Orcinus ater ( Cope in Scammon, 1869 )

Morin, Phillip A., McCarthy, Morgan L., Fung, Charissa W., Durban, John W., Parsons, Kim M., Perrin, William F., Taylor, Barbara L., Jefferson, Thomas A. & Archer, Frederick I., 2024, Revised taxonomy of eastern North Pacific killer whales (Orcinus orca): Bigg’s and resident ecotypes deserve species status, Royal Society Open Science (231368) 11 (3), pp. 1-23 : 16-17

Orcinus ater ( Cope in Scammon, 1869)

Etymology

In Latin, ater means black or dark, which probably refers to the largely black colour of this species.

Synonymy

Orca ater Cope in Scammon, 1869: 22 ; original designation.

Common name

We are planning on engaging with North American Indigenous tribal groups and expect to eventually have a consensus common name, but in the meantime, we suggest continued use of ‘resident killer whale’ so as to maintain consistency.

Type specimen

USNM 594672

No type specimen was preserved from the original description [125], so we have designated a neotype. The neotype is a physically mature male (total length 698 cm) in the U.S. National Museum of Natural History Marine Mammal Collection. The skull (CBL 1019 mm) is deposited under museum number USNM 594672 and was collected by C.H. Fiscus, H. Kajimura and M. Keyes on 28 February 1967. The skull was previously in the NOAA National Marine Mammal Laboratory collection as NMML 0089. It is illustrated in figure 7 b . mtDNA control region haplotype (160 bp) ‘SR’ [SWFSC ID 39071 in 142] unambiguously identifies this specimen as a resident killer whale. Skeletal morphological analysis of this specimen was included in Fung [103]. Detailed measurements of the type specimen are in electronic supplementary material, table S2.

Type locality

The neotype specimen was collected in Yukon Harbor , Puget Sound , Washington, USA (lat: 47.53, lon: −122.52).

Diagnosis

Resident killer whales are slightly smaller than Bigg’s, with total lengths not exceeding 725 cm in a moderate sample [103,143,145]. They have a shorter-based, more falcate dorsal fin that tends to be more rounded at the tip. The post-dorsal fin saddle patch generally does not extend forward much past the mid-point of the dorsal fin base. Individuals show a variety of saddle patch types, including closed (with no invasion of black), open (with a large invasion of black extending from above) and cupped (with a small ‘scoop’ or notch of black invading the saddle from above; e.g. [40, 79]). These correspond to all five patch types of Baird and Stacey [22].

Resident killer whales have smaller and more gracile skulls than Bigg’s, with the following measurements showing significantly lower average values: condylobasal length, postorbital width, occipital width, width of rostrum at base and length of mandible [103]. While individual measures show modal differences with some overlap, PCA of both cranial and mandibular morphology resulted in distinct clusters for Bigg’s and resident species ([103]; figure 2 ). Genetic analyses of mtDNA are diagnostic, based on fixed sequence differences ( table 1 ), while nuDNA allele frequency differences allow diagnosis based on cumulative assignment probability (e.g. assignment tests or PCA; figures 3 and 4 ).

Description

This is a species of killer whale, reaching total lengths of at least 725 cm in males and 644 cm in females [145]. The basic killer whale body plan includes a robust, spindle-shaped body, with a tall dorsal fin positioned at the centre of the back, large paddle-shaped pectoral fins, wide flukes with a slightly convex trailing edge and a blunt head ( figure 8 b ). There is a short, poorly defined beak. This species shows extreme sexual dimorphism, with males growing much longer and heavier than females; males also develop a tall (up to 2.25 m [149,150]) erect dorsal fin that may actually cant forward (making it look like it was put on backwards), and much larger flukes and pectoral fins. There is a straight mouthline, ending in a slight downturn at the gape. Resident killer whales show the basic colour pattern found in all members of the genus: most of the body is dark grey to black, with a white ventral field that branches into lobes extending up and back along the caudal peduncle. There are large white post-ocular patches, and a light grey to white ‘saddle patch’ originating behind the dorsal fin. The undersides of the flukes are mostly white, as is the entire lower jaw, but the flippers and dorsal fin are black on both sides. There are 10–14 large, conical teeth in each tooth row [144].

Comparison to other taxa

The resident killer whale is one of three proposed species of killer whales (genus Orcinus ) globally. It is endemic to the North Pacific Ocean and adjacent seas, and it preys primarily on bony fish, especially North Pacific salmon Oncorhynchus spp. (as opposed to marine mammals and elasmobranchs). It can be distinguished from other killer whale species and ecotypes by its genetic profile, its morphology and colouration (see Diagnosis above), and also acoustically.

Distribution

Resident killer whales occur in inshore and continental shelf waters, primarily from Oregon in the eastern Pacific to eastern Russia and northern Japan in the western Pacific [48,54,147,148,151]. Pods occasionally move southwards down the U.S. west coast to as far as Monterey Bay, CA [152]. They also inhabit the Okhotsk Sea and the southern Bering Sea ( figure 1 ).

Figures

Figure 1. Expected range maps for (a) resident and (b) Bigg’s killer whales, including locations of samples used for mitogenome analysis (figure 5a, resident n = 106, Bigg’s n = 93) [15]. Distribution ranges have been inferred based on published identifications of individuals that are identified by ecotype [48–53]. Sample distributions cover the known ranges of both ecotypes, with the exception of residents of Oregon and northern California,and both ecotypes off northern Japan (Hokkaido) inthe western Pacific [48,54].Sample maps for microsatellite data arein electronic supplementary material,figure S2.

Figure 2. Canonical variate 1 and 2 plots for cranial shape features that distinguish among ecotypes for (a) skull morphology (resident (n = 17), Bigg’s (n = 13) and offshore (n = 6)) and (b) dentary bone morphology (resident (n = 21), Bigg’s (n = 12) and offshore (n = 8) specimens) (reprinted from [103]).

Figure 3. PCA plot of first two principal components based on (a) 88 SNPs: offshore (n = 3), resident (n = 11), Bigg’s (n = 30) from data in Morin et al. [15]; (b) 26 microsatellites:offshore (n = 5), resident (n = 250), Bigg’s (n = 116) (samples genotyped at ≥20 loci) [56]; unpublished); (c) 3678 RADseq SNPs: offshore (n = 7), resident (n = 52) and Bigg’s (n = 37) populations [57,62]; (d) 1 00 000 (subset from 6 371 282) SNPs from 147 high-coverage genomes of offshore (n = 7), Bigg’s (n = 14) and resident (n = 126) samples from multiple geographically and behaviourally defined subpopulations (Alaska, northern and southern resident populations) (based on subset of SNP genotype data from [113].See Supplementary Materialsfor methods and data set information.

Figure 4. Structure assignment probability plots for K = 3 groups from (a) 26 microsatellites: offshore (n = 5), resident (n = 250), Bigg’s (n = 116) samples genotyped at ≥ 20 loci) (56; unpublished); (b) 3340 RADseq SNPs (polymorphic in sample set): offshore (n = 7), resident (n = 52) and Bigg’s (n = 37) populations [57,62]. Vertical bars represent the individual assignment probability for each group inferred by Structure (groups identified by shading), with samples sorted by a priori ecotype assignment. See electronic supplementary material for methods and data set information.

Figure 7. Photographs of neotype skullsfor (a) Orcinus rectipinnus (USNM 594671) and (b) Orcinus ater (USNM 594672).

Figure 8. Vertical images of (a) an adult male Bigg’s killer whale (BKW) from the West Coast Transient population of Bigg’s killer whales and (b) an adult male resident killer whale (RKW) from the sympatric Southern Resident population of resident killer whales. Images are scaled to the estimated asymptotic lengths of 7.3 m [20] and 6.9 m [145], respectively. Vertical images were collected using an octocopter drone usingmethods described by Durban et al. [146], provided by John Durban and Holly Fearnbach.