NOTARBARTOLO-DI-SCIARA NATURAL HISTORY OK MOBrLA 



feeding on planktonic Crustacea still appears 

 to be a feasible foraging technique for M. tara- 

 pacana, judging from the size of its branchial 

 sieve as it compares with the average-sized crus- 

 tacean prey. 



SUMMARY AND CONCLUSIONS 



Four species ofMobula were found in the south- 

 ern Gulf of California. The most abundant spe- 

 cies, M. thurstoni , was present year-round, but 

 only the smaller individuals were seen during the 

 winter. The bulk of the population, including the 

 adults, appeared in early spring. Numbers began 

 declining in July. Mobula japanica , the second 

 most abundant species, was comprised of only 

 large individuals; numbers progressively in- 

 creased from March throughout July. Large M. 

 japanica were rare in winter, but were occasion- 

 ally caught then. Mobula tarapacana is the rarest 

 mobulid in the area, yet its presence as a summer 

 and fall visitor is well known and predictable; it 

 is believed by the local fishermen to be more 

 abundant farther offshore. All three species share 

 a similar pattern of peak summer seasonal abun- 

 dance. The reverse is true for Mobula munkiana , 

 it being most abundant in winter, and almost to- 

 tally absent during the rest of the year. It is not 

 known where any species goes when not seen in 

 the area. Seasonal migrations within the 

 epipelagic habitat to different areas of the 

 Panamic region are likely, but unverifiable be- 

 cause of the present lack of knowledge of the oc- 

 currence of identified Mobula species south of the 

 Gulf of California. Alternatively, devil rays may 

 spend part of the year in midwater, or near the 

 sea bottom, therefore disappearing from sight and 

 reach. 



There is a striking similarity between the 

 array of mobulid species found in the Gulf of Cali- 

 fornia (and probably along the Pacific coast of 

 tropical America) and the mobulid fauna from the 

 tropical waters off west Africa. The family is rep- 

 resented in both areas by Manta birostris and by 

 four species oi Mobula : M . thurstoni; M Japanica 

 (reported from west Africa as M. rancureli by 

 Cadenat 1959); M . tarapacana (reported as M. 

 coilloti for African waters by Cadenat and Ran- 

 curel 1960 and Stehmann 1981); and a small gi-e- 

 garious form, represented in the Gulf of Califor- 

 nia by M . munkiana and off west Africa by the 

 closely related M . rochebrunei (Notarbartolo-di- 

 Sciara 1987). Tropical coastal areas off west 

 America and west Africa are known to be among 



the most productive tropical waters in the world, 

 because of comparable large-scale atmospheric 

 and oceanographic circulation patterns (Sverdrup 

 et al. 1942). It is conceivable that the ecological 

 similarity between these two regions is reflected 

 in similar faunal associations, especially as far as 

 low levels of the tropic chain (e.g., plankton- 

 feeding vertebrates) are concerned. 



The Gulf of California presents a unique envi- 

 ronment in the eastern Pacific Ocean, with ex- 

 treme annual water temperature ranges, wind- 

 induced mixing and upwellings, and subsequent 

 great productivity (Roden 1964; Brusca 1980). 

 Upwelling is caused along the peninsular coast by 

 the southerly winds prevailing during the 

 warmer months. This environment apparently 

 creates optimal conditions for the existence of the 

 euphausiid shrimp A^yc^/p/?a/?es simplex , which is 

 found in great abundance in the neritic habitat 

 between spring and midsummer, before the in- 

 tense August heat causes a decline in its numbers 

 (Brinton and Townsend 1980). The following data 

 are combined in Figure 6 to provide an overview 

 of the possible relationship between the seasonal- 

 ity of predator and prey in the study area: a) the 

 relative importance of Nyctiphanes simplex and 

 Mysidium sp. in the diet of M. thurstoni; b) the 

 relative abundance of A'^. simplex; the occurrence 

 of M. tarapacana (c) and M . munkiana (d) in the 

 catch; and the relative abundances of M .japanica 

 (e) and M . thurstoni (f) (no data on the biology of 

 Mysidium sp. are available). 



Young M . thurstoni and all M . munkiana ex- 

 amined in winter appeared to subsist largely on 

 Mysidium sp., whereas adult M. thurstoni and 

 M .japanica caught during the warmer months 

 fed exclusively on N . simplex . An extreme degree 

 of feeding specialization was evident in all mobu- 

 lid species in which quantitative analyses of the 

 stomach contents was possible; most prey forms, 

 other than N . simplex and Mysidium sp., were so 

 rare that they were probably ingested acciden- 

 tally. Stenophagy was linked to feeding special- 

 ization in another myliobatiform species, the 

 mollusk-feeder Rhinoptera bonasus (Schwartz 

 1966; Smith and Merriner 1985). These results 

 suggest that devil rays are highly efficient in lo- 

 cating and selecting their preferred food. They 

 may be aided during this behavior by their prey's 

 habit of swarming. Competitive interaction is to 

 be expected between sympatric species-pairs 

 which are closely related both taxonomically and 

 ecologically. Food-resource partitioning is known 

 to occur in sympatric species-pairs of skates 



63 



