The public profile of jellyfish blooms has increased in recent years, probably reflecting a real global increase in occurrence or severity, although local effects may be quite the opposite (Mills 2001). During the past couple of years, large swarms of jellyfish have bloomed in the Gulf of Oman. They have caused considerable concern in Oman, where they have blocked many seawater intakes, and in Iran, where they have hindered fishing. Trawls (mesh-size 80 mm) targeting bony fish have returned catches comprising as much as 90% jellyfish (by weight). The jellyfish are commonplace in surface waters above the thermocline. Abundant jellyfish also have been reported from Yemen and the problem may be widespread in the Arabian Sea.

Results of a trawl in the Gulf of Oman.

Understanding jellyfish blooms has been hindered by a general lack of knowledge of, for example, jellyfish ecology, historical patterns of abundance, oceanography, and the effects of variation in local ecosystems and changes in global climate. Understanding also has been hindered, at least in this case, by the lack of a reliable identification of the jellyfish, and it was in this context that the jellyfish and blooms were brought to the attention of a wider audience via the cnidarian newsgroup in late-2002. Since then, photographs and some morphometric data exchanged via the internet have allowed us to tentatively identify the jellyfish from Iran and Oman as Crambionella orsini. This is consistent with the previously reported geographic range of C. orsini, from the Red Sea to India, including the "Iranian Gulf" (Kramp 1961). This suggests, therefore, that the problems may be attributable to natural fluctuations of an endemic species rather than, say, recent explosion of a weedy invasive.

Researchers in Iran are gathering weather information and data describing the chemical and physical oceanography of the region to investigate possible relationships with the abundance of jellyfish in recent years. A more robust identification of the species is also intended. Increased knowledge may alleviate some of the concern about possible future impacts of the jellyfish on coastal industries in the Arabian Sea. Certainly, a better understanding of factors influencing their occurrence should allow measures to be put into place that will mitigate detrimental effects. As C. orsini is considered to be an edible jellyfish, there may even be a silver lining.

 

 

Photograph of the underside of the bell, showing several features commonly used in identifications of jellyfish.

Preliminary measurements of four jellyfish off the Iranian coast reveal the following. The mean bell diameter of specimens examined was approximately 9 cm (range 7 cm to 10 cm; the jellyfish from Oman at the top of the page is between 10 cm and 15 cm bell diameter.) All specimens had 8 rhopalia. The number of lappets between adjacent rhopalia was 8, 12, or 16, and could vary within single jellyfish. Pigment was absent between lappets. The oral arms/disk lacked scapulets. The orals arms are three-winged in cross-section and have triangular clubs (in cross section). The length of the oral arms was, on average, approximately 0.6 * bell diameter (range 0.56 to 0.68). Most of these features are consistent with the description of Crambionella orsini given by Kramp (1961; see also Vanhöffen 1888). Deviations may be due, for example, to differences in methods or geographic variation among jellyfish. Confirmation of the preliminary identification is pending.

(Above) an oral arm amputated from the oral disk of C. orsini.
(Below) Sketches of cross-sections along lines A and B.