. Exp. Biol. (1962), 39, 483-490 483 Vith 1 text-figure Printed in Great Britain EXPERIMENTS ON CHOICE OF SUBSTRATA BY SPIRORBIS LARVAE (SERPULIDAE) BY P. H. D. H. DE SILVA Department of Zoology, University College of Swansea* {Received 22 April 1962) INTRODUCTION At Swansea there are three closely related species of Spirorbis occurring each on a characteristic substratum: S. boreaUs Daudin on Fucus serratus, S. coraltinae de Silva and Knight-Jones (1962) on Corallina officinalis and S. tridentatus Levinsen on rocks and stones. This pattern in their natural distribution suggests a high degree of sub- stratum selectivity by the larvae at the end of their pelagic stage. It is known from previous studies that larvae of S. borealis tend to select various substrata, settling readily and abundantly in the laboratory on F. serratus, F. vesiculosus, F. platycarpus and Laminaria saccharina but sparsely on Uha lactuca, Pelvetia canaliculate, Ascopkyllum nodosum, Himanthalia lorea, Rhodymema pahnata, ascidians, shells and stones (Gar- barini, 1936; Gross & Knight-Jones, 1957). To investigate the settling behaviour of S. coralUnae and S. tridentatus, adults of the various species were collected from the shore at Mumbles Head and Bracelet Bay, Swansea, during the summer and generally during periods of neap tides, when larvae are most common (de Silva, 1962). Many of the adults were incubating embryos, some of which would hatch readily if the adult tubes were broken in the laboratory. Larvae obtained in this way seem to be just as discriminating during their search for a substratum as are those which hatch without artificial aid (de Silva, 1958). CHOICE BETWEEN PAIRS OF SUBSTRATA The method followed was similar to that used by Knight-Jones (1951). Crystal- lizing dishes, wiped clean and containing about 100 ml. of sea water were placed in narrow boxes painted white inside. These boxes were kept near a window with their long axes directed towards it so that the two sides of each dish were equally illu- minated. The experimental substrata were placed on opposite sides of each dish, the positions being reversed in alternate dishes, to compensate for any small undetected differences in illumination. Varying numbers of freshly liberated larvae were added to these dishes. The first two series of experiments involved larvae of S. borealis. In the first series these were offered a choice between F. serratus and Corallina officinalis (Table 1). In the second (Table 2) the choice was between F. serratus and a small stone which had been kept in sea water for a few days to allow it to develop the film of micro -organisms without which stones are very unfavourable for the settling of this species. Of the • Present address: Colombo National Museum, Ceylon. 31-2 484 P. H. D. H. DE SILVA three substrata offered to S. boreatis larvae in these experiments, F. serratus was the most favoured, filmed stones being somewhat less favourable and C. officinalis very unfavourable. The total number of larvae settling on C. officinalis was less than 2 % Table 1. Settlement of Spirorbis borealis in forty-three experiments involving choice between a piece of Fucus serratus and a piece of Corallina officinalis in the same dish No. of larvae settling on: (a) Fucus serratus 6, 22, 22, 27, 26, 37, 14, 22, 25, 22, 29, IS, 13, 17. 17. 23, 67, 68, 20, 66, 68, 14, 11, 23, 25, 10, 25, 26, 17, 38, 19! 32. 35. 19. ". 15. 13. 63, 10, 84, 52, 72, 56- Total 1297 (6) Corallina officinalis o, o, o, o, o, 1, o, o, o, o, o, o, o, o, o, o, o, o, o, o, o, o, 6, 3, 3, o, O, 2, I, O, O, O, O, O, O, O, O, O, O, o, 1, o, 1. Total 18 Table 2. Settlement of Spirorbis borealis in nine experiments involving a choice between a piece of Fucus serratus and a filmed stone in the same dish No. of larvae tettling on: (a) Fucus serratus 3, M, 57. 55, 65, 36, 77, 71, 69. Total 457 - (b) Filmed stone 127, 41, 26, 49, 3, 24, 9, 1, 15. Total 295 Table 3. Settlement of Spirorbis coraUinae in fourteen experiments involving choice between a piece of Fucus serratus and a piece of Corallina officinalis in the same dish No. of larvae settling on: (a) Fucus serratus o, o, 1, o, o, o, o, o, o, o, o, o, 1, o. Total 2 (6) Corallina officinalis 3. 5. 6, 4, 7, 7, 1, 13, 3, 7, 2, 2, 2, 1. Total 63 Table 4. Settlement of Spirorbis tridentatus in (I) fifteen experiments each giving five larvae a choice between Fucus serratus and a filmed stone, and (II) fifteen similar experi- ments involving choice between Corallina officinalis and stones (I) No. of larvae settling on: (a) Fucus serratus o, o, o, o, o, o, o, o, o, o, o, o, o, o, o. Total o (6) Filmed stone 4. 2, 5, 4. 4, 4. 5. 4. 2, 3, 3, 1, 4, 4, 3. Total 52 (II) No. of larvae settling on: (a) Corallina officinalis o, o, o, o, o, o, o, o, o, o, o, o, o, o, o. Total o (6) Filmed stone 5. 4, 4, 5. 5, 5. 3, a. 4, 2, 3, 3, 5, 3, 2. Total 55 Choice of substrata by Spirorbis larvae (Serpulidae) 485 of the total number settled. However, this alga was chosen most loyally by larvae of S. coraltinae (Table 3), whereas F. serratus was extremely unfavourable to this species, the number settling on this alga being less than 5 % of the total number settled. The comparatively poor settlement recorded in Table 3 was partly due to these experi- ments having been carried out under bright light. It was found subsequently that better results could be obtained by using dim light. The larvae continued to select Corallina, however, no matter how the illumination was varied. In similar experiments with S. tridentatus larvae (Table 4) all the larvae settled on filmed stones, showing that both F. serratus and C. officinatis are very unfavourable for the settling of this species. They will, however, settle to some extent upon these algae if no other substratum is available. Indeed, when offered a choice between F. serratus and an unnlmed stone, which had been freshly immersed in sea water, more settled on the Fucus than on the unfilmed stone. This avoidance of unnlmed surfaces may help to prevent them from settling on scoured or abraded surfaces where they would probably soon be damaged by further scouring. CHOICE BETWEEN SEVERAL SUBSTRATA In their natural habitats a variety of other substrata are available to the larvae. In order to simulate more closely these conditions further experiments were carried out with the help of a turn-table apparatus of the sort used by Crisp & Ryland (i960). In this all the experimental substrata are placed round the periphery of a single dish, which is rotated slowly. The substrata offered in each dish were of similar size and the rotation ensured that they were all similarly illuminated. The results (Table 5) indi- cated that F. serratus, TJha lactuca and filmed stones are most favourable for the settling of S. boreaUs whereas other algae, including F. vesiculosus and C. officinatis, are unfavourable. Table 5. Settlement of Spirorbis borealis in ten experiments involving choice between thirteen different substrata in the same dish. A hundred larvae were added to each dish Experiment number... 123456789 10 Totals Enteromorpha intatinalis 0028611110 20 Ulva lactuca I o 17 14 39 24 o o 213 100 Cladophora sericea 0000000000 o Fucus vesiculosui 0100051101 9 F. serratus o o o 3 13 18 17 14 28 20 113 Ascopkyllum nodosum 0100001000 2 Pdvetia canalictdata 47216720000 38 Corallina officmalis 0110000000 2 Laurencia pitmatifiaa 62940110000 41 Chondrus crispus 40714 12 0013 32 Furcellaria fastigiata 0401300000 8 Polyides rotundus 101 10 200010 15 Filmed stone 8 o 1 23 15 415 4 19 17 106 In natural conditions S. borealis is much more common on F. serratus than on U. lactuca. It therefore seemed desirable to carry out further experiments involving choice between these two algae. The results of these showed clearly that S. borealis larvae choose F. serratus in preference to Ulva lactuca (Table 6). Further experiments with S. coralUnae larvae (Table 7) showed that C. officinatis 31-3 486 P. H. D. H. DE SILVA on which this species typically occurs is the most favoured substratum, Chondrus crispus and filmed stones being much less favourable. On certain shores S. corallinae occurs on both C. crispus and Gigartina stellata but it has not been observed so far on rocks and stones. Table 6. Settlement of Spirorbis borealis in fifteen experiments involving choice between a piece of Fucus serratus and a piece of Ulva lactuca in the same dish. Ten larvae were added to each dish No. of larvae settling on: (a) Fucus serratus 4. 8, 8, 6, 3, 7, 9, 5, 2, 5, 6, 7, 4, 4, 4. Total 82 (6) Ulva lactuca o, o, o, o, o, o, o, 3, 4, 1, o, 1, o, o, o. Total 9 Table 7. Settlement of Spirorbis corallinae in ten experiments involving choice between thirteen different substrata in the same dish. About seventy-five larvae were added to each dish Experiment number... 133456789 10 Total Enteromorpha mtestinalis 0061120000 10 Ulva lactuca 0000050011 7 Cladophora sericea 0000000000 o Fucus vesiculosus 0000000000 o F. serratus 0000000000 o Ascophyllum nodosum 0000000000 o Peivetia canalicvlata 0101000010 3 Corallina officmalis 5 3 33 30 20 26 23 18 18 21 197 Laurencia pitmatifida 0000000001 1 Chondrus crispus 8 10 00000382 31 Furcellaria fastigiata 0000000000 o Polyides rotundus 0000000000 o Filmed stone 3123005405 23 Table 8.