Tane (1968) lh: 1+3-63

1+3

LIMPET MOVEMENTS An investigation into some aspects of limpet movements, especially the homing behaviour.

By T. W. Beckett*

INTRODUCTION

Several overseas workers (Stephenson 1935, Patella granularis; Thorpe, 1956 P a t e l l a depressa) have shown that some species of limpets can r e l o c a t e an e s t a b l i s h e d home on the shore.

Two common Auckland p a t e l l i d l i m p e t s , Cellana ornata (Dillwyn l 8 l 7 ) and Cellana radians (Gmelin 1791) were observed to determine t h e i r movements and homing a b i l i t y .

The m a j o r i t y of the work was conducted i n the Takapuna area, w i t h s u p e r f i c i a l observations around Auckland's east and west coasts. (Figure l ) .

At Takapuna C. ornata occurred from extreme high water s p r i n g (E. H. W. S. ) to the mid-tide l e v e l . (Crassostrea glomerata zone) w h i l e C. radians occurred from high water neap (H. W. N. ) to extreme low water spring (E. L. W. S. ).

I n i t i a l observations i n d i c a t e d that the behav­iour patterns v a r i e d w i t h h a b i t a t . Homing behaviour was a property of C. ornata though i t was not always shown by C- ornata occupying e i t h e r permanently wet c r e v i c e s , or the undersides of rock overhangs. To standardise c o n d i t i o n s only specimens on seaward slo p i n g rock faces of 30° - 70° from the h o r i z o n t a l were marked and observed.

Marking methods: The dry s h e l l and the rock surface were each marked with the same number (Roman numerals) and a dot marked on the rock at the

^Department of Zoology, University of Auckland.

1*1*

F ig 1 LOCALITY

Scale of miles

0 5 10 15 20 25

•45

a n t e r i o r edge of the s h e l l . A durable marking mater­i a l was d i f f i c u l t to f i n d , but Gala "More Red" No. 6 n a i l p o l i s h was found to remain on the rocks f o r up to 60 days.

Observations on marked and unmarked specimens showed no apparent i l l e f f e c t s on limpets marked i n the above procedure.

INVESTIGATION OF HOMING BEHAVIOUR

a) The Presence of Homes on the Shore: A home was taken to mean the s p e c i f i c area to which a limpet r e g u l a r l y returned.

To determine whether homes e x i s t e d on the shore at Takapuna, nineteen specimens were marked, and t h e i r subsequent p o s i t i o n s recorded at successive low t i d e s . As the r e s u l t s were intended to encompass the average limpet p o p u l a t i o n , n e i t h e r the highest nor the lowest specimens were ever marked.

S i x groups of limpets were marked.

Group 1. Three C. ovnata from E. H. W. S. i n the Chamaesipho ornata barnacle zone. (One l a r g e , 27 - 33 m. m. long; two medium, 20 - 26 m. m. long)

Group 2. Four C. ornata from H. W. N, on bare rock sur­faces. ( A l l medium s i z e ) .

Group 3. Three C. ornata from M. T. L. (Crassostrea glomerata zone). (One l a r g e , one medium and one s m a l l , 11 m. m. long ) .

Group k. Four C. radians from H. W. N. ( A l l medium s i z e , 28 - 32 m. m. l o n g ) .

Group 5- Two C. radians from M. T. L. (Both medium s i z e ) .

Group 6. Three C. radians from L. W. N. (Two medium, one s m a l l , 15 m. m. l o n g ) .

These s i x groups were observed at i n t e r m i t t e n t

1+6

low t i d e s during the day and night f o r periods of nine days (Groups 1+, 5 and 6) to three months (Groups 1, 2 and 3 ) .

The r e s u l t s may be summarised as f o l l o w s : i ) C. ornata over 12 m. m. long were each found to

occupy a confined area on the rock face t o which they r e g u l a r l y returned when the t i d e receded. This area i s the animal's home. For each i n d i v i d u a l t h i s c o n s t i t u t e s the home.

i i ) C. ornata smaller than approximately 12 m. m. long d i d not possess a home.

i i i ) No specimens of C. radians possessed a home.

One day v i s i t s were made t o Whatipu, P i h a and t o Motutapu, Motuihe and Ponui I s l a n d s , each v i s i t c o i n ­c i d i n g w i t h a low t i d e p e r i o d .

Specimens of both species were marked at Goat I s l a n d , St. Leonard's Beach and Cheltenham, and observed over a number of v i s i t s t o determine i f homes e x i s t e d on these shores.

These r e s u l t s support the observations made at Takapuna, i . e. Adult C, ornata possess a home. Adult C. radians do not.

Discussio n on the s t r u c t u r e of the home i n C. ornata.

Several authors (Stephenson, 1935, P- granularis, Thorpe 1956, P. depressa and F r e t t e r and Graham 1962, P. vulgata) mention t h a t c e r t a i n limpets possess a home, and th a t t h i s may be v i s i b l e as a scar on the rock surface. A scar i m p l i e s e i t h e r a depression eroded i n the rock, presumably by the s h e l l margins and/or the f o o t , or a s e c r e t i o n exuded onto the rock surface t o d i s t i n g u i s h i t from the surroundings.

No tru e limpet scars were seen at Takapuna, nor was any s e c r e t i o n or d i s c o l o r a t i o n of the rock sur­faces noted where a home e x i s t e d . (Sypharochiton pelliserpentis possessed a home at Takapuna which

hi

was i n the form of a scar with a w h i t i s h s e c r e t i o n at i t s base. This type of home was observed at Canterbury by Johns ( i 9 6 0 ) ) .

At Takapuna i n dense areas of C. columna, C. ornata homes were r e a d i l y recognisable when the animals moved. They appeared as ov a l bare rock patches i n the midst of the surrounding barnacles corresponding i n s i z e to the animals' s h e l l s .

C. ornata a l s o occurs on bare areas of b a s a l t at Takapuna. Here they s t i l l returned t o a home, although the home i s i n d i s t i n g u i s h a b l e from the surrounding rock surfaces.

At St. Leonard's beach, where the rock i s Waitemata sandstone, d e f i n i t e depressions i n the rock surface were noted, and tr u e scars were t h e r e f o r e present. The P a r n e l l g r i t north of Cheltenham beach, and some of the s o f t e r West Coast rocks a l s o showed some s c a r s , but these were g e n e r a l l y not w e l l d e v e l ­oped.

Whether a scar i s present or not i s th e r e f o r e e n t i r e l y dependent on the type of rock on which the limpet has i t s home. The absence of a recognisable scar does not appear to a f f e c t the homing a b i l i t y of C. ornata.

At Takapuna C. ornata had a more permanent home than d i d specimens of P. vulgata observed by Orton (1929). Orton noted that scars were o f t e n abandoned by the animals that formed them, and th a t there was u s u a l l y some movement of the limpet p o p u l a t i o n from one place t o another, e s p e c i a l l y on smooth damp rocks.

At Takapuna eighteen C. ornata were marked at low t i d e on l H . 3. 67. By the 20. 7. 67, s i x t e e n were s t i l l on t h e i r o r i g i n a l homes - the two that had moved were both small ( l l m. m. l o n g ) , and both had moved up the shore.

b) The Importance of the Home t o C. ornata.

HQ

A l l a dult C. ornata observed i n the areas studied possessed a home, i n d i c a t i n g that homing i s an i n t e ­g r a l part of t h i s limpet's behaviour.

Over a p e r i o d of four months at Takapuna, s i x t e e n C. ornata continued t o r e t u r n to t h e i r i n d i v i d u a l homes. I f limpets are removed from the rock surface they may s u c c e s s f u l l y e s t a b l i s h a new home, implying that the behaviour patterns are f l e x i b l e .

Homing ensures that the limpets remain evenly d i s t r i b u t e d over the shore, r a t h e r than densely aggregated as i n the high t i d a l s n a i l Nerita melanotragus.

The home, and scar i f present, has been assumed to be e f f e c t i v e i n reducing water l o s s through d e s i c ­c a t i o n . Observations and experiments showed t h a t C. ornata does not d i e i f kept o f f i t s home, nor does C. ornata minimize water l o s s by e x a c t l y f i t t i n g i n t o i t s home, as has been c l a s s i c a l l y described i n the European P. vulgata.

The home probably represents a p o s i t i o n on the shore where the limpet i s most secure from i n t e r ­s p e c i f i c competition ( f o r space), predation and d e s i c ­c a t i o n .

c) General Movements: L i t t l e exact data could be found i n the l i t e r a ­

t ure as to the extent of limpet movements. Stephenson (1935) i s reported t o have found that P a t e l l a granularis t r a v e l s up to f i v e f e e t and th a t "accuracy of performance" (to return) " f a l l s o f f w i t h d i s t a n c e . " F r e t t e r and Graham (1962) report that "the whole journey w i l l cover two f e e t or so, sometimes f o l l o w i n g a zigzag path. "

Stephenson (1935) found that limpets f r e q u e n t l y returned t o t h e i r homes by a d i f f e r e n t path to that taken on the outward journey. F r e t t e r and Graham (1962) noted that limpets o f t e n but not i n v a r i a b l y returned along t h e i r outward path.

h9

Presumably these movements were noted when the animals were submerged, but no mention was ever made of t h i s v i t a l f a c t .

P r e l i m i n a r y observations on C. ornata and C. radians at Takapuna i n d i c a t e d that there are a var­i e t y of types of movements, and th a t the extent of these movements was g e n e r a l l y much l e s s than f i v e f e e t .

The d i f f e r e n t types of movements were v e r i f i e d by t i m i n g the animals (with a stop watch) over a f i v e centimetre distance.

The four types of movements observed were: i ) Normal submerged feeding movements. Both

species show a slow (C. radians 0. 5-2. 5 cm. / min. ) (C. ornata 0. 2-1. 5 cm. /min. ) type of movement i n which the head sways from side t o side and the ra d u l a i s presumably scouring the substrate f o r food.

i i ) Feeding on damp surfaces at night when the t i d e i s out. Both species show a much slower speed of movement than when submerged.

i i i ) Returning movements. Only observed i n submer­ged C. ornata r e t u r n i n g t o t h e i r homes. A much f a s t e r (1. 5-3. 0 cm/min. ) and more d i r e c t movement.

iv ) Submerged "avoidance" movements. Only observed with two submerged C. radians. On 19. -+. 67 two C. radians were seen to be approached by two Haustrum haustorium, each of these t h a i d whelks pouncing on one limpet. The limpets both sped away at 8. 0-10. 0 cm/min. (A s i m i l a r r e a c t i o n could not be a r t i f i c i a l l y produced using a thumb and "pouncing" on C. radians).

During the i n v e s t i g a t i o n s i n t o the presence of homes on the shore the s i x C. ornata i n Groups 1 and 3 were observed at i n t e r m i t t e n t low t i d e s during the day and night over a p e r i o d of two months (March-

50

A p r i l 1967).

The r e s u l t s of these observations showed that i n general C. ornata were on t h e i r homes i f low t i d e occurred during the day, whereas they o c c a s i o n a l l y moved o f f t h e i r homes i f low t i d e occurred at n i g h t .

To i n v e s t i g a t e the extent and d i r e c t i o n of a c t u a l movements, three groups of limpets were marked on the shore.

Group A: Ten C. ornata from E. H. W. S. Group B: Eight C. ornata from M. T. L. {Crassostrea glomerata l e v e l ) . Group C: Ten C. radians from M. T. L.

When the t i d e was out observations on the pres­ence of homes on the shore revealed that C. radians on and below the C o r a l l i n a " t u r f " l e v e l d i d not a c t u a l l y remain s t a t i o n a r y . Because observations on movements were u s u a l l y done underwater, i t was only p o s s i b l e t o study upper shore animals a c c u r a t e l y .

A l l members of each group were i n a l i n e down the shore, on which slope and aspect were equivalent. Each limpet was marked and i t s movements from the i n i t i a l p o s i t i o n recorded at t h i r t y to s i x t y minute i n t e r v a l s .

Observations of movements i n these three groups were done on a b r i g h t day ( t i d e i n) 19-3. 67, d u l l day ( t i d e i n ) 19- 1+-67, night ( t i d e i n) 9. ^. 67 and night ( t i d e out) 2U. U. 67.

The r e s u l t s of these observations are shown i n Table 1, where the average maximum distances t r a v e l l ­ed by each group away from the home s i t e or marking p o s i t i o n are recorded.

In a d d i t i o n t o the preceding observations the p o s i t i o n of f i v e specimens was checked every f i f t e e n to t h i r t y minutes under a l l environmental c o n d i t i o n s .

I t was p o s s i b l e t o p l o t on graph paper the

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a c t u a l path followed by each limpet. A sample p l o t i s given i n Figure 2. In the Figure the home (or i n i t i a l p o s i t i o n i n C. radians) of each animal i s represented by an ov a l . Movements above the home on the shore are represented by dotted tracks above the o v a l s , and movements to the r i g h t , l e f t or below are similarly shown.

Group Bright Day 19. 3. 67

(tide In)

Dull Day 19. 4. 67

(tide In)

Night 9. 4. 67

(tide in)

Night 24. 4. 67 (tide out)

Average max. distance (cm) moved from marking post.

A B C

16. 3 16. 1 40. 6

8. 0 11. 3 43. 4

17. 9 13. S 40. 8

10. 0 27. 8 32. 6

TABLE 1. THE EXTENT OF LIMPET MOVEMENTS

These observations on general movements may be summar­i s e d as f o l l o w s : i ) Both species moved whenever t o t a l l y submerged,

t h i s movement being i n v a r i a b l y above the home {C. ornata) or i n i t i a l p o s i t i o n (C. radians).

i i ) Both species moved on moist rock surfaces at n i g h t , the d i r e c t i o n of t h i s movement oft e n being below the home i n C. ornata.

i i i ) Neither species was ever observed moving on open rock surfaces during the day.

i v ) The speed of movements: C. radians moves at almost double the speed of C. ornata when both are feeding.

v) The extent of movements: C. radians t r a v e l l e d f u r t h e r than C. ornata from the marking p o s i ­t i o n .

v i ) The d i r e c t i o n of movements: Both species tended to show o r i e n t a t e d movements i n a set pat t e r n on outward and r e t u r n journeys r a t h e r than random wanderings.

v i i ) C. ornata were u s u a l l y on t h e i r homes every time the t i d e receded, unless moist c o n d i t i o n s

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F i g 2 L IMPET TRACKS

C. ornata C. radians

• \ V i 50,

40 1

Tide in / Tide out', /

i T

i

Tide in Tide out

30

20

10

0 CMS

10 20 30

CMS

40 50

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occurred at night. v i i i ) C. ornata d i d not n e c e s s a r i l y f o l l o w the same

paths on t h e i r journeys t o and from t h e i r homes. The higher l e v e l specimens seemed l e s s l i k e l y to r e t u r n by the same path than lower l e v e l specimens.

i x ) C. radians returned towards t h e i r i n i t i a l p o s i ­t i o n , as the t i d e ebbed, becoming s t a t i o n a r y when the rocks d r i e d .

d) Discussion on the Observed Behaviour P a t t e r n s , and some of the Experimental work undertaken. The stimulus i n i t i a t i n g submerged movements i n

both species was wetting by the incoming t i d e . C. ornata high up the shore showed a f a s t e r response t o w e t t i n g , than C ornata lower down the shore. This behavioural d i f f e r e n c e could be p r e d i c t e d , f o r the 'higher l e v e l specimens have r e l a t i v e l y l e s s feeding time a v a i l a b l e .

Movements during submersion (Tide i n ) . The subsequent submerged movements were i n v a r i ­

ably above the home or the i n i t i a l p o s i t i o n , i n both species. The d i r e c t i o n a l movements observed may be i n t e r p r e t e d as avoidance of d e s i c c a t i o n , i . e. f o l l o w ­in g the water up the shore, and r e t r e a t i n g down the shore as the t i d e ebbs.

The extent of these submerged movements was i n general much l e s s than the three t o f i v e f e e t obser­ved by previous workers. The distances C. ornata moved were r a r e l y l a r g e r than 30 c, m. s., while C. radians tended to move double t h i s distance (Table l ) .

The higher l e v e l C. ornata, by moving as soon as they were submerged, and not r e t u r n i n g t o t h e i r homes t i l l the t i d e receded, managed to feed f o r s i m i l a r periods of time to C. ornata lower down the shore.

Measurements taken d i d not i n d i c a t e that h i g h l e v e l C. ornata moved s i g n i f i c a n t l y f a s t e r than d i d

lower l e v e l specimens. Higher l e v e l C. ornata fed over new ground continuously, as they returned t o t h e i r homes by a path d i f f e r e n t to that taken on the outward journey.

The t e r m i n a t i o n of these submerged movements i n C. ornata occurred when the t i d e receded past the l e v e l of the home. High l e v e l specimens were o f t e n l a t e r r e t u r n i n g to t h e i r homes ( i n r e l a t i o n t o the t i d e l e v e l ) than those lower down the shore. C. radians r e t r e a t e d down the rock face as the t i d e ebbed, coming to r e s t only when the rock face d r i e d .

As noted p r e v i o u s l y C. ornata showed a r e t u r n i n g movement, which was i n i t i a t e d by the animal being exposed to the a i r f o r s e v e r a l seconds - o f t e n as a wave receded. The limpet i n v a r i a b l y returned d i r e c t ­l y t o i t s home.

Movements on damp rock surfaces (Tide o u t ) . Only r a r e l y d i d e i t h e r C. ornata or C. radians

above M. T. L. {Crassostrea glomerata l e v e l ) move on open rock faces i n daytime during low t i d e . C. rad­ians occupying permanent high l e v e l rock pools a l s o stopped moving when the t i d e was out during the day.

Stephenson (1935) and Yonge (l9*+9) r e f e r to limpets moving over rock surfaces on damp days, but no such movements were ever observed at Takapuna.

To determine whether movements could be induced by pouring s a l t water over C. ornata, f i v e g a l l o n s were slowly poured over f i v e members of Group B on a b r i g h t day, d u l l day and at night. The remaining three members of the group were c o n t r o l s . Movement o f f the home could only be induced at n i g h t , i n contrast to the r e s u l t s of Arnold (1957) who found that P. vulgata moved o f f t h e i r homes during low t i d e i n the day i f splashed w i t h s a l t water.

C. ornata and C. radians were found t o move over damp rock surfaces at night.

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The i n i t i a t i o n of these movements depended on the amount of moisture present on the rocks. I f the rocks were dry no movement took p l a c e , however i f r a i n f e l l on these dry rocks limpets f r e q u e n t l y l e f t t h e i r homes on feeding excursions.

In C. ornata the d i r e c t i o n of movements on damp rocks at night was often below t h e i r homes. (Figure 2). The advantages of moving below t h e i r homes at night are the p o s s i b i l i t y of grazing over new ground, and the le s s e n i n g of d e s i c c a t i o n f u r t h e r down the shore.

As i n t h e i r submerged movements high l e v e l C. ornata showed a greater tendency to r e t u r n t o t h e i r homes by a path d i f f e r e n t from that taken on the outward journey.

The extent of movements on damp rock surfaces at night was much l e s s than submerged movements.

The stimulus t e r m i n a t i n g such movements was un c e r t a i n . I f daybreak, (and the consequent increase i n drying c o n d i t i o n s ) occurred before the next high t i d e , the C. ornata were a l l r e s i t e d on t h e i r homes by s u n r i s e . In one instance s i x C. ornata were observed o f f t h e i r homes at 03-+5, yet by Okh^ a l l were r e s i t e d on t h e i r homes although no l i g h t i n t e n ­s i t y , humidity or temperature d i f f e r e n c e s were detected between the two observation times.

I f the incoming t i d e covered the C. ornata b e f o r e , or j u s t a f t e r daybreak, the limpets occasion­a l l y remained o f f t h e i r homes, and the t i d e covered them before excessive drying c o n d i t i o n s p r e v a i l e d . C. ornata at the highest l e v e l s on the shore o f t e n returned t o t h e i r homes at daybreak, regardless of the s t a t e of the t i d e .

e) Observed Movements i n r e l a t i o n to Homing:

Location of the home occurred i n C. ornata from distances up to 30 cms. both when submerged, and when

56

exposed to the a i r . C. radians always returned t o ­wards i t s i n i t i a l p o s i t i o n .

Recognition of the home i n C. ornata.

C. ornata i n v a r i a b l y returned to t h e i r own homes. Their o r i e n t a t i o n was to t h e i r own p a r t i c u l a r home, f o r they o f t e n moved over other limpet homes on t h e i r r e t u r n t r i p s .

Behaviour on reaching the home. Thorpe (1956) describes a P. vulgata r e t u r n i n g

to i t s home from 20 cms. Thorpe s t a t e s , " i t appeared completely o r i e n t a t e d throughout, showing apprecia­t i o n of the topography of the environment, and was not dependent on a s i n g l e guiding stimulus. " Shuff­l i n g movements have a l s o been observed by various authors (Yonge 19^9) and imply the limpet attempting to f i t i n t o a scar on the rock surface.

None of the 72 C. ornata observed r e t u r n i n g to t h e i r homes ever showed such a movement. A l l s e t t l e d down on t h e i r homes very promptly.

O r i e n t a t i o n on the Home.

C. ornata had no constant alignment on i t s home, fr e q u e n t l y being found l80° out of alignment. This l a c k of alignment undermines the idea that limpets always f i t e x a c t l y i n t o a scar on the shore.

In C. ornata the method of o r i e n t a t i o n d i d not invo l v e a t r i a l and e r r o r b a s i s of f i t t i n g the s h e l l i n t o the surroundings. Nineteen C. ornata had the outer edge of t h e i r s h e l l s c a r e f u l l y chipped away w i t h a small s c r e w d r i v e r , y et a l l continued t o r e t u r n to t h e i r own homes ac c u r a t e l y .

Cases of limpets r e t u r n i n g to the wrong homes.

Only once was a C. ornata observed r e t u r n i n g t o a neighbour's home. When the r e s i d e n t limpet r e t u r n ­ed i t attempted to move the i n t r u d e r by pushing i t f o r approximately twenty minutes. (Pushing has a l s o

57

been observed by Stephenson (1935) w i t h P. gvanulavis\ When pushing proved unsuccessful and the rocks d r i e d , the r e s i d e n t took up a p o s i t i o n alongside the i n t r u d ­ing neighbour, corresponding t o the distance between t h e i r two homes.

On being covered by the next high t i d e the in t r u d e r moved o f f on a feeding e x c u r s i o n , while the res i d e n t limpet returned immediately to i t s own home and d i d not feed over t h i s p eriod. As the t i d e receded the i n t r u d e r returned to i t s c o r r e c t home without going near i t s neighbour.

S i m i l a r r e a c t i o n s were a r t i f i c i a l l y produced by p l a c i n g " f o r e i g n " C. ornata on " r e s i d e n t " l i m p e t s ' homes.

A l t e r i n g the Shape of the Home • Homes i n the C columna zone were e a s i l y en­

larged using a wire brush. This a c t i o n d i d not pre­vent the limpet r e c o g n i s i n g the p o s i t i o n of the home.

The removal of the home, using a c o l d c h i s e l while the animal was o f f on a feeding e x c u r s i o n , c u l ­minated i n the animal r e t u r n i n g t o the general area where i t s home was, and searching f o r s e v e r a l minutes before s e t t l i n g down i n the near v i c i n i t y .

f ) The Accomplishment of Homing i n C. ornata:

A b r i e f review of the distance receptors a v a i l a b l e to an archeogastropod limpet such as Cell-ana i s necessary t o determine how homing could be accomplished.

F r e t t e r and Graham (1962) consider that i n P a t e l l a the body i s wholly r e c e p t i v e to o v e r a l l chemical s t i m u l a t i o n and contact, w i t h the eyes, osphradia, t e n t a c l e s and s t a t o c y s t s being the most important s p e c i a l receptors.

Eyes: S i t u a t e d at the base of the ceph a l i c t e n t a c l e s , these are of the simplest nature known i n

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gastropods, comprising an open v e s i c l e w i t h n e i t h e r lens nor cornea. Such simple organs could he used f o r the d e t e c t i o n of l i g h t , and as a guide to i t s d i r e c t i o n .

Tentacles: Both ce p h a l i c and e p i p o d i a l t e n t a c ­l e s , are almost c e r t a i n l y of importance i n a t a c t i l e sense, being i n constant motion while the animal i s moving. They are a l s o p o s s i b l y o l f a c t o r y receptors. The presence of a t r u e osphradium i n P a t e l l a i s debated (Spengel l 8 8 l , Bernard 1890), but may be regarded as w e l l e s t a b l i s h e d ( F r e t t e r and Graham, 1962).

A l a r g e number of f a c t o r s i n f l u e n c i n g limpet (and other gastropod) homing can be suggested. These may be broadly subdivided: i ) S t i m u l i which could a f f e c t homing. i i ) Methods of n a v i g a t i o n .

i ) S t i m u l i which could a f f e c t homing: S t i m u l i such as g r a v i t y , barometric pressure,

temperature, s a l i n i t y , water pressure, v i b r a t i o n caused by water movement and b i o l o g i c a l clocks were r e j e c t e d as having no d i r e c t e f f e c t on homing i n C. ornata.

Humidity i n the present study was measured with a hygrometer placed on the rock surface. Although t h i s gives a good i n d i c a t i o n , i t does not a c c u r a t e l y measure the water present on the rock. The values obtained f l u c t u a t e d considerably from place t o place and day to day.

The moisture of the rock surface appeared t o determine whether movements occurred at night w i t h the t i d e out.

i i ) Methods of Navigation:

L i g h t : Newell (1953) has shown that i n Littorina littorea the eyes serve as a l i g h t compass, and the animal o r i e n t a t e s t o the l i g h t d i r e c t i o n .

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C. ornata homed i n the day or n i g h t , and t h i s homing a b i l i t y was not a l t e r e d over the night of the 2U/25. -4. 67 when a t o t a l lunar e c l i p s e occurred at 0200.

Chemoreception: Morgan (189-+), Pieron (1909) and Hewatt (19-+0), have suggested that limpets retrace t h e i r outward t r a c k s when r e t u r n i n g t o t h e i r homes. Davis ( l895) considered that the sense data used were probably o l f a c t o r y , and were appreciated by the marginal p a l l i a l t e n t a c l e s .

C. ornata u s u a l l y returned t o i t s home by a d i f f e r e n t path from that taken on the outward journey, so i t appears that homing was not achieved by r e t r a c ­i n g a t r a i l , as by response to chemical substances p r e v i o u s l y l a i d down.

On two occasions the rock surface between a C. ornata and i t s home was v i g o r o u s l y scrubbed w i t h a wire brush. The animals i n v a r i a b l y continued to home accur a t e l y .

The p o s s i b i l i t y t hat the home was l i b e r a t i n g some chemical substance t o which each animal homed was a l s o i n v e s t i g a t e d . The homes of seven C. ornata were scrubbed, using detergent and a wire brush, while the re s i d e n t limpets were on feeding excursions. In a l l cases the limpets' homing a b i l i t y was unaffected.

The Presence of Adjacent animals, and zones of feeding: C. ornata at Takapuna f r e q u e n t l y e x i s t i n groups of three to s i x , and a l l the i n d i v i d u a l s make far-ranging excursions over the others' homes when feeding. Experiments were conducted i n v o l v i n g the removal of a l l but one animal from four such groups. Five days l a t e r the four remaining limpets were s t i l l homing acc u r a t e l y .

This would dispose of the p o s s i b i l i t y considered that each C. ornata might be able t o appreciate the density of the food source surrounding i t s home,

6o

g r a z i n g away from i t s home t i l l i t reached areas where the food became scarcer (another limpet's t e r r i t o r y ) . On encountering l e s s food i t would r e t u r n t o i t s own area.

R e o r i e n t a t i n g the home •

Hewatt (19**0) found that i n the l a b o r a t o r y limpets could home to a scar r e o r i e n t a t e d i n p o s i t i o n .

Experiments at Takapuna on r e o r i e n t a t i n g the homes of C. ornata while the animals were absent were i n c o n c l u s i v e . A l l animals managed t o r e t u r n i f the home was moved through 180° i n the same p o s i t i o n , but i f i t was e l e v a t e d , or moved away from i t s o r i ­g i n a l p o s i t i o n many animals f a i l e d to f i n d i t .

On the 19'h. 6l three C. ornata were moved (on t h e i r home) on a piece of b a s a l t t o a p o s i t i o n 10 metres away at a s i m i l a r l e v e l on the shore. On being covered by the t i d e one moved o f f the rock on to "new" ground, and never returned, e v e n t u a l l y e s t a b l i s h i n g a home 27 cms. away on an adjacent rock face. The other two stayed on t h e i r homes f o r the f i r s t t i d e . On the next t i d e both these limpets r e s t r i c t e d any movements to the piece of b a s a l t t h e i r homes were on; but by 25. 1+-67 both were ranging over the "new" t e r r i t o r y and r e t u r n i n g t o t h e i r homes.

Removal experiments-

Several workers, Morgan ( I 8 9 M , Pieron (1898) and Stephenson (1935) have conducted removal e x p e r i ­ments, moving the limpets from the rock surface t o a d i f f e r e n t p o s i t i o n , and determining the number of limpets which returned t o t h e i r homes.

Twenty submerged C. ornata were q u i c k l y removed from the rock surfaces when o f f t h e i r homes, and replaced on the same rock not more than 20 cms. above t h e i r homes. The animals tended to move i n the d i r ­e c t i o n they were f a c i n g when they reattached to the rock. I t was observed that many C. ornata d i d not

6 i

readhere to the submerged rock surface a f t e r removal, and these limpets were c a r r i e d out to sea by a reced­ing wave. Observation of these animals i n d i c a t e d that they d i d not r e a t t a c h to the s u b s t r a t e , and they pre­sumably died.

I t i s probably best to consider removal as being a h i g h l y unnatural phenomenon d i s r u p t i v e t o limpet o r i e n t a t i o n , and almost c e r t a i n l y culminating i n the death of the animal i n i t s n a t u r a l environment.

DESICCATION EXPERIMENTS

Two d e s i c c a t i o n experiments were conducted using C. ornata and C. radians t o determine i f there was any p h y s i o l o g i c a l d i f f e r e n c e between or w i t h i n each species.

a) D i f f e r e n t s i z e d specimens c o l l e c t e d from the same l e v e l and placed upside down i n dry a i r showed that smaller specimens of both species l o s t weight more r a p i d l y than large specimens. A number of small C. radians l o s t approximately 50% of t h e i r t o t a l weight over a twenty-two hour p e r i o d . A l l recovered on being placed i n s a l t water.

The r e s u l t s confirm observations on the shore -that no small limpets ever occur on open dry rock face s , though they feed over these areas at high t i d e . As the t i d e ebbs the smaller specimens i n v a r i a b l y r e t i r e to a c r e v i c e . No C. ornata under 11 m. m. long were ever seen to possess a home.

b) Twenty s i m i l a r s i z e d (22-26 m. m. long) limpets of both species were c o l l e c t e d from four l e v e l s on the shore. A l l were exposed t o d e s i c c a t i o n i n the a i r on glass f o r twenty-four hours, w i t h hourly weighings over the f i r s t ten hours.

There were no s i g n i f i c a n t d i f f e r e n c e s i n percen­tage weight l o s t w i t h i n or between species from d i f f ­erent l e v e l s on the shore.

SUMMARY

( i ) Some i n d i v i d u a l s of C. ornata possess a home, to which they r e g u l a r l y r e t u r n .

( i i ) C. radians was never observed to possess a heme. ( i i i ) The r e s u l t s obtained suggested that C. ornata

d i d not use l i g h t , chemoreception or topograph­i c a l memory as primary homing methods.

( i v ) No p h y s i o l o g i c a l d i f f e r e n c e s between or w i t h i n C. ornata and C. radians over t h e i r t i d a l range were detected from a d e s i c c a t i o n e x p e r i ­ment i n the l a b o r a t o r y .

The numerical r e s u l t s are a v a i l a b l e from a Zoology IIIB p r o j e c t "Limpet Movements" i n the B i o l o ­g i c a l Sciences L i b r a r y , U n i v e r s i t y of Auckland.

ACKNOWLEDGMENTS

Thanks are due t o Dr. P. R. Bergquist f o r super­v i s i n g the study; and t o Professor J . E. Morton f o r c r i t i c a l l y reading the s c r i p t .

REFERENCES

Arno l d , D. C. 1957. The Response of the Limpet Pat­e l l a vulgata to waters of d i f f e r e n t s a l i n i t i e s . J. mar. Biol. Ass. U. K. Z6: 121-129-

Bernard 1890 from F r e t t e r and Graham (1962) British Prosobranch Molluscs. Ray S o c i e t y , London.

Fra e n k e l , G. S. and Gunn, D. L. 19*+0 O r i e n t a t i o n of Animals. Dover Press. London.

F r e t t e r , Vera and Graham, A. 1962 British Prosobranch Molluscs. Ray S o c i e t y , London.

Hewatt, W. G. 19^0 Observations on the homing Limpet Acmaea scabra. Am. Mid. Nat. 24: 205-8.

•Johns, P. M. i 9 6 0 Chiton p e l l i s e r p e n t i s . M. Sc. t h e s i s . U n i v e r s i t y of Canterbury.

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Morgan, C. L. 189-+ Introduction to comparative Psy­chology. London.

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Newell, G. E. 1958 Papers on littorina. J. mar. Biol. Assos. U. K. 37, 229-239.

Orton, J . H. 1929 Observations on P a t e l l a vulgata. PT. I I I Habitat and Habits. J. mar. Biol. Ass. U. K. 14, 277-289.

P i e r o n , H. 1909 from Thorpe 1956. Learning and instinct in animals. Methuen. London.

Southward, J . A. 1965 Life on the Sea Shore. Heine-mann. Educational Books Ltd. London.

Stephenson, T. A. 1935 from thorpe 1956. Learning and instinct in animals. Methuen. London.

Spengel, l 8 8 l from F r e t t e r and Graham 1962. British Prosobranch Molluscs. Roy. Soc. London.

Thorpe, W. H. 1956. Learning and instinct in animals. Methuen. London.

We l l s , M. H. 1956 Learning in Marine Invertebrates. Advances i n Marine Biology. V o l . 3.

Yonge, CM. 19-+9 The Sea Shore C o l l i n s 1 New Natur­a l i s t . London.