Tuatara: Volume 25, Issue 2, January 1982
Snail Eating Behaviour of the Tunnel Web Spider — Porrhothele Antipodiana — (Mygalomorphae: Dipluridae)
Snail Eating Behaviour of the Tunnel Web Spider
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.
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.
|Weight of live snail||Weight of Snail Remains||Change in Weight|
|Mean wt. loss: 0.775g|
|Weight of spider prior to feeding||Weight of spider after feeding||Change in Weight|
|Mean wt. gain: 0.28g|
|Total body length||Width of abdomen prior to feeding||Width of abdomen after feeding||Change in width|
|Mean increase in abdominal width: 2.6mm|
|Spider body length (mm)||Snail wt. (g)||time to complete feeding (hours)|
|Mean time to complete feeding: 12.5 hours|
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.
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.
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.
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.
One of the central questions concerning snail predation was: How can a spider kill and eat a snail which has retreated into its shell? Detailed observations were made to answer this question. These revealed that the following phases were important in the process:
|1.||Detection and recognition of the snail as prey;|
|3.||keeping the fangs embedded in the snail as it retreated into its shell;|
|4.||remaining attached to the snail while it produced much foamy substance;|
|5.||protracted feeding following the death of the snail.|
A diagrammatic portrayal of these phases is given in Fig. 4.
It was clear that the spider was faced with several problems when trying to kill a snail. First, a large snail would drag the spider with it as it retreated into its shell after the strike had been made. Sometimes the spider lost its hold during this movement and would not pursue the snail into its shell. Second, the snail retreated deep into its shell and sometimes the spider lost its hold at this stage. Third, the snail usually began to produce foam from its pulmonary aperture soon after the strike had been made. This production of foam was the main deterrent to the spider: more spiders relinquished their hold when they began to be covered in the foam than at any other stage. In some instances the foam production was so extensive that a week elapsed before the last traces of foam had been removed from a spider's palps and legs.
The snails seemed to be quite resistant to spider venom. Even a bite from a large spider which lasted for several minutes, and which would have killed any large insect or even a mouse, appeared to have little effect on most snails. The time for death to occur if the spider managed to hold on with its fangs, seemed to be in the vicinity of 30 minutes. At about this time the spider was able to partly withdraw the snail's body from the shell, so presumably prior to this the snail was still providing some resistance to the spider.
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.
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.
|No spiders tested||Snails eaten|
|Ix uticus martius||5||-|
|Shell diameter (mm) (to nearest 5mm)||Number|
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.page 81
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.
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.
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.