NOTARBARTOLO-Dl-SCIARA: NATURAL HISTORY OF MOBULA 



found was Pupulina cf. minor (Copepoda: Caligi- 

 dae) from the skin. 



Among M. miinkiana's distinguishing features 

 are its neritic preferences combined with its so- 

 cial habits. This is the only mobulid species in the 

 Gulf of California that was consistently seen in 

 schools. It is not known whether this is a seasonal 

 behavior, or a permanent ethological feature of 

 the species. Schools appear as a conspicuous dark 

 patch, sometimes a few tens of meters in diame- 

 ter, as they slowly cruise along the coastline in 

 shallow water. The presence of the school is often 

 also highlighted by the frequent, simultaneously 

 leaping individuals, which betray its position 

 from a long distance. Similar behavioral traits 

 (schooling and leaping) have been reported for 

 two closely related species from the Atlantic, 

 M. hypostoma (Bancroft 1829; Coles 1910, 1916a) 

 and M. rochebrunei (Cadenat 1960). During 

 leaps, M. munkiana occasionally reached a 

 height of about two DWs. Two types of leaps were 

 observed: rising vertically head first and landing 

 flat with the belly on the sea surface with a loud 

 clap (breach), and spinning one to three times 

 around the main transverse body axis (somer- 

 sault). 



A salient feature of M. munkiana's ecology in 

 this area is its winter occurrence when all other 

 mobulids are absent or at their lowest numbers. 

 Mobula munkiana apparently subsists then 

 chiefly on the mysid shrimp, Mysidium sp., which 

 is also the main food for young wintering M. 

 thurstoni . However, M . munkiana frequents the 

 area occasionally in summer: two specimens were 

 caught by surface gill net in Bahia de la Ventana 

 in July 1983. Even during the season in which it 

 is most abundant, M. munkiana is seen in 

 "pulses", as its occurrence at any particular loca- 

 tion is spotty. It may occur in large numbers at 

 one location for a few days, and then be absent for 

 1 or 2 weeks. This observation suggests the possi- 

 bility that M . munkiana lives in large concentra- 



tions, perhaps composed by several schools, which 

 travel along the coast. A similar phenomenon was 

 observed off the Senegal coast by Cadenat (1960) 

 in M . rochebrunei, a species which is closely re- 

 lated to M. munkiana both morphologically and 

 ecologically. 



It is conceivable that mobulids in the northern 

 half of the eastern tropical Pacific mate and give 

 birth in summer, based on the few term and near- 

 term embryos found in summer in M . thurstoni 

 and M . japanica, and from anatomical evidence 

 of mating activity in adult males M . thurstoni 

 and M .japanica . That such a hypothesis can also 

 be extended to M . munkiana is supported by lack 

 of reproductive activity in any of the specimens 

 collected during the fall or winter, and that new- 

 born and young-of-the-year are missing from the 

 sample. This evidence corroborates the hypothe- 

 sis that the local waters are a wintering ground 

 for M. munkiana, which then migrates into an 

 unknown area (perhaps the northern Gulf of Cali- 

 fornia) during the warmer season for mating and 

 pupping. 



The possible causes of this ecological difference 

 between M. munkiana and the other mobulids 

 are many, and open to speculation. Mysid abun- 

 dance may be declining in summer in the south- 

 ern Gulf of California, and M . munkiana perhaps 

 migrates to areas where this crustacean or re- 

 lated species abound during the warmer season. 

 Alternatively, M. munkiana could be excluded 

 from this region in spring by competition with the 

 incoming, larger M. japanica and adult M. 

 thurstoni. Finally, M. munkiana may be moving 

 during the summer into an area which is more 

 suitable for its reproductive needs. Unfortu- 

 nately, this recently discovered species is very 

 little known, and it has been reported only from 

 the Gulf of California and Ecuador, although its 

 distribution probably extends to other coastal 

 areas of the tropical east Pacific (Notarbartolo-di- 

 Sciara 1987). 



Table 6^ — Prey species found in three stomachs of Mobula munkiana ranked by decreasing 

 Index or Relative Importance (symbols as m Table 2). Identifiable copepod species included 

 Undinula vulgans, Rhincalanus nasutus, and Scolecithnx danae. The stomatopods found 

 were "erhithrus" larvae. One unidentified food item was a fragment of a larger crustacean, 

 probably an euphausnd. 



Prey species 



N 



°/oN 



M 



°/oM 



%F 



IRI 



%IRI 



Mysidium sp. 293.40 97.80 287.40 95.80 100.00 19.360 



Stomatopod larvae 4.65 1.55 9.06 3.02 66.67 304 



Copepoda 1.59 0.53 1.44 0.48 66.67 67.3 



Other 0.37 0.12 2.10 0.70 66.67 54.7 



97.84 

 1.54 

 0.34 

 0.28 



61 



