Experiments involving encounters between the snail Helix aspersa and the diplurid spider Porrhothele antipodiana confirm that not only does this spider kill snails, but it also spends many hours feeding off the body of the snail. Spiders can maintain body weight for at least two months when fed exclusively on a diet of snails. It is suggested that snails could be quite important as a source of food and fluid for these spiders over the drier months of summer and autumn.

Introduction

In an investigation of the prey of Porrhothele antipodiana (Laing, 1973) it was reported that snails* formed 7.5 per cent by number of the prey remains found in the tunnels of this spider. At the time of publication, the question of whether P. antipodiana consumed the body of the snail was left open. Subsequent experiments and direct observations on encounters between snails and spiders have been carried out in an attempt to answer the foregoing question.

Methods

Mature female spiders were selected for the experiments. They were kept in individual plastic containers 20cm x 20cm and 8cm deep. A covering of soil was provided on the bottom of each container. Glass lids were attached with lumps of plasticine to give a small air gap at the edges of the lid.

The snails were all Helix aspersa; this being the common species found in the open country and urban areas where P. antipodiana flourishes. Both the snails and the spiders were weighed before each encounter. (Note: snail page 76

Fig. 1 Weight changes to three P. antipodiana individuals fed on a diet of snails for eight weeks.

weights included the shell.) In addition to weighing the spiders, total length and abdomen width were also measured as an extra check for any ingestion which might take place. Each spider was provided with one snail at a time. Observations were made several times daily, and also at night.

Results

Of the five spiders used in the experiments, all killed snails and three proved to be especially proficient snail killers. Weighings of the snails after the spiders had finished with the bodies revealed that a substantial weight loss was always involved (Table 1). In conjunction with the weight loss of the snails, the spiders involved always showed a weight gain (Table 2). There can be little doubt therefore, that the spiders did ingest some of the snail.

In addition to the changes in body weight of the spider, evidence of ingestion could be obtained from measurements of its abdomen. These always increased after a spider had fed on a snail. (Table 3).

Observations on four instances of snail predation confirmed that feeding was a protracted event. (Table 4).

Having determined that P. antipodiana could both kill and eat snails, the next step was to feed a small group of spiders on a snail-only diet for an extended period of time. The three spiders which had killed snails regularly were used for this experiment. Each spider was offered a snail twice weekly. Weighings of the spiders were carried out weekly.

A control group was not used in this particular experiment as it was known from previous experience that P. antipodiana individuals kept in the same containers but without food were in a very poor condition at the end of four weeks. A body weight loss of up to 20 per cent was common page 77

Fig. 2 A large female P. antipodiana (25mm body length) holding tightly to its snail prey at the moment of the strike.

Fig. 3 The snail has partially retreated into i1ts shell and is producing large amounts of foam from its pulmonary aperture. Although the spider is becoming covered with foam, it is still maintaining its hold.

page 78 along with severe shrinkage of the abdomen and a general weakening of the spider. Therefore if the spiders in the current experiment failed to show a weight loss, it should indicate that they were capable of maintaining themselves satisfactorily on a snail-only diet (at least for the eight weeks of the experiment).

The three spiders fed solely on snails did not, in fact suffer any undue weight loss after eight weeks (Fig. 1) and their general condition was very good when the experiment was terminated.

Experiments with other spiders

Six other species of spiders were tested with small snails to determine if snail eating was widespread amongst spiders. The results of these investigations are given in Table 5. In the case of Cantuaria sp. a deeper container was provided so that this spider could construct a burrow and operate in its normal feeding manner.

From the limited number of species tested it would appear that snail eating is not widespread. In fact most spiders appeared to find the initial contact with the snail a disturbing event and would not proceed with the strike, thus confirming the results of Bristowe's experiments (1941). page 79

Fig. 4 Diagrammatic representation of the sequence of events involved in the killing and eating of a snail by P. antipodiana: (a) the detection of the snail moving over the sheet web; (b) the launching of the strike at the snail; (c) the retreat of the snail into its shell; (d) the spider maintaining its hold on the snail and following it into the outer portion of the shell; (e) the dead snail being partially withdrawn from its shell to facilitate feeding.

page 80

TABLE 5. Results of experiments to see which of a range of spider species would utilise snails as food. (X indicates positive record of spider killing and feeding on Helix aspersa).

	No spiders tested	Snails eaten
Spider
Fam. Dipluridae
Porrhothele antipodiana	5	X
Hexathele sp.	2	-
Fam. Ctenizidae
Cantuaria sp.	2	-
Fam. Clubionidae
Miturga sp.	5	-
Fam. Theridiidae
Archaearanea veruculata	5	X
Fam. Dic+ynidae
Ix uticus martius	5	-
Fam. Pisauridae
Dolomedes major	5	-

TABLE 6. Diameter of Helix aspersa shells recovered from 20 webs of P. antipodiana in Northland, Wellington.

Shell diameter (mm) (to nearest 5mm)	Number
10	11
15	8
20	9
25	6
30	3

Discussion

The investigations reported here indicate that snails are definitely used as food by P. antipodiana The question remains, however, how important are snails in the diet of these spiders? It is probable that snails could be useful as a source of fluids to P. antipodiana due to the high fluid content of their bodies. The protein mass in the foot masculature would also be useful to the spider, providing it could be digested. Maintenance of body weight by the three spiders over two months indicates that this is probably the case.

That only Porrhothele and Achaearanea ate snails is worth noting, for these two spiders also incorporate both slaters and millipedes in their diet. This itself is an unusual feature because these animals, like snails, are not taken as prey by the majority of spiders.

There are at least two, and probably more, of the Australian mygalomorphs which feed on snails. Raven (1978) recorded finding empty snail shells in the webs of the diplurid Bymainiella boycei The same observer has also reported (1979) the finding of an individual Cethegus (F. Dipluridae) actively feeding on Helix aspersa. Raven was also able to elicit a feeding response from the theraphosid Selenocosmia crassipes by offering it live Helix aspersa.

It seems very likely that further investigations of the mygalomorphs will reveal a widespread use of snails as prey items.

Snails would be valuable as prey over the drier months of late summer and autumn, and the eight-week time span of the feeding experiment described in this paper relates especially to this feature: eight weeks is the length of time we might expect a spider such as P. antipodiana to have to survive possible dehydration. The ability to accept unusual prey of high water content could well be a significant advantage during the dry months.

The size of the snail most often eaten was in the 10-25mm shell diameter range (Table 6.) Searches carried out around Wellington through summer and autumn confirmed that snails of this size were indeed present in the localities occupied by P. antipodiana.

It is clear that the behaviour of P. antipodiana when killing a snail is very important. In particular, this spider allows itself to be dragged into the outer portion of the shell and be covered with mucus. Moreover, it usually stays there until the snail's resistance ceases, thus revealing a tenacity uncommon in spiders. Very little appears to be known about the foamy mucus and its probable value in deterring predators.

The discrepancy between the weight loss from the body of the snail caused by the spider killing and eating it (Table 1) and the weight gain shown by the spider after feeding on the snail (Table 2) is quite marked. This could be due partly to evaporation of fluids from the dead snail during the protracted feeding period. It could also be due partly to seepage of digested snail remains onto the substrate rather than all digested fluids going into the spider's mouth. From the information given in Table 3 it seems that the abdomen of the spider is capable of accommodating large quantities of digested food. Whether its elasticity is great enough to cope with the large volume of material from a snail's body is unknown.

Acknowledgements

I would like to thank the following: Dr F. Climo, National Museum, Wellington, for his helpful comments on the snails; Dr R. R. Forster, Otago Museum, Dunedin, and Dr. R. Jackson, Zoology Dept., Canterbury University, for reading the rough draft and giving me their comments on it; and Mr R. J. Raven, Queensland Museum, for his observations on the Australian mygalomorphs, which helped give perspective to the New Zealand observations.

References

BRISTOWE, W. S. 1941: The Comity of Spiders Vol II. The Ray Society, London.

LAING, D. J. 1973: Prey and Prey Capture in the Tunnel Web spider Porrhothele antipodiana. Tuatara 20:2 57-64.

—— 1975: The Postures of the Tunnel Web Spider Porrhothele antipodiana: a Behavioural Study. Tuatara 21: 3 108-120.

—— 1978: Studies on Populations of the Tunnel Web Spider Porrhothele antipodiana, Part 1. Tuatara 23: 2 67-81.

—— 1979: Studies on Populations of the Tunnel Web Spider Porrhothele antipodiana, Part II. Tuatara 24: 1 1-21.

RAVEN, R. J. 1978: Systematics of the Spider Subfamily Hexathelinae (Dupluridae: Mygalomorphae:Arachnida) Aust. J. Zool., Suppl. Ser., 65: 1-75.

—— 1979: Personal Communication.