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University of the Pacific Theses and Dissertations Graduate School

1979

Shore-level size gradients in Tegula funebralis (A. Adams) : Shore-level size gradients in Tegula funebralis (A. Adams) :

seasonal changes influenced by interaction of predator seasonal changes influenced by interaction of predator

preference and prey behavior preference and prey behavior

Daniel Victor Markowitz University of the Pacific

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Recommended Citation Recommended Citation Markowitz, Daniel Victor. (1979). Shore-level size gradients in Tegula funebralis (A. Adams) : seasonal changes influenced by interaction of predator preference and prey behavior. University of the Pacific, Thesis. https://scholarlycommons.pacific.edu/uop_etds/2017

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SHOR E>LCVEL S I ZE GRADIENTS IN

Te9.~1la fun~bral is ( A . Ad a ms ) :

Sea sona l cha nges i n flue nced by inte r action o f predator prefer e nce a nd p rey beh avior .

A 1'he3iS

Prese;;ted to

the Facult y of the Grad~a te School

Univer si ty o f the Pac i fi c

In P a rtia l Ful f il l ment

of the Requirement s fo r the Degr ee

Mas t e r o f Science

by

Da n ie l v. ~arkow i t z

~1ay '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 ter-Teg~) c: inte r acti0!1 are re -

examined. Reproductive port i ons o f T. funeb.:.·aJ:.~.£ pop-·

u l ations arc shown to be immune to s eas tar pre d at ion

through a combination of pre d ator preference for larger

snai l s and a withdra wal behavior t hat favors the escape

of smaller snail s after capture by a seas t a r . Experi-

mental addition of p; ochra~ in winter causes chang~s

in the intertidal d istribut ion of T . funebralis similar

to those observe d during the summe r increase ii1 seastar

numbers . :~ t i s suggested that these r esul t s s u ppl a n t the

hypothes i.s : tha t lowe r e d pre r e productive mortal ity influ e nc e3

formation a nd ma intena nce of v ertica l size gr a d ient s in the

lower intertidal .

ACKNOvlLEDGEf·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 ur t hered 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 crit~clsm of the other students and staff of the Marine

Station was i nva lu able i n many d iff i cult situations . My

parents deserve special recognition f or their mor.a l a nd

fina ncial support without. which I coul d not have eve n b egun

th is 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 grad -

i ents in juvenile mortality in intertidal rcgj.ons (Vernei j

1972), hut Bertness ( 19 77) 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 ind ividual fecundi ty due to

lowered dcHsi t y ar.d greater food a vail ability out•tJeigh the

di sadvantages of habitat overlap with thepredatory seastar ,

Pisaster ochraceus (Brandt) in the lower intertidal .

Paine (1969 , 1971) demonstrates t he effects of physio l og ica l

str ess on larger snails in the upper intertida l but does n J t

accoun t fo ~~ the maintenance o f h igh densitie 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~~~-~

predat i on .

Obrebski ( MS ) h as determined that lower intertid~ l

T . fun cbralis migrate up\·:a rds concurrently v1i th s c a :.; onal

incrc<Jsc s in ~>custar abund <.t ncc . l!c <:l lso rc:orc~ :; <:t :-; ;;ocL::.( .}

2

fluctu a tions in s nai l size and a rgues that lower escape

velocities of smal ler snail s tend . to s ubstantia te

Vermei j' s (1972) hypothesis. The prese nt study shows

'I'. f une br.u.l is migr a t ion i n a nothe r loca1. i ty . Ne\·J infor­

mation on seas tar feedi ng 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 explanation

to T. fu~e~E~!i~ s ize gradients . In particular, smal ler

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°5 6 ' 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

wa s 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 lowes t 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 thorough ly 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 drat s were a l so used to measure snai l 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 col l 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 shel l 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 with in quadrats. The computations were

performe rl ~ith the aid of a computer program adapted from

Sakal and Rohlf (1969).

, . . )

Pisastcr och rnceus ABUNDANCE :

Seastars increase in abundance and maximum tidal

leve l to a peak in midsumme r and decline thereafter

(see Fig·...~re 1) .

Tegul~ funeb~2l is SHELL SIZE:

Th e snail shell siz e data for the entire tr ansect

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 var iation ( p <. 05)

between months for sizes of snails found in the lowest

squ3.r c 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 ar iation occurs at all leve ls in the

analysis of the snai l abundance d a la. Figure 3 shows the

mean o f abundance for the e ntire transec t 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 significa 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

'.j.. sl .... e . One was the seaward 30 m of the previously descr ibed

( 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 tr a 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

(Th e re were no seastars in the higher areas at these times . ) .

'I' • .f_~1_1 ety.a.li~ \vere subsampled for s:t e ll size and abundance

in squares A and B of the transectf. . Ten 625 cm2 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 th e 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.:t r

abu ndance was checked and augmented as neccessary on the

three fo l lowing days . Snail size and abundance was sampl e 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 leve l design witl1 levels 2s above minus the lowest

level .

9

SEAS'rAH ABUNDANCE DUf<ING FIELD f;XPERH'Ir:;NTS :

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 EXPERIMENTS :

The increase in size in square A of each transec t

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 actual l y consumed by foraging

seastars is a key factor in the understanding of the

dynamic s of the snail population . Paine (1 969) estimated

the numbers of T . funebralis per u nit a r ea consumed by

seastars, but did not inc l 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 fie l 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 ~alf of a divided ~ater table with a screened

top . Twenty s n ai ls, 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 snai l 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.

Resul ts were collec t 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 mea 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.

Comparison of the field data to the s nai l 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~counter­

ed snail popul ations with a lower mean size tha n t hat found

in the lowe st squa re (A) of the tra ns ect.

Visual observation of the l aboratory predat ion e xper­

ime nts sho~ed ma ny insta nce s of a s ~astar r e l eas ing small

snails a ft e r cap t ure . Also a g r e at•=r propor t i on o f sma ll

snails were f ound in witl1dra wn pos ~.c ion indic at i ng suc cess

of the escape r esponse a s de sc r 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, fo l lowing

capture by a seastar. This behavior was observed when

snai l s that appeared to have been eaten during feedi ng

e x periments were subsequently found to be alive. The

following experiment was performed to test the effec tiveness

of this escape response .

The shel l 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 operat i 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 es igne d for analysis with a three

l e vel ANOVA . The leve ls from hjghest to low~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 repl icates within treatments .

The difference between size cl&sses eaten is similar to

the data presented earl ier and is also significant

( P<. 001 ).

14

DISCUSS ION :

Limits on lower distr i but i ons of i ntertidal a ni mals

se t 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 iora l and mor9hologica l

aspects of a mobi l 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

intertida l . This correlation of t h e influ ence of seastars

on t urban snai l migration in two local ities is fur ther

supported by the f ield and laboratory experime n ts .

The fie ld expe riments demons trate t h at P . ochraceus

h as a measurabl e s h ort term effec t on T . funebralis size

distribution . The seastars prefer the l a rge r snails .

~fuil e larger snails h ave faster runn ing speeds ( Obrebski ,

MS) , t h e smaller snai l s , through ~ combina t ion o f b eh avioral

and mor~ho logica l charac teristics ~ escape predation b y

P . · ochraceus . Previous n a tural h is tor y studies ( Paine

1976 ; La ndenberger 1967 , 1968 ; Day ton 1971 ; J il l son 1 9 73 ;

Ma u zey 1 966 ) gen erally assume that escape from predation

occurs only by h aving a l arge r size or by migrati ng into

areas inacc~ssible to t h e pred ators . The fnct that all

s nu.i l 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 . funebr.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 during 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 proport ion of the population

that is reproductive is relatively immune from seas t 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 exp l ain the sel F:c t i ve mechanisms actual l \'

matntaining small snails in the uppe r intertidal, sinc e th e y

are able to escape digestion when caught. Other preda tors

eat snail s . Smaller Tegula are more susceptible to preda-

tion by crabs than large r ones (H. Daley , pers. communication) .

It is possible that the tendency for sma ll snai l 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 detail ed 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 prov i 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~ Paci f 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

organizat ion: The provision and subsequent utilization

of spac0 in a rocky intertidal community . Ecol . Monog~ .

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 Th esis, 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 pacif i c starfish, P i saster . PhD . d isser­

tation , Univers ity of California Santa Bar bar a . 164 pp .

Land e nberger , D.E . 1968 . Studies on se l ective fee d 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 .

Obrebsk i , s . MS . Seasonal migration a nd escape from

predat ion in Tegul a f u nebralis .

Paine , R. T . 1.969 . The Pis~te£-T~ula 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 popu lat ion of

the herbivorous Gastropod Tegulc; funebral is . Limnology

and Oce~nography, Vol . 1 6 1 pp . BE - 98 .

Paine, R. T . 1 976 . Size limi ted predation : An observationa l

and ~xpe:rimental approach with the Myti_lus-Pisaster:

interaction . Eco l 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 .

Vermei j, G. J . 1972 . I n traspecific shore-le v e l size gradients

in intertidal mol luscs . Ecolog~· , Vol . 53, pp . 693 - 700 .

Yarnall , .:;· eL . 1964 . The responses of Te~~ _funebra_li~

to sta:~ :F ishes a nd predatory sn<:t ::..1 s. Ve l iger, Vol . 6

( Suppl), pp . SS-58 .

19

Figure 1: Numbe 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

--------------~~~~~--------

10·

60·

so-

40-

30-

20-

10·

<,

J -~ I

~ ~

M 1\ M J J Jy 5 27 21 6 23 5

~ . l

y

u ~ i! . [~ ~ ·:. . -

Jy A 26 17

SAMPLING DATE

Sq C

Total

';

~ ' : ~. .•

~ -~. J ·' ~ ~ .\. ... -~~ .LL.~

0 N N D .J 16 2 28 12 :l6

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! l lJJ 20 T ·-w ~ I <t Q

.,.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

SAM P LING DATE

23

Figure 3: Means and 95% confidence limits of

Jequla funebralis abundance for the entire

field transec t at each sampling date. (March

1978-January 1979) Nov.28-Jan.26 samples based

on squares A and B only.

N E

240

~ ·> .... Cl 'J

: 16l <( z C/)

l

M A fVj ,. ~ 27 21

24

_,_ __ .I J Jy Jy 6 23 5 26

SAMPLING

,..---~-- --+--A 0 N N D J 17 16 2 2 8 12 26.

' 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)

N r: -

26

240

" >-t- 1.60

z <l: lJJ 80 ~ . J

I ~~ ~~~·--~--L~-

M A M J J Jy · 5 2']' 2 1 6 23 5

SAMPL I NG

"T

J r

Jy 26

A 0 N N D J 17 16 2 2B 12 26

DATE

27

Figure 5: Seastar numbers during field exper-

iments. Solid bars indicat e number present.

Clear bars show those additional seastars

added on that sampling date.

28

Transect 1 Sq A

30

2

,. 10 -~

~fl · .. ·t ::·~

'

t .

. ~·

Tri!nsect 2 Sq A

30

20 !gl

10

::: '~1 ." I •

. . t'

ttl D J 23 2.r1

FEBRUARY MAR CH

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 fi e ld expe r­

imertts. Dotted lines show times of experimental inc~ease i n seastar numbers.

2

19

18

17

D J

19

18

17

30

Transec t 1 -Sq A

T"l I --r--22 24 27

FE{3RUARY

Transect 2

! I I 22 24 27

FEBRUAR Y

D/\TE

23 25 2 7

MARCH

Sq A

I I I

-..J --.--·-· 23 25 27

MARCH

31

Figure 7 : Total numbers of Tegula funebra l 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 .

z hl .... <~ l!J

f(! L:J m ,, , ~~~ ;.)

~~

20

15

10

5

2or

15

5

.. ~

u- 12·.s - 1"7- 21.s­.. 1 2 .4 1 6. 9 2 1.4· :2 6

SIZE CLASS mm

33

Table 1: Experimenta l data for Tegula funebra l is

size preferred in l aboratory by feeding

Pisaster ochraceus . Data are d aily means

of totals presented in Fi gure 7.

')

I .

34

SHELL 1 2 3 4 SIZE CLASS 8-12.4mm 12.5-16.9mm 17-21.4mm 21.5-26mm

TANK A

Mean # eaten per du.y 0.12

Mean % survival97 _65 per day

TANK B

Mean # eaten per day 0.05

Mean % survival 98.8 per day

0.65 0.82 1 . 0

87 . 05 83.5 80

0.1'7 0.65 0.76

96 . 5 87 . 05 84.7

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

E E 20[ 0 l!J ~ :;) C/)

2: 0 ( .)

t!J N C!)

lS z <J: w ~!

• I • f

10 .

± n=59

July 5

36

n=ll

July 20

SAMPLE

I n ='73

Lab

~ 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) <t 5 w en

> .. Ctl

z 20 TREATED SNAILS !.:tJ

·~' <-"'! t>.J

l,t".:

i't" .. w C! t ~ ;:) 1.0 z

5

3 4

SI ZE CLASS

Table 2: Survivorship data for laboratory

experiments to test wi t hdrawa l response

of Tequ l a funebral~~ to P. ochraceus

predation.

1

I ~

Mean daily Mean % s urvival % survival for t~eatment

------------------------~------~------------Tank 1 77.14

•rreate d Snai l s 76.07

Tank 3 75

Tank ·2 85

Control Snails 86.07

Tank 4 87.14