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Chapter 4 Commensalism: Meal Sharing
- Hong Kong University Press, HKU
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£ Chapter 4 Commensalism: Meal Sharin g Commensalism i s the term most glibly applie d t o animal associations an d in many instances wrongly so . The term 'commensal' was first coined by van Beneden in 1876 and was use d i n th e contex t o f 'messmate' . Th e concep t encompasses al l thos e relationship s base d aroun d a gastronomic hospitality, that is a unilaterally beneficial and certainly non-harmfu l associatio n grounde d i n nutrition . Such a clea r definitio n ha s bee n expande d o f lat e t o encompass th e concept s o f shelter , protection , transport , warning and so on. In almost all associations, it is impossible to be certain that only a single benefit is derived. Thus, as we have seen with aegism , ther e i s alway s th e possibilit y o f som e nutritional side-benefit, or some other, more abstract, benefit such a s enhance d longevity , tha t give s advantag e t o th e species if perhaps not quite so obviously to the individual. Thus, in the previous chapter it was made clear that aegism is principally , bu t no t exclusively , concerne d wit h protection. In this chapter, a series of associations is studied which are principally concerned with nutritional benefit to the commensal, but with no harm to the host, although by the very nature of th e intimacy between th e two species, some other benefits may accrue. Foremost among these are protection and the provision of aerating respiratory currents. Commensal associations are among the most difficul t to study, because, generally speaking, for the active partner to depend upon and yet have no adverse impact on the host, it must be very small. Similarly, to gain the extra advantages of protectio n an d t o exploi t hos t respirator y currents , smallness an d obscurit y becom e additionall y important . Thus, commensals are usually tiny, cryptically coloured or shaped, ofte n hidde n i n respiratory o r feedin g structure s and wher e thei r hos t i s a burrowe r the n the y ar e als o immured in deep tunnels. Animals which live in crevices, holes o r burrow s ca n b e calle d anachorete s an d th e phenomenon anachoresis. The hosts of many commensals are anachoretes especiall y i n marine sand s and mud s and on coral reefs. The commensal can thus be referred t o as a secondary anachoret e ye t posses s characteristic s quit e different fro m thos e o f it s host . Mos t commensa l associations are poorly known because of behavioural and anatomical adaptations to life with a host which relies on, for example , deep burrowin g fo r protectio n an d ar e thu s difficult t o catch. Commensalism, however , i s no w know n t o b e a widespread phenomeno n i n th e animal kingdo m an d th e extent to which such associations are exceedingly common among intertidal and shallow coastal water communities of the tropic s an d sub-tropic s i s becomin g increasingl y apparent. For specie s to evolve a commensal life-style , certai n general host prerequisites are necessary. First, the life-style of the host must be such that food is unlimiting for it; that is, food mus t be i n suc h abundanc e tha t it can be share d without detriment. Second, the host must occupy a microhabitat that is more or less permanent over a long period of time, tha t is , i t i s a permanen t anachorete . Thus , opportunistic inhabitant s o f unstabl e environments , fo r example shifting sands, are unsuitable as hosts because the short and long-term effects of instability preclude the casual interaction and, thus, the eventual evolution of a permanent relationship. Third , however , th e habita t mus t no t b e s o suffocatingly ric h in sediment and nutrients, as for example with slopp y mud , tha t th e commensa l canno t kee p itsel f clean of the excessive load. Fourth, the host must usually be large in relation to the commensal. This may be in part so that the host does not notice its companions, but...