Tane (1968) lh: 25-k2 25

THE FEEDING BEHAVIOUR OF ROSTANGA RUBICUNDA (MOLLUSCA, NUDIBRANCHIA).

By A. M. Ayling*

INTRODUCTION

Most members of the order Nudibranchia are spec• ialised carnivores feeding on sessile and encrusting animals such as hydroids, polyzoans, Porifera, ascldians and alcyonarians. All have different means of feeding eg. scraping, tearing or sucking and differ• ent modifications especially in the buccal mass. The least specialised of these grazing carnivores are the members of the Doridacea which feed on sponges. Most are brightly coloured either for camouflage when on the food sponge or to serve as a warning for predat• ors. The bright red Rostanga rufescens of Britain feeds on the encrusting red sponge Microciona in the order Poecilosclerida (Morton) and the very similar R. pulchra (MacFarland) of North America feeds on a sponge of the same order Ophlitaspongia penata (Cook 1962). The New Zealand species R. rubicunda (Cheeseman) occurs commonly on Westmere reef, Auckland, in association with three very similar sponges.

Microciona coccinea Holoplocamium neozelanicum Ophlitaspongia seriata

The feeding of R. rubicunda in relation to these three sponges was investigated using a number of techniques.

The food of carnivores and scavengers is frequ• ently local and specific and thus chemoreception from a distance is undoubtedly important in feeding behav• iour. R. pulchra is attracted to Ophlitaspongia pennata by chemotaxis (Cook 1962) and it was thought that the same was probably true of the relationship between R. rubicunda and one or more of the above ^Department of Zoology, University of Auckland. 26 mentioned sponges. The small size of this dorid and its ease of collection make it possible to attempt the type of feeding behaviour experiments used by Stehouwer (1952), Braams and Geelen (1953) and Cook (1962) to determine its food preferences and other aspects of its feeding behaviour. This method was substantiated by making spicule mounts of gut contents and faeces to determine the sponges eaten and also by observing the animal in the field.

FIG. 1. ROSTANGA RUBICUNDA.

EXTERNAL CHARACTERS

Rostanga rubicunda (Cheeseman l88l) is a small bright scarlet nudibranch that seems to be very close to the British R. rufescens and the North American R. pulchra, MacFarland. The mantle is covered with minute, closely packed, erect tubercules and the foot 27 extends a short distance posteriorly to this. The rhinophores are completely retractable and have 12 broad laminae which run obliquely upwards. The apex is a small projecting flat topped style. The rhino• phores of nudibranchs were developed following the loss of ctenidia and the osphradium as a replacement sensory area and are clubbed and finely plicate to increase the sensory epithelium. They contain relat• ively large lobed ganglia associated with the recep• tors. In Rostanga the rhinophores seem to be used for detecting the presence of the food sponge at a distance ie. by chemotaxis (Cook 1962) although Augersberg (1922) after experiments with food extrac• ts concluded that the rhinophores were not involved with chemoreception at a distance. However, placed as they are in an anterior, elevated position it would be fair to suggest that the function of the rhinophores is similar to that of the osphradium they replace ie. to test the water flowing over them for contained chemicals, either favourable (food) or un• favourable. The gills (branchiae) are also retract• able, eight in number, erect and bipinnate.

ECOLOGY

Rostanga rubicunda can be found occasionally in the sublittoral fringe on most protected rocky shores in the North Auckland area, eg. Eastern beach, Waiwera, Takapuna, Bonaccord harbour, Leigh. It is, however, limited in numbers in most of these areas by the scarcity of its food. But on Westmere reef which straddles the main tidal stream of Auckland Harbour the detritus rich waters which surge twice daily across it provide ample nourishment for a prolific growth of sponges. The three food sponges of Rostan• ga rubicunda occur quite commonly amongst these and this slug occurs in considerable numbers at this lo• cality. In this area the slugs are not found as part of the under-stone fauna but prefer more open situa• tions even though their food sponges are often pres• ent under stones. Approximately 50% of the Rostanga population were found feeding on the red food sponges. The sponges eaten by 20 of these feeding slugs were 28 identified using spicule mounts. II were feeding on Ophlitaspongia 7 " " " Microciona 2 " " " Holoplocamium

Rostanga was found in considerable numbers over the entire period during which they were examined, ie. from early March to September. Powell (1957) states that it also occurs commonly over the months September to November. Thus it is probable that this animal may be found throughout the year with equal abundance. The smallest slugs collected were about 10 mms. in length and the largest 33 mms. with an average of 15-20 mms.

FEEDING BEHAVIOUR

Materials and Methods

Rostanga rubicunda3 as has been pointed out, is always found where its food sponges are present and as sponges occur in regions where there is consider• able water movement it should be possible for the animal to trace its food by detecting secretions from the sponge and following them through the current to their source, ie. by chemotaxis. As Rostanga feeds on at least three sponges the response to each sponge would be expected to increase with the slugs' prefer• ence for that sponge as food. To test this an arti• ficial current flow was set up in the laboratory and arranged so that the slugs could be given a choice between two sponges. To achieve this seawater was run into two bowls in which the various sponges could be placed and then siphoned out into a single shallow tray in which the slugs to be tested were placed. The water was allowed to overflow from the tray and run to waste as it could not be recirculated without mixing the sponge "odours". The apparatus use is shown in Fig. 6, and diagramatically in Fig. 2.

(i) Using this apparatus five fairly similar sponges from Westmere were tested against blank controls and against each other to find out which were eaten by Rostanga and the order of preference 29

MCOMINO 5CAWATER

SLUG TRAY

'lilt

OVF»FLOW

* INFLOW

FIG. 2. CURRENT FLOW APPARATUS. amongst these.

These sponges were: - Ophlitaspongia seriata Holoplocamium neozelanicum Microciona coccinea Hymeniacidon perleve Suberites cupuloides

Other aspects of Rostanga's feeding behaviour were tested using this apparatus, viz. (ii) Gregariousness - a few of the slugs themselves were placed in one of the sponge howls to see the slugs in the tray were affected at all. 30

(iii) The "fouling" effects of other encrusting org• anisms and algae found associated with the sponges on the reaction of the slugs was inves• tigated to see if the presence of secretions from other organisms masked those from the sponge and confused the slugs' sensory mechan• isms. (iv) The effects of varying the current speed were tested by regulating the siphoning height be• tween the bowls and the tray. (v) The effects of light or dark On the slugs' reactions were tested to see if feeding activ• ity was greater during the day than at night or vice versa. (vi) The minimum amount of sponge needed to give a definite reaction was investigated.

To see if the slugs could detect the sponges without water movement another apparatus was set up in which the sponge secretions could only reach the slugs by diffusion. In this the two sponge bowls were placed at either end of the tray containing the slugs; the three linked by water bridges. These bridges were made as short as possible to minimise the diffusion path (see Fig. 3. ). After five 3 hr. experiments using sponges that evoked a considerable reaction in the current flow apparatus and 35 speci• mens of Rostanga in the tray none of the animals had responded. It is thus probable that water movement is necessary to enable Rostanga to detect and seek out its food.

Results Each experiment was run for three hours and at the end of that time the animals in each sponge bowl were counted. The number of animals remaining in each tube was also noted and both figures recorded as percentages. The slugs were removed from the bowls as soon as they reached them to prevent feeding on the sponges. 31

SIPHON *—z^T^ SPONGE SPONGE BOWL •OWL

FIG. 3. DIFFUSION APPARATUS

Table 1.

Controls

Run no. Bowl Contents of bowl % in bowl %in tube

I. A. nil 2. 7 0 B nil 1. 1+ 0 2 A nil 0 0 B nil 0 0

Feeding preference experiments

3 A nil 0 1. 1+ B l+. 55gm. Ophlitaspongia 26 13. 7 1+ A l+. 55gm. Ophlitaspongia 31. 5 5-5 B nil 0 0 5 A 2. 25gm. Ophlitaspongia 20. 6 l+. l B 2. 3gm. Ophlitaspongia 15-1 9. 6 6 A nil 1. 1+ 1. 1+ B lgm. Holoplocamium 6. 9 7 A 3-71gm. Holoplocamium 2. fk 6. 9 B nil 0 1. 1+ Feeding preference experiments.

Gregariousness

"Fouling" effects 33

(Table 1. continued)

Run no. Bowl Contents of bowl % in bowl % in tube

"Fouling" effects

22 b Microciona 23. 3 10

Variation of current speed

23 A Microciona with current 0 0 velocity of 0. 86 m. p. h. B Microciona with current 26 10 velocity of O. 58 m. p. h. 2k A Microciona with current 3 5 velocity of 0. 75 m. p. h. B Microciona with current 25 8 velocity of 0. 58 m. p. h. 25 A Microciona with current 27 3 velocity of 0. 7 m. p. h. B Microciona with current 7 1 velocity of O. 58 m. p. h.

Standardisation of results (for current speed). As they stand the results show comparisons be• tween only two of the current velocities at a time and in order to compare all four velocities directly they are standardised against one value ie. O. 58 m. p. h. in run 2k. For example: - A current of 0. 58 m. p. h. in run 25 was compared with 0. 7 m. p. h. - 7 slugs responded to 0. 58 m. p. h. and 27 to 0. 7 m. p. h. If, however, 25 animals had responded to 0. 58 m. p. h. as in run 2k then 96 would have chosen 0. 7 m. p. h. etc.

Table 2.

Current velocity (m. p. h. ) O. 58 0. 7 0. 75 0. 86

Run 23 26 - - 0 Run 2k 25 - 3 - Run 25 7 27 - - (Table 2. continued)

Current velocity (m. p h. ) 0. 58 0. 7 0. 75 0. 86

Standardisation 25 96 3 0

See accompanying graph of results (Fig. k).

CURRENT VELOCITY (M. PH. )

FIG. 4. CURRENT VARIATION

Table 3.

Light and dark

Run no. Bowl Contents of bowl % in bowl % in tube

26 A Microciona 8 16 (conditions: - light] B nil 0 0 27 A nil 0 0 B Microciona 8 10 (conditions: - dark) Minimum amount of sponge 28 A nil 3. 3 0 35

(Table 3. continued)

Run no. Bowl Contents of bowl % in bowl % in tube

28 B 0. 37gm Ophlitaspongia 30 21. 7 29 A 0. 2Ugm Ophlitaspongia 11. 7 18. 3 B nil 0 3. 3 30 A 0. 13gm Ophlitaspongia 5 16. 7 B nil 1. 6 0 31 A nil 0 3. 3 B 0. 06gm Ophlitaspongia 3. 3 10

See accompanying graph of results (Fig. 5).

1 L 1 I

01 0-2 0-3 04

WEIGHT OF SPONGE IN BOWL (GMS. )

FIG 5 VARIATION OF SPONGE WEIGHT, 36

Fig. 6. This is a drawing (taken from a photograph) of the current flow apparatus. The two sponge howls are on the right and the shallow slug tray on the left. Run 12 was in progress and had been running for about 60 minutes with Holoplocamium in bowl A and Ophlitaspongia in bowl B. 15-20 slugs are clustered around the entrance to tube B and k can be seen mov• ing up it, (arrowed). In contrast to this only a few animals are in the vicinity of tube A, and none in it. This is a vivid demonstration of Rostanga's ability to detect and differentiate between the very similar sponges on which it feeds.

DISCUSSION

(i) Feeding Preferences Runs 1 and 2 were controls with no sponge in either bowl. These runs ensured that any reaction 37 from the slugs in the experiments was caused by the sponge being tested and was not a positive reaction to the current. As can be seen from the results the number of animals reaching the bowls is very small in 1, possibly due to random movement of the slugs about the tray, and nil in 2, indicating that Rostanga does not show positive rheotaxis.

Runs 3, h and 5 using Ophlitaspongia seriata in either bowl and then in both bowls show that Rostanga can detect this sponge by chemotaxis. These results also indicate that the slugs show little or no pre• ference for either of the two bowls used. This is important if comparisons are to be made with two different sponges in the bowls.

Holoplocamium neozelanicum: - In runs 6 and 7 this sponge also evokes a reaction from the slugs but the numbers eg. 12. k% and 9. 6% as opposed to 31% and 39% indicate that Ophlitaspongia is more easily detected and hence probably preferred as food.

In runs 8 and 9, Microciona coccinea was used and the reaction - 1+9-2 and 57• 5% - was greater than either Ophlitaspongia or Holoplocamium.

Thus Rostanga appears to feed on all three of the Poecilosclerid sponges mentioned and the order of preference which has been deduced from the magni• tude of response may be put at: - Holoplocamium Ophlitaspongia Microciona increasing preference •

Runs 10 and 11 involve Hymeniacidon perleve and Suberites cupuloides respectively and the slugs showed no reaction to either of these sponges which although somewhat similar to the others in external appearance belong to different orders.

In runs 12 and 13 the responses to Ophlitaspon• gia and Holoplocamium were compared by putting one in tray A and the other in tray B. As can be seen 38

from the results Holoplocamium, the least preferred sponge was almost completely ignored in the presence of the more powerful stimulus from Ophlitaspongia.

Runs lk, 15 l6 and IT are comparisons between Ophlitaspongia and Microciona. Microciona is prefer• red but the difference between these two sponges is not as great as between Ophlitaspongia and Holoplo• camium.

Thus the order of preference put forward on the basis of runs 3 and 9 is borne out by these further trials and it is also established that there is a greater difference in response between Holoplocamium and Ophlitaspongia than between Ophlitaspongia and Microciona. ie. Holoplocamium—* Ophlitaspongia—• Microciona increasing preference

Gut contents and faeces were examined from a number of animals collected in their natural habitat and spicule mounts made to determine which sponges had been eaten. Spicules from all three of the poecilosclerid sponges used in the experiment were found indicating that Rostanga does indeed feed upon these three sponges.

(ii) Gregariousness In runs 19 to 21 five Rostanga were placed in one of the sponge bowls and nothing in the other to see if the slugs remaining in the tray were attracted to their fellows. In two cases there was no response, indicating that the presence of animals in the tube or sponge bowl during the experiments was not an attracting factor to the slugs still in the tray and did not affect the results. In the field this would mean that slugs already feeding on a piece of sponge do not attract other animals to them. It is possible however, that the damage done to a sponge during the feeding process would release a greater amount of the attracting substances into the water and thus facilitate detection by other slugs. Thus when 39 the slugs are found actively feeding on a piece of sponge on the shore there is often more than one in• dividual present and occasionally up to five. In the other run, three of the slugs crawled down the tube to the main tray thus exhibiting a negative rheotaxis (response to current). This was also indicated when the sponge was taken out of the trays at the end of a run when slugs were still actively crawling up the tube. Many of these reversed direction when the stimulus was removed and made their way back to the tray.

(iii) 'Fouling' Effects In run 22, Microciona was placed in both trays but in one it was associated with other organisms commonly found at Westmere Reef. eg. Sargassum undulatum Sargassum serratifolium Codium adhaerens Watersipora subovoidea A simple ascidian The very close results for both trays show that these organisms did not affect the slugs' response to the sponge. Thus the animals would be able to detect their food in their natural habitat where the sponges are growing amidst a profusion of these other organ• isms.

(iv) Variation of current speed Four different current speeds were compared (runs 23 to 25); the results standardised against one value and graphed. The graph shows that there is an optimum current flow which elicits the greatest res• ponse from the animals. There are two possibilities to explain the fall in response above the optimum current flow: - 1. It is possible that the sponge secretions would be too diluted to permit easy detection especially in the presence of the more powerful stimulus in the lower velocity current. 2. The slugs may have a negative response to the ho higher current velocities. The fall below the opti• mum is more difficult to explain but could be due to nonmaintenance of directional flow in the tray, from the low velocity tube. Thus, although the secretions from the low velocity tube were more concentrated the slugs ignored them in favour of the stronger current which could be traced more easily.

(v) Effects of Light and Dark In run 26, (Table 3), Microciona was placed in one bowl and k x 100 watt bulbs with reflectors placed 3ft above the apparatus to provide an intense source of light. In run 27, Microciona was placed in the bowl and the experiment left in darkness. The res• ponse was similar in both cases indicating that Rostanga probably detects its food and feeds equally efficiently regardless of whether it is day or night.

(vi) Minimum amount of Sponge Four runs were made (runs 28 to 31) using steadily decreasing weights of Ophlitaspongia. The results show that the slugs could respond to a piece as small as 0. 06gm, ie. a 2mm cube of sponge and this at a distance of 52cm with a current flow of O. 58 m. p. h. The graph of response against weight (graph 2) shows that an increase in the weight of sponge leads to a steady increase in response possibly due to differ• ences in the individual sensitivity of the animals to the sponge secretions. In the field where the majority of the sponge masses are very much larger than 0. 06gm most sponges in an area would be open to detection and thus feeding by Rostanga and size of a sponge does not limit its availability.

SUMMARY

1. Rostanga rubicunda feeds on three very similar sponges in the order Poecilosclerida and can det• ect and then locate these by chemotaxis. Rostanga shows definite preferences amongst these, preferr• ing Ophlitaspongia seriata to Holoplocamium neoze- lanicum and preferring Microciona coccinea to both 1+1

of these. 2. Rostanga responds to very small concentrations of the stimulating substances and located a piece of sponge 0. 06gms in weight at a distance of 52cm. 3. Rostanga does not show gregariousness and the slugs are not attracted to each other during feeding activities. k. Rostanga could distinguish the sponge secretions amongst those from the encrusting animals and algae, ie. in the type of situation encountered in their natural habitat. 5. There is an optimum current velocity at which Rostanga shows a maximum response to the sponges. 6. Rostanga shows equal feeding activity both day and night.

REFERENCES

Morton J. E. 1967 -+th. ed. Molluscs Hutchinson. Cook E. F. 1962 Food choices of Rostanga pulchra and Archidoris. Veliger, 4 (U). Braams W. G. and Geelen H. F. M. 1953 Preferences of some nudibranchs for certain coelenterates. Arch• ives Neerlandaises de Zoologie 10 (3). Stehouwer H. 1952 Preferences of Aeolidia papillosa for the sea anemone Metridium senile. Archives Neerlandaises de Zoologie 10. Forrest J. E. 1953 Dorid feeding. Proc. Linn. Soc. Land. 164. Powell A. W. B. 1961 1+th ed. Shells of N. Z. Whitcom- be and Tombes. Suter H. 1913. Manual of N. Z. Mollusca. Government Printers (Wellington). Augersburg 1922. Chemical and physical stimulation of rhinophores. J. Expl. Zool. 29. Miller M. C. 196l. Distribution and food of nudi• branchs of the Isle of Man. J. Anim. Ecol. SO. 1+2

Wilber K. and Yonge CM. 196U & 1966 Physiology of the Mollusca Vol. 1 and 2. Academic Press.