Heteropoda
Geographic and vertical distribution - General patterns

Although we are uncertain about the occurrence of a number of species in the major oceans, our present understanding of global distribution patterns is that most species are cosmopolitan; 12 of 21 species of Atlantidae, 4 of 9 Carinariidae, and all Pterotracheidae.

(He. table Atlantidae)
(He. table Carinariidae)
(He. table Pterotracheidae)

These distributional data are provisional, insofar that they are valid only for the species recognized today. The latest descriptions of species new to science (van der Spoel and Troost, 1972; Richter, 1990, 1993; Seapy and Richter, 1993) show us that we cannot be certain whether or not we really know all extant heteropod species. Moreover, large areas of the oceans are still poorly investigated (among these the South Atlantic), and intensive studies in these regions may lead to the detection of still more species.

The Pacific (32 species) and Indian Oceans (31 species) have a higher species richness than the Atlantic Ocean (22-24 species). Only one species (Atlanta fragilis) is limited to the Atlantic Ocean, while two species (Carinaria japonica and Atlanta californiensis) are restricted to the Transition Zone faunal province in the North Pacific Ocean (Seapy, 1974; Seapy and Richter, 1993). Five atlantids (Atlanta echinogyra, Atlanta gibbosa, Atlanta meteori, Atlanta plana, Atlanta turriculata) and four carinarids (Carinaria cithara, Carinaria cristata, Carinaria galea, Pterosoma planum) have Indo-Pacific distributions.

Nearly all heteropods are epipelagic (Tesch, 1949; Thiriot-Quiévreux, 1973), although two species of Pterotrachea (Pterotrachea coronata, Pterotrachea scutata) and one species of Carinaria (Carinaria lamarcki) appear to be mesopelagic and undergo nocturnal vertical migration into the epipelagic zone (Pafort-van Iersel, 1983). Vertical distributions of heteropod larvae in the Mediterranean were reported by Thiriot-Quiévreux (1968) and Richter (1968, 1973b). In the latter study, vertical migration by heteropod larvae was recorded. Seapy (1990a) reported on day-night distributional patterns of atlantids (11 species) and pterotracheids (2 species) from the epipelagic zone off Hawaii. While diel differences could not be distinguished for upper epipelagic species, lower epipelagic species commonly migrated toward the surface at night. Michel and Michel (1991) reported on vertical distributions of heteropods from the Florida Straits. Low abundances of large atlantids (size range = 4 to 8.5 mm) were obtained, which can be attributed to the coarse mesh (3 mm) of the MOCNESS trawl. Nonetheless, their results for the atlantid Atlanta peroni supported the conclusion of Seapy (1990a) that this is a lower epipelagic species that migrates into the upper epipelagic at night.

Heteropods occur in moderately low to low abundances. Highest abundances have been recorded for atlantid species, although several pterotracheids (particularly Firoloida desmaresti) can be as abundant as many of the atlantids in a given location. Generally, however, atlantids are numerically dominant, followed by pterotracheids and carinarids. Quantitative abundance data (numbers per unit volume of water filtered) are limited, and the following examples are illustrative of this pattern. In Hawaiian waters the mean densities of eleven atlantid species ranged from less than one to a maximum of 25 per 1000 mö3, and two pterotracheids ranged from less than one to 7 per 1000 mö3 (Seapy, 1990a). In the California Current the carinarid Carinaria japonica occurred at densities of less than one animal per 1000 mö3 during most of the year, increasing during summer months to 1-10 per 1000 mö3 (Seapy, 1974). In the mid North Atlantic Pafort-van Iersel (1983) reported 0.2 to 3.3 individuals per 10,000 mö3 for two species of Carinaria and 0.2 to 9.8 per 10,000 mö3 for five species of Pterotrachidae. From the Florida Straits, Michel and Michel (1991) found that two pterotracheid and two carinarid species ranged from less than one to a maximum of 6 per 10,000 mö3 and less than one to 3 animals per 10,000 mö3, respectively.