An Introduction to the Invertebrates, Second Edition - tiera.ru

WHATARE THE SPECIAL FEATURES OF PARASITIC CRUSTACEANS?17713.4 What may limit the size of Crustacea?Aquatic arthropods can become much larger than terrestrial ones:the exoskeleton is supported in water, and respiration by gills, unlikeaerial respiration, does not demand small size. Accordingly manymarine crustaceans, bottom-living, pelagic and even intertidalspecies, have been able to profit from the ecological and metabolicadvantages of increase in size.The largest examples of many groups are found in the Antarctic.There are no decapod crustaceans, but their place is taken by a verylarge number of amphipods and isopods. Amphipods in particularhave been studied by scientists from the British Antarctic Survey,who investigated the finding that the largest polar species havebodies five times the length of amphipods in warmer waters. Thesolubility of oxygen in sea water is considerably increased at lowtemperature: it is 1.82 times greater at 0 °C than at 30 °C. Increasedavailability of oxygen, rather than a direct effect of low temperature,was postulated as the factor allowing size increase. The effectsof low temperature and high oxygen content can be distinguished bycomparing freshwater lakes at different altitudes: in Lake Titicaca(altitude 3809 metres, in the Andes) the oxygen content is reducedby high altitude, and also by relatively large quantities of dissolvedsalts. There the maximum length of amphipods is 22 millimetres.In Lake Baikal (in the lowlands of Russia, with no measurable saltcontent) the amphipods are 80 90 millimetres long, four timeslarger than in Lake Titicaca. From the temperature difference alone(12 °C in Lake Titicaca, 6 °C in Lake Baikal) only half this increasein size would be expected. This is strong evidence that oxygenavailability rather than temperature is the limiting factor formaximum size in amphipods.Amphipods do not have very efficient mechanisms of acquiringoxygen. It diffuses into the blood over the gills and is carried partlyin solution in the blood and partly in combination with haemocyanin,but the circulatory system is restricted, with few lateralbranches. With this system it is not surprising that availability ofoxygen limits the size of amphipods (obviously this finding cannot begeneralised to other crustaceans).At the other end of the scale, the minimum size is not relatedeither to oxygen supply or to temperature; it probably depends onthe minimum possible size for an invertebrate egg.13.5 What are the special featuresof parasitic crustaceans?Parasitism has arisen many times separately in Crustacea.Ectoparasites, living mainly on the gills or the outer surface of fish,