I. Characters and Taxonomic Relationships
The Atlantic spotted dolphin is not always spotted. A large heavy-bodied form found along the coast on both sides of the Atlantic (formerly called Stenella plagiodon along the U.S. coast) may be so heavily spotted as to appear white from a distance, but a smaller more gracile form occurring in the Gulf Stream and out into the central North Atlantic can be lightly spotted or entirely unspotted as an adult .A constant diagnostic external feature of S. frontalis is a spinal blaze sweeping up into the dorsal cape; this distinguishes it from the very similar pantropical spotted dolphin, S. attenuata, also found in the tropical Atlantic. In addition, the peduncle does not exhibit the dorsoventral division into darker upper and lighter lower halves present in S. attenuate!. The calf of the heavily spotted form is born unspotted, with a three-part color pattern of dark dorsal cape, medium-gray lateral field, and white ventral field. Spots first appear at 2-6 years and increase in size and density up to 16 years (Herz-ing, 1997).
Figure 1 Atlantic spotted dolphin in the Gulf of Mexico.
The beak is of medium length (intermediate between those of Tursiops truneatus and S. attenuata) and sharply demarcated from the melon. The dorsal fin is tall and falcate. Measured adults range from 166 to 229 cm in body length (n = 106) and weigh up to 143 kg (n = 37) (Nieri et al, 1999). Weight at length is greater than for S. attenuata.
As in S. attenuata, T. truneatus, and T. aduncus, the skull is characterized by a long rostrum, distal fusion of maxillae and premaxillae in adults, convergent prernaxillae, large rounded temporal fossae, and arcuate mandibular rami. Tooth counts are 32—42 in the upper jaw (n = 115) and 30-40 in the lower (n = 107) vs 35-48 (n = 315) and 34-47 [n = 315). This species and S. attenuata overlap in all skull measurements as well as in tooth counts. Both species vary greatly geographically. Some specimens of the two species can be identified only with multivariate analysis. However, vertebral counts for the two species do not overlap [67-72 (n = 52) in S. frontalis vs 74-84 (n = 75) in S. attenuata].
Taxonomy of the spotted dolphins was long confused, with specimens of this species and the pantropical spotted dolphin (S. attenuata) classified or identified under various permutations of the nominal species S. attenuata, S. frontalis, S. pla-giodon, S. froenatus, S. pernettyi, and S. dubia (e.g., see Her-shkovitz, 1966). A revision recognized one pantropical species (S. attenuata) and a second species endemic to the tropical Atlantic (S. frontalis), both highly variable geographically in size, tooth size, and color pattern. While the skull of the Atlantic spotted dolphin shows close affinities with that of the pantropical spotted dolphin, the two species did not emerge as sister taxa in a cladistic phylogenetic analysis based on cytochrome b mtDNA sequences (LeDuc et al, 1999). S. frontalis was imbedded in a strongly supported polytomic clade with S. eoeruleoalba and S. elymene (sister taxa), Tursiops aduncus, and Delphinus spp. T. truneatus was a sister taxon to this clade, with the resulting higher clade imbedded in the five-part polytomic delphrnine clade with S. attenuata, S. longirostris, Sousa ehinensis, and Lagenodelphis hosei. Despite a high degree of cranial similarity, this wide phylogenetic separation suggests that the similarity represents either convergence (homo-plasy) or retention of primitive character states (plesiomorphy). The interspecific relationships in color pattern may accord better with the molecular phylogeny; e.g., the pattern of head stripes in S. frontalis is closer to those of T. truneatus and T. aduncus than that of S. attenuata.In any case, the existing genus-level taxonomy of the group badly needs revision; Stenella is presently polyphyletic and Tursiops para-phyletic (LeDuc et al, 1999). A cladistic analysis of morphology (not yet attempted) is in order.
II. Distribution and Ecology
This species is endemic to the tropical and warm-temperate Atlantic; it is not known to occur in the Pacific or Indian Oceans. The range extends from about 50°N to about 25°S (Jefferson et al, 1993). In the western Atlantic, the large heavily spotted form inhabits shallow, gently sloping waters of the continental shelf and the continental-shelf break, usually within or near the 200-m curve but occasionally coming close to shore in pursuit of prey (Davis et al, 1998; Wirrsig et al, 2000). It is usually replaced in nearshore waters by the coastal form of the bottlenose dolphin, Tursiops truneatus. Shallow water (6-12 m) over sand flats is utilized as habitat in the Bahamas (Herzing, 1997). A wide variety of prey items has been recorded, including small-to-large epipelagic and mesopelagic fishes and squids and benthic invertebrates; diet may differ between coastal and Gulf Stream forms. Sharks are the only known predators, but it is probably also preyed on by killer whales and other small-toothed whales.
III. Behavior and Life History
Dives to 40-60 m and lasting up to 6 min have been recorded, but most time is spent at less than 10 m (Davis et al., 1996). Behavior of the Atlantic spotted dolphin has been studied extensively in the Bahamas (e.g., Herzing. 1997; Herzing and Johnson, 1997), where it associates closely with bottlenose dolphins during foraging and traveling. Schools may be segregated by age and sex and fluctuate in size and composition, consisting of up to 100 individuals.
Little is known of the life history of this species. Age at sexual maturation is estimated at 8-15 years in females (Herzing, 1997). First parturition is associated with the mottled phase of spotting development. The average calving interval is about 3 years, with a range of 1-5 years. Nursing has been observed to last up to 5 years. Average first-year natural mortality in a study in the Bahamas was 24%.
IV. Interactions with Humans
This species does not do well in captivity; most captive animals have died within a year or less, many refusing to eat. It is killed incidentally in fisheries in Brazil, the Caribbean, the western North Atlantic, and Mauretania, mostly in small numbers ( Nieri et al, 1999).
