Heteropoda
Distribution in the South Atlantic Ocean

Records of heteropods from the South Atlantic Ocean are very limited, and do not enable us to make any statements about biogeographical patterns. Although South Atlantic waters were sampled during the Bonite Expedition, Souleyet (1852a) did not give exact station locations for his species. Tesch (1949) reported eight species from the South Atlantic (Protatlanta souleyeti, Carinaria lamarcki, Cardiapoda placenta (only near the equator), Cardiapoda richardi, Pterotrachea coronata, Pterotrachea hippocampus, Pterotrachea scutata, and Firoloida desmaresti). These records are of limited value, however, since only a few stations were included in the South Atlantic during the Dana Expeditions. From nearshore waters in the eastern South Atlantic, Frontier (1968) named 13 species; Atlanta lesueuri, Atlanta gaudichaudi, Atlanta inclinata, Atlanta helicinoides, Atlanta fusca, Atlanta turriculata, Oxygyrus keraudreni, Protatlanta souleyeti, Carinaria lamarcki, Cardiapoda placenta, Pterotrachea coronata, and Pterotrachea scutata. From the western South Atlantic Magaldi (1977) examined 107 station samples collected by the R/V El Austral during December 1966 and January 1967 off the coasts of Brazil and Uruguay, between the equator and 35°S. He reported on nine species of Atlantidae (Oxygyrus keraudreni, Atlanta peroni, Atlanta gaudichaudi, Atlanta fusca, Atlanta lesueuri, Atlanta inclinata, Atlanta inflata, Atlanta meteori, and Atlanta helicinoides) and three Pterotracheidae (Pterotrachea hippocampus, Pterotrachea minuta, and Firoloida desmaresti). No Carinariidae were recorded, although he subsequently (Magaldi, 1984) reported a single specimen of Cardiapoda richardi collected by the R/V Atlantis II in March, 1971 at 37°48'S, 52°14'W.

He. table 2

In the numerous plankton samples taken by the R/V Meteor during Cruise 51 in the equatorial Atlantic Ocean (South Equatorial Current) between January and June
1979 (unpublished), all species of heteropods described in this paper were found.

He. table Atlantidae
He. table Carinariidae
He. table Pterotracheidae

The same collections provided data on the vertical distribution and vertical migration patterns for one carinarid and four pterotracheids (He. table 2). Cardiapoda placenta and Firoloida desmaresti were limited essentially to the upper 100 m of the water column, and the latter species was about four times as abundant as the former. Pterotrachea coronata was collected in all depth intervals above 250 m, but was most abundant below 100 m, while Pterotrachea scutata was only captured below 100 m, as was Pterotrachea hippocampus (with the exception of a single individual between the surface and 30 m). Because these data are from a single vertical profile, they should not be used to indicate ranked abundances of carinarid and pterotracheid species. For example, in February 1979 323 specimens of Firoloida desmaresti were captured (13 stations and 64 samples), which contrasts with the two Firoloida desmaresti collected in June 1979 from a comparable number of stations and samples. Examination of these samples for atlantids revealed that about 70% were found between the surface and 50 m, and more than 60% were between the surface and 30 m. About 25% were caught between 50 and 100 m, and only 5% below 100 m. An exception to these results was Atlanta fragilis, which lives deeper than other atlantids and appears to undergo a nocturnal vertical migration. In 27 tows (14 during the day and 13 at night) a total of 68 adult specimens was caught. During the day 30 were captured at depths between 100 and 250 m and none were present in the 0-30 m samples. At night 33 specimens were present in the 0-30 m samples, while 5 were in the 100-200 m samples.

Evaluation of neuston tows during Cruise 51 of the R/V Meteor suggests that heteropods belonging to different families, genera and species form hugh swarms at the surface, consisting exclusively of mature males of comparable size. The numbers of individuals belonging to three different species (Atlanta oligogyra, Protatlanta souleyeti and Firoloida desmaresti) from neuston tows and oblique (0-250 m), open net tows are compared inHe. table 3. For each species all of the specimens examined from the neuston tows were mature males, while the specimens from the oblique tows were a mixture of larval stages and males and females of different sizes. Seapy and Richter (1993) reported comparable neuston tow records for Atlanta californiensis off Southern California. Surface aggregation of animals of not only one species, but also of only one sex and age points to a purposeful act. The digestive tracts of the animals were empty, so there is no evidence that males assemble at the surface for feeding purposes. Although we know literally nothing about the sexual behavior of heteropods, we speculate that such swarms are somehow involved in mating. Hypothetically, a female swims into an all-male swarm at the surface, couples with a male, and drops beneath the surface to mate (similar to the mating behavior seen in mosquitos; discussed in Seapy and Richter, 1993). We have no knowledge of how long males remain at the surface; swarms may form intermittently or they may break up more or less passively with the slow movements and mixing of water masses of different origin and composition. Adult male atlantids were found by Seapy (1996) to serve as the exclusive substratum for egg attachment by the pleustal water skater Halobates sericeus off Hawaii. The only logical explanation for this selectivity is that only adult males were available to the water skaters at the ocean surface. All-male swarming behavior is not comparable with any other form of reported aggregation behavior (whether in heteropods or other pelagic taxa), since they include both males and females, and could include larval stages.

The R/V Meteor Cruise 51 data also provide an indication of the high densities that atlantids can achieve in the tropical Atlantic Ocean. All recognized larval stages are included in these data and account (in most cases) for 80-99% of the totals: Atlanta inflata (171 per 100 mö3), Protatlanta souleyeti (80 per 100 mö3), Atlanta peroni (72 per 100 mö3), Oxygyrus keraudreni (68 per 100 mö3), Atlanta helicinoides (67 per 100 mö3), and Atlanta oligogyra (41 per 100 mö3). These maxima should only be used to suggest an approximate ranking and magnitude of difference in abundances. Actual abundances can vary in relation to location, seasons, tow depths, etc. It is well known that the thecosome pteropods, which overlap spatially with the atlantids in the epipelagic zone, are far more abundant. In the above samples the highest maxima among the thecosomes were 2,041 per 100 mö3 (for Limacina inflata) and 302 per 100 mö3 (for Creseis virgula). Even a group of neritic origin, the pelagic larvae of Architectonicidae, showed a higher frequency (395 per 100 mö3).