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Tuatara: Volume 20, Issue 2, March 1973

A Note on the Occurrence of Small Fishes in Sponges

A Note on the Occurrence of Small Fishes in Sponges

According to local fishermen, sponges containing small fishes are sometimes found in Tasman Bay, New Zealand. This was confirmed on February 9, 1970, by a trawl catch from that area, taken at a depth of 40 m and containing, among other things, many small tarakihi, Cheilodactylus macropterus (Bloch and Schneider), and a quantity of the sponge Suberites australiensis Bergquist. I found small fishes in the exhalant cavity of two sponges: one contained three tightly packed tarakihi, the other a scaly gurnard Lepidotrigla brachyoptera Hutton. All these fishes were between 10 and 12cm long. They sat in the cavity with the head inwards, and were almost completely enveloped by the sponge. On the outside only the tails could be seen, protruding through a narrow opening (Figs. 1-3). The inner end of the cavity occupied by the scaly gurnard had the same size and characteristic angular shape as the head of the fish itself, so that the cavity was almost a page 110
Fig. 1: Outside view of Suberites australiensis Bergquist containing three small tarakihi, Cheilodactylus macropterus (Bloch and Schneider). The vertical cleft in the middle of this specimen is a result of opening the sponge for inspection of its contents before the photograph was taken; the fishes remained in situ. The sponge is shown joined together with plastic tape.

Fig. 1: Outside view of Suberites australiensis Bergquist containing three small tarakihi, Cheilodactylus macropterus (Bloch and Schneider). The vertical cleft in the middle of this specimen is a result of opening the sponge for inspection of its contents before the photograph was taken; the fishes remained in situ. The sponge is shown joined together with plastic tape.

Fig. 2: Cross-section of the specimen shown in Fig. 1.

Fig. 2: Cross-section of the specimen shown in Fig. 1.

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Fig. 3: The same cross-section as in Fig. 2, but with the fish removed, showing size and shape of the exhalant cavity.

Fig. 3: The same cross-section as in Fig. 2, but with the fish removed, showing size and shape of the exhalant cavity.

mould of the fish. When I opened the sponges the fishes were dead but quite fresh. It seems likely that they were alive when the sponges were caught by the net and that they died as a result of the subsequent rapid journey to the surface; most of the other fish in the catch were also dead when brought on deck.

It is unlikely that these fishes used the sponges for shelter, because then they would have been more likely to occupy the cavity with the head outwards. The cavities were so narrow that, once inside, the fishes could not have turned round. A more likely explanation follows from the fact, mentioned to me by Professor P. R. Bergquist, that the exhalant canals of Suberites australiensis are sometimes inhabited by small animals such as worms. Tarakihi feed on this kind of prey and may thus be attracted to enter the exhalant cavity of the sponge.

Suberites australiensis has much contractile tissue around the exhalant cavity. If irritated by the presence of a fish inside the cavity, the sponge may contract in an effort to rid itself of the source of irritation, with the result that the fish becomes trapped and will remain so unless the sponge relaxes again. Both tarakihi and scaly gurnard have spines which may make it difficult for them to move backwards through a narrow opening in spongy material, and wriggling movements intended to free the fish may have the effect of pushing it further inwards and of irritating the page 112 sponge. The trapped fishes may stay alive for some time, being supplied with a continuous flow of water by the normal activity of the sponge. This appears to have been the case in the instances described; the perfect fit of the sponges around the fishes suggested that these had become trapped well before the sponges were captured by the trawl.

The tendency to explore narrow openings in search of food has been observed in other fishes. For instance, the yellow-eyed mullet, Aldrichetta forsteri (Curier and Valenciennes), can be caught in baited milk bottles, and a ‘bait-catcher’ (a small cylinder with two funnel entrances) sold commercially in New Zealand operates on the same principle. The presence of one fish in such a trap seems to attract others, because many can be caught at a time. This may account for the presence of three fishes in one of the sponges. I have also found a tarakihi of 12cm trapped inside a somewhat larger salp (Salpa sp.) which shows that this phenomenon is not restricted to sponges but can also occur in other animals.

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