
date by thoroughly e xa mining the transect. Snails we r e 2 counted in 10 randomly chosen 625 em quadrats i n each of
squares A,B,C and 0 at each sampling date . From 2 to 10 of
these qua drats were a l so used to me asure snail shell size
depe nding upon t h e abundance of snRils .
Shell size: \\'U.S Jileasurcd \•d .lh vcrn:i cr cu.J i:x:.::::: t.o the
near est. 0.1 mm u s inC] t h o method of Paine ( 1.9C, ')) . All di:.tl il was collected in the field and no snails or seastars were eve~ removed from the transect.
T. funebralis s ize a nd abundance data was coll ected using a nested analysis of v ariance (ANOVA) design.
Abundance data is compared in a two level analysis with the highest treatment leve l as sampling dates , then square s within dates, and finally quadrats within squares as the lowest level. Snail shell size data was taken i n a three level design with levels from highest to lowest: Sa mpling dates,· squa res within dates, quadrats within squa r es, and shell sizes of snails within quadrats. The computations were performe rl ~ith the aid of a computer program adapted from
Sakal and Rohlf (1969). ,. .)
Pisastcr ochrnceus ABUNDANCE :
Seastars increase in abundance and maximum tidal leve l to a peak in midsumme r and decline thereafter
(see Fi g·...~re 1) .
T egul~ funeb~2 l is SHELL SIZE: The snail shell siz e data for the entire tra nsect shows no significant variation b e tween months, but highly significant variation in the lowe r l evels of the analysis. Further analysis of the individual squa res in a two level d es i gn shows a significant variation ( p <. 05) between months for sizes of snails found in the lowest squ3.rc sampled (square A). Figure 2 shows that the mean size of snails in square A reaches a maximum in midsummer and declines through the fall .
The corre lation between the number of seastars and the size of snails in each squa re was positive and signi- ficant (r= 0.23, N=1230 , p~.001). The correlation betweer size of snail s and tidal l evel ( s~uares) was negative and significant (r= - 0 . 23, N~887, p~.001).
Significant v ariation occurs at all leve ls in the analysis of the snail abundance d a la. Figure 3 shows the mean o f abundance for the e ntire transect a t each s a mpling du l e . Figure ·1 shO\vS the snuil ubunciuncc mcuns for the ___,,..._,..._,..._...... ______--- .. 6
lowest squure meusurcd ( squa~c A) . The graphs show the
decline in density through the eurly summer months to a
midsummer low und s light recovery in numbers through the
full .
The correlation between se~star numbers per square and
s nail abundance within squ5r~s was negutive and sig nifica nt
(r= -0.203, N~4SO, p<:.001). 7
EXPERH~f::i ,J'l'AL INC H.E/\SE OF Pi sa .s t c r och racci..l S l'.BUND/·.NCE
IN WINTER :
Two transects we r e established at the Palomarin sl'.j...... e . One was the seaward 30 m of the previously described
( see Field Sampling) transect (Transect 1). A second tran- sect (Transec t 2) wa s establishe d in a comparable area 50 meters to the North of the first .
Both tra nsects were sampled monthly for the two months prior t o expe rimental manipulation in February and March .
Sampling vras performe d as in the field sampling section.
Se~stars w~re counted in each of the t hree 10X1 0 m squa res of each transect as well as in the area above the transects
(The re were no seastars in the higher areas at these times . ) .
'I' • .f_~1_1 ety. a. li ~ \vere subsampled for s:te ll size and abundance 2 in squares A and B of the transectf. . Ten 625 cm quadrats were randoffily selected and all snajls found in each quadrat were counted and measured .
On Fel,ruary 22 another sample was taken. At this time l aboratory maintained specimens of Pisa ster ochra ceus were brought to the field site . These seastars we re added to transe ct 1 such that total numbers in each of the squares was comparab le to peak summer abunda nce . Resident s e astars were left. in place and included in the totals . Areas adjacent to each square were also see d e d with seastars to
limit the effect of some latera l movement of the added
<:mim•J. l s . Trc.tn.s0ct:?. \ •Jc.ts usc:d a s o. control a r c2 . Sc c·lst.:tr abundance was checked and augmented as neccessary on the three fol lowing days . Snail size and abundance was sample d
2 and 5 days after t h e tr~nsplant in each transect .
On March 23 the above experiment was repeated using transect 1 as a contr.ol and adding seustars to transect 2 .
Snail size and abundance was sampled 2 and 4 d ays after the transplant.
The sampling of snail shell size was in a three level nested ANOVA design with levels : AMong sample dates, between sq~ures within dates , between quadrats wihtin squares, and snails within quadrats . Snail abundance was in a two level design witl1 levels 2s above minus the lowest level . 9
SEAS'rAH ABUNDANCE DUf Figure 5 shows the numbers of seastars present in square A of each transect for the duration of the two field experiments as wel l as data from the two previous months . !£_gul~ funebrcilis ABUNDANCE DUIUNG FIELD EXPERif·iENTS: There vJas no significant change in density in either transect during the period of either experiment . Abundance was comparable in both transects . Tegula funebralis SIZE DURING FIELD EXPERIMEN TS : The increase in size in square A of each transect after expe1·imental manipulation (Figure 6) was significant (p<. o.s). There was no significc.nt change in size of snails when each ~ransect was measured as a control (Figure 6) . Square B of both transects showed no significant changes . 10 The size of snail that is actuall y consumed by foraging seastars is a key factor in the understanding of the dynamics of the snail population . Paine (1969) estimated the numbers of T . funebralis per u nit a r ea consumed by seastars, but did not incl ude specific data on the size range preferred by the seastar . Studies to determine the s eastars preference i n s nail size a r e described below. Snail she l l h eld by seastars i n feeding position wer e measured in July 1978 at the Pal omarin fiel d site . All feeding se.:tstars encountered i n a wide portion of the area were u sed. In addition , six simil a rly sized seastars were placed, 3 in each ~a lf of a divided ~ater table with a screened top . Twenty s n ails, 5 each o f 4 size classes were placed in each half . At the e nd of each 24 hour period a ny con- _ sumed snai:s wer e removed meqsu red and replaced with snail s of the sam ·~ size class . The design was a nalysed with a tvJO level .\NOVA vJith levels : Among replicates , size c l asses within r eplicates , and daily survivQl within size classes. Results were collect ed for 17 d ays. 11 SIZE PREFERENCE: The data (Table 1 , Figures 7 & 8) shows that s e asta rs pre fer the larger sized snails. Table 1 shows a signi ficantly (p~.001) higher survivorship in the s maller size classes . The me a n sizes of snails eaten in the field are compare d to those found in the laboratory in figure 8 . It is appa rent that larger sized snails are consumed in the field. The difference b e tween laboratory and field res u l ts is probably an artifact of the laboratory conditions. Compa rison of the field data to the s n ail sizes found i n the tra nsect (see section 1) shows that the s izes consumed are grea t e r tha n most average sizes found in the f i eld . The field s amples were taken when tl1e seastars were s pr ead through the ir peak range of tidal h e ight and thus e ~count e r ed snail popul ations with a lower mean size tha n t hat found in the lowe st squa re (A) of the tra nse ct. Visual observation of the l a boratory predation e xper ime nts sh o~ed ma ny insta nce s of a s ~as tar r e l easing small snails a fte r cap t ure . Also a g r e at•=r proport i on o f sma ll snails were f ound in witl1dra wn p os ~.c ion indic ati ng suc cess of the escape r esponse a s de scr iued in the n e xt s e ction. 12 STUDIES ON NEVJ ASPECTS OF ESCr\PE BEHAVIOI~: During laboratory observations,of Pi.saster ochraceus predation on Teau~_Q. funebr?-lis , it Has discovered that the snails withdrew their entire body and operculum into the shell past the first third of the opening whorl, fol lowing capture by a seastar. This behavior was observed when snail s that appeared to have been eaten during feeding e x periments were subsequently found to be alive. The following experiment was performed to test the effectiveness of this escape response . The shell opening was ground as a treatment to remove the protection of the first quarter whorl . Snails were size classed and Darked before grindin g and used at least a day after the operati on. No difference in running speed was found in snails tested before and after grinding. Two seastars were p l aced in each of four ten gallon g l ass aquaria with running seawater and screened tops . Seastars were randomly selected from a group of roughly equivalent wet weights and sizes . Each tank was supplied with twerty snails 5 each of 4 size classes . Two tanks held normal snails and the other two held treated snails. All tanks were checked daily for 7 days and consumed snails were scored and replaced by live snails of the same size class and type . The experiment was d esigne d for analysis with a three l e vel ANOVA . The leve ls from hjghest to lo w~st were: 1. 3 Among treatments , replicates within treatments , size classes within replicates , and survival within size classes. EFFECTIVENESS OF lrliTHDRA\vi.L BCH.I\VIOR : The data (Fig ure 9, Table 2) sho~,oJs the decreo.se in survival caused by the treatment. The 10% difference in survival vras significant (p<. OS) while t h ere vJas no sig nificant differe~ce between replicates within treatments . The difference between size cl&sses eaten is similar to the data presented earl ier and is also significant ( P<. 001 ). 14 DISCUSSION : Limits on lower distri buti ons of i ntertidal a ni mals set by predation a r e widely reported ( Conne ll 1975; Paine 19 76 ; Bros 1978) . The present study e mphasizes behaviora l aspect s of a predator and the behav ioral and mor9hological aspects of a mobil e prey t hat a llows the latter to e xtend their range int o that of the f ormer . Field data from the Pt . Reyes area agree with the observations by Obrebski ( MS ) indicating t hat a seasona l change in ! · funebralis d istri- bution occurs c oncurrently with the upward migration of P . oc~~~~e~ and a change in snail d e ns i ty i n t h e lower intertidal . This correlation of t h e influ ence of seastars on t urban snail migration in two local ities is further supported by the f ield and laboratory experime n ts . The field expe riments demonstrate t h at P . ochraceus h as a measurabl e s h ort term effect on T . funebralis size distribution. The seastars prefer the l a rge r snails . ~fui l e larger snails h ave faster runn ing speeds ( Obrebski , MS) , t h e smaller snail s , th rough ~ combina t ion o f b eh avioral and mor~h o l ogica l charac t eristics ~ escape predation b y P . · o chraceus . Previous n a tural h istor y studies ( Paine 1976 ; La ndenberger 1967 , 1968; Day ton 1971 ; J ill son 1 9 73 ; Ma u zey 1 966 ) gen erally assume that escape from p redation occurs only by h aving a l arger size or by migrating into areas inacc~ssible to t h e pred ators . The fnct that all s nu.il s !lave a runnin g r espon se ( l·'cder 1963 ; Yarnall 196!1 ) , 15 and smaller snails may be only rarely consumed by seastars , suggests an · a lternative explanation for the observed gradients and migration in T . fun ebr.a ~ is . Physiological factors (Paine 1971) cause larger snails to migrate l ower where they can be better maintained due to the great er abundance of food resources . Due to increasing numbers of encounters with seastars some larger snails may be eaten and smaller snails move upward r e sulting in the observed increase in average snail size and the corresponding decrease i n snail abundance d uring upward seastar migra tior!. The reproductive characteristics of T . funebralis provide a mechanism for the maintenance of these distribution pat t erns despite a l 2rge snail overlap with the predator . Paine ( 1971) shov;s that T. funebraJ is are fully mature at sizes greater than 18 mm , well below the seastars preferred prey size of 20- 23 mm . Thus , a proportion of the population that is reproductive is relatively immune from seast c.i.r predation. The tendency for small0r snails to migrate into the upper intertidal due to their escape response to seas tars does not expl ain the selF:c t i ve mechanisms actual l \' matntaining small snails in the uppe r intertidal, sinc e the y are able to escape digestion when caught. Other preda tors eat snails . Smaller Tegula are more susceptible to preda- tion by crabs than larger ones (H. Daley , pers. communication) . It is possible that the tendency for sma ll snail s to migrate uni·/ .:'l rcl ~; due i:o the !1rcscncc of SC'i1Stars j s ma :L n taincd 1.6 because it aids in their r emoval from the range of activity of other predators to which they have less effective escape responses and with which seastar distributions are correlated. Experimental manipulation of seastar populations i n other localities should provide further corroborative evidence for the foregoing hypothesis and help to define the effects of other changes as possible causitive £actors in T. funcbralis miaration. Further experimentation is -~ -·---- - .... also needed to verify assumptions about the effects of other pred<1tors. As suggested above, a complex system of intera ctions results in a size gradient in the vertical distribution of 2> i~l?.ra:::j:E. intertidal populations. These facts supply further knowledge to support a detailed evolutionary ex planation as suggested by Bertness (1977). The simpler explanatiort of lowered post-larval prereproductive mortality in higher areas suggested by Vermeij (1972) is considere d insufficient. Rather, a lowered predation effect on part of the rep~oductive portion of the snail population provi d e s an explanation more congruent with the new data and na t u r a L history information presented h ere . 1. 7 H.EFI:::RENCJ:;S: Bertness , M. D. 1977. Behavioral and ecological aspects of shore level size gradients in Tl!ais lame llosa and Thais ~marginat~ . Ecology, Vol.58 , pp. 86- 97 . Bros, W. E. 1978 . Vertical zonation of three species of California limpets (Acmaeidae) as a function of predation . MSc Thesis, Unive rsity of the Pacific~ Pacif ic Marine Station. 98pp. Connell, J . H. 1975 . "Some mechanisms producing structure in natural communities: a mode l and eviden=e from f ield exper imEmts." Exp . t'larine Biology pp. 46 0--489 . Dayton, P . r~ . 1971. Competition, disturbance, and commun ity organization: The provision and subsequent utilization of spac0 in a rocky intertidal community . Ecol . M ono g~ . Vol. 41, pp.106-128. Feder, H.M. 1963. Gastropod defensive responses and their effectiveness in reducing predation by starfishes . Ecology Vol . 44, pp . SOS-512. Jillson, D.A . 1973 . The predatory behavior of Pisaster ochraceus. f·1Sc The sis, University of the Pac:i.fic , Pacific Marine Station. 121 pp. Landenb erger , D.E. 1967. A study of predation and predatory b e havior in t h e pacifi c starfish, P i saster . PhD . d isser tation , University of California Santa Bar bar a . 164 pp . Lande nberger , D.E . 1968. Studies on sel ective feed ing in the pac :i.f ic start i sh 2::> i s r1st~.£ in southe rn Ca.L :i.for nia . Ecology Vol. ' l~) , pp . 1.0G2 - 107 5 . 18 Mauzey, K. P . 1 966 . Feeding behavior a nd r eproductiv e cyc l es i n Pisaster ochraceus . Biol . Bu ll . Vol . 131, pp.12 7-144 . Obrebski , s . MS . Seasonal migration a nd escape from predation in Tegula f u nebralis . Paine , R. T . 1.969 . The Pis ~te£- T~u la interacti,on: prey patche s, predator food preference , and intertidal community structure . Ec o logy Vol . 50 , pp . 950- 961 . Paine , R.T. 1971 . Energy f low in ~ natural population of the herbivorous Gastropod Tegulc; funebral is . Limnology and O c e~nograph y, Vol . 1 6 1 pp . BE - 98 . Paine, R. T . 1 976 . Size limi ted pred ation: An observational and ~ xp e:rime nt a l approach with the Myti_lus-Pisaster: interaction . Ecol ogy , Vo l . 57 , pp w858- 873 . Sokal , R. R. a nd F . J . Rohl f , 1969 . Biometr:y,. ~v . H . Freeman and Company , Ne w York . 776pp . Vermeij, G. J . 1972 . I n traspecific shore-le v e l size gradients in intertidal mo l luscs . Ecolog~· , Vol . 53, pp . 693- 700 . Yarnall , .:;· eL . 1964 . The responses of Te~~ _fun ebra_li~ to sta: ~ :F ishes a nd pred atory sn<:t ::..1 s. Ve l iger, Vol . 6 ( Suppl), pp . SS -58 . 19 Figure 1: Numb e rs of Pisaster ochraceus found in the 5 squares of the field transect at each sampling date. (March 1978- January 1979) ------~----~20 10· Sq 0 ------~~~~~------Sq C Total 10· ~ . l 60· so- 40- <, 30- '; J y -~ 20- I ~. .• ~ ~ 10· ~ -~. J ~ i! ·' : [~ .\. ~... -~~ .~ ~ ·-:. ' . .LL.~ M 1\ M J J Jy Juy A 0 N N D .J 5 27 21 6 23 5 26 17 16 2 28 12 :l6 SAMPLING DATE 21 Figure 2: Means and 95% confidence limits for size of Tegula funebralis sampled in Square A of the field transect at each sampling date. (March 1978-January 1979) 22 25 Squar·c A E E ,- Q! lJJ 20 w T ·~- l .,.J ..J LIJ ~ (/) 15 z ~ l.lJ .~ ~ 10 --- - .J-~~- • _...~ --7-- ·..J.-... -.1...---·-'------...l- ...... _ -,.J__ _ --~. ______.. _ _ ·l--~--~~ ---- l\~ A M J J · Jy Jy I\ 0 N N D J 5 27 21 6 23 5 26 17 16 2 2·8 12 26 SAMP LING DATE 23 Figure 3: Means and 95% confidence limits of Jequla funebralis abundance for the entire field transect at each sampling date. (March 1978-January 1979) Nov.28-Jan.26 samples based on squares A and B only. 24 240 N E ~ ·>.... Cl 'J : 16l l <( z C/) _,_ __ ,..--- ~-- --+-- M,. A fVj .I J Jy Jy A 0 N N D J ~ 27 21 6 23 5 26 17 16 2 2 8 12 26. SAMPLING UJ.\TE ' 25 Figure 4: Mea.ns and 95% confidence limits for Tegula funebralis abundance in square A of the transect at each sampling date. (March 1978- January 1979) 2 6 240 N r:- >- "t- 1.60 z I "T J ~~ ~~~ ·--~--L~ - r M A M J J Jy · Jy A 0 N N D J 5 2']' 2 1 6 23 5 26 17 16 2 2 B 12 26 SAMPLI NG DATE 27 Figure 5: Seastar numbers during field exper- iments. Solid bars indicate number present. Clear bars show those additional seastars added on that sampling date. 28 Transect 1 Sq A 30 2 ,. 10 -~ f: :·l~ ·. t t. ~' .~· Tri!nsect 2 Sq A 30 20 !gl ::.": 'I •~ 1 10 .. t' ttl D J 23 2.r1 FEBRUARY MARCH DATE 29 Figure 6: Mean and 95% confidence limits of Tegula funebrali~ shell size found in square A of the transec t s during the two fie ld expe r imertts. Dotted lines show times of experimental inc~ease i n seastar numbers. 30 Transec t 1 -Sq A 2 19 18 17 T"l I --r-- D J 22 24 27 23 25 2 7 FE{3RUARY MARCH Transect 2 Sq A 19 18 ! I I 17 I I I -..J --.--·-· 22 24 27 23 25 27 FEBRUAR Y MARCH D/\TE 31 Figure 7 : Total numbers of Tegula funebral is consumed by Pisaster ochraceus during 17 day experiment where prey were available in 1 : 1:1:1 ration of each s i ze c l ass . 20 15 10 5 z ..~ ....hl <~ l!J 2or f(! L:J m,, , ~~~ ;.) 15 ~~ 5 u- 12·.s - 1"7- 2 1. s .. 1 2 .4 1 6. 9 2 1.4· :2 6 SIZE CLASS mm 33 Table 1: Experimental data for Tegula funebral is size preferred in l aboratory by feeding Pisaster ochraceus . Data are d aily means of totals presented in Fi gure 7. ') 34 I. SHELL 1 2 3 4 SIZE CLASS 8-12.4mm 12.5-16.9mm 17-21.4mm 21.5-26mm TANK A Mean # eaten 0.12 0.65 0.82 1 . 0 per du.y Mean survival _ % 97 65 87 . 05 83.5 80 per day TANK B Mean # eaten 0.05 0.1'7 0.65 0.76 per day Mean survival % 98.8 96 . 5 87 . 05 84.7 per day 35 Figure 8: Means and 95% confidence limits of snail. sizes consumed by Pisaster ochrcceus in ,2 field and 1 lab samples. (See text) 1 36 ± n=59 E E 20[ n=ll 0 l!J I n ='73 ~ :;) C/) 2: 0 ( .) t!J N C!) lS z • I • f 10 . July July Lab 5 20 SAMPLE ~ I Figure 9 : Numbers of Te gula funebralis eaten by seastars in experiment to test effectivne ss of withdrawal respons e . Size classes are the same as those listed in Table 1 . 1~ CONT OLS 10 (fJ c:: c.1: 1- tl) 5 3 4 SI ZE CLASS Table 2: Survivorship data for laboratory experiments to test wit hdrawa l response of Tequl a funebral~~ to P. ochraceus predation. 1 I ~ Mean daily Mean % s urvival % survival for t~eatment ------~------~------Tank 1 77.14 •rreate d Snail s 76.07 Tank 3 75 Tank ·2 85 Control Snails 86.07 Tank 4 87.14