SUMMARY Prostaglandins (PG) are oxygenated derivatives of C20 polyunsaturated fatty acids including arachidonic and eicosapentaenoic acids. In mammals, these compounds have been shown to play key roles in haemostasis, sleep-wake regulation, smooth muscle tone, and vaso-, temperature and immune regulation. In invertebrates, PGs have been reported to perform similar roles and are involved in the control of oogenesis and spermatogenesis, ion transport and defence. Although there is often a detailed understanding of the actions of these compounds in invertebrates such as insects, knowledge of their mechanism of biosynthesis is often lacking. This account provides a critical review of our current knowledge on the structure and modes of biosynthesis of PGs in invertebrates, with particular reference to aquatic invertebrates. It emphasises some of the most recent findings, which suggest that some PGs have been misidentified. Prostaglandins in invertebrates can be categorised into two main types; the classical forms, such as PGE 2 and PGD 2 that are found in mammals, and novel forms including clavulones, bromo- and iodo-vulones and various PGA 2 and PGE 2 esters. A significant number of reports of PG identification in invertebrates have relied upon methods such as enzyme immunoassay that do not have the necessary specificity to ensure the validity of the identification. For example, in the barnacle Balanus amphitrite , although there are PG-like compounds that bind to antibodies raised against PGE 2 , mass spectrometric analysis failed to confirm the presence of this and other classical PGs. Therefore, care should be taken in drawing conclusions about what PGs are formed in invertebrates without employing appropriate analytical methods. Finally, the recent publication of the Ciona genome should facilitate studies on the nature and mode of biosynthesis of PGs in this advanced deuterostomate invertebrate.

SUMMARY In this study the influence of the dietary level of the fatty acid arachidonic acid (ArA, 20:4n-6) was determined on the acute stress response and osmoregulation of adult gilthead seabream Sparus aurata L. Seabream were fed a diet containing either 0.9% or 2.4% of total fatty acids as ArA for 18 days before being subjected to a 5 min period of net confinement. Prior to this stressor, a subgroup of fish from both dietary treatment groups was treated with acetylsalicylic acid (ASA), an irreversible blocker of cyclooxygenase (COX). This would indicate whether any effects were caused by an enhanced synthesis of prostaglandins derived from ArA. The highest ArA levels were found in the kidneys, and these were further enhanced by dietary ArA-supplementation. In gill tissues, there were significant changes in all selected fatty acid classes 24 h after confinement, except for the docosahexaenoic acid (DHA, 22:6n-3): eicosapentaenoic acid (EPA, 20:5n-3)ratio. ArA feeding strongly reduced the cortisol response to confinement,which was partially counteracted by ASA treatment. ArA also attenuated the stress-associated increase in plasma osmolality and, in combination with ASA,enhanced the osmolality and plasma chloride levels, but reduced plasma sodium levels after confinement. Furthermore, ArA enhanced the branchial Na + , K + -ATPase activity both before and after confinement, whereas feeding ASA diminished this effect. It appeared that the effects of ArA-supplementation could not always be ascribed to an increase in prostaglandin synthesis. It is advisable to determine the long-term effects of replacing fish oils in commercial diets with vegetable oils that contain no long-chain fatty acids, particularly in carnivorous/marine species with low fatty acid elongation and desaturation activities. The effects of a low dietary intake of ArA (and other polyunsaturated fatty acids) should be studied over a longer term, taking into account any consequences for the health of the fish.