Orcinus rectipinnus ( 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 : 44-46

Orcinus rectipinnus ( Cope in Scammon, 1869)

Etymology

In Latin, recti means right or upright, and pinna means fin, feather or wing, most likely referring to the tall, erect dorsal fin of males.

Synonymy

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

Common name

We propose continued use of the common name, ‘Bigg’s killer whale’, for this species, to honour Dr. Michael A. Bigg (1939–1990), who pioneered the study of North Pacific killer whales in the 1970s. This ecotype was formerly known as the ‘transient killer whale’.

Type specimen

USNM 594671

No type specimen is extant from the original description (Cope in Scammon [125]), so we have designated a neotype. The neotype is the skull of a physically mature male (total length 731 cm, CBL of cranium 1124 mm) in the U.S. National Museum of Natural History Marine Mammal Collection, deposited under museum number USNM 594671. The skull was previously in the NOAA National Marine Mammal Laboratory collection as NMML 0082. It is illustrated in figure 7 a . mtDNA control region haplotype (160 bp) ‘T’ (SWFSC ID 39064 in 142]) unambiguously identifies this specimen as a Bigg’s killer whale. 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 was collected by J.E. Eckberg on 22 September 1966, near San Francisco , CA, USA .

Diagnosis

Bigg’s killer whales differ from residents in growing to somewhat larger sizes, and having a widerbased, more triangular dorsal fin that is more pointed at the tip. The dorsal fin also tends to be less falcate (even in females). The saddle patch behind the dorsal fin extends further forward than it does in residents, usually to well past the mid-point of the dorsal fin base, and may appear larger compared to that of residents [40,79]. Virtually all saddle patches are closed (with no significant invasion of black), and many are rounded, with their forward extensions not ending in a point. These correspond to the ‘smooth’ or ‘bump’ patch types of Baird and Stacey [22].

Bigg’s killer whales have longer and more robust skulls than residents, with the following measurements showing significantly greater 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, canonical variate analysis of both cranial measures and measures of mandibular morphology yields distinct clusters for the two ecotypes ([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 830 cm in males and 710 cm in females [20] and weights of 6600 kg in males and 4700 kg in females [143]. It has the basic features of the killer whale body plan: a robust body with a tall dorsal fin near the centre of the back, large paddle-shaped flippers, broad flukes with a slightly convex trailing edge and a blunt head with a short, poorly defined beak ( figure 8 a ). There is sexual dimorphism, with males growing much larger than females, and near sexual maturity developing a tall (up to at least 1.5 m), erect dorsal fin and much larger flukes and pectoral fins. The mouthline is straight, with a small downturn at the gape. The basic killer whale colour pattern is largely dark grey to black, with a white ventral field that has lobes extending up and back along the tail stock, a white post-ocular patch, and a light grey to white ‘saddle patch’ behind the dorsal fin. The lower jaw and the undersides of the flukes are mostly white, but the entire flippers and dorsal fin are black. The areas of light and dark are generally well defined, with a crisp border. Each tooth row of both upper and lower jaws contains 10–14 large, conical teeth [144].

Comparison to other taxa

Bigg’s 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 marine mammals (as opposed to bony fish 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

Bigg’s killer whales occur throughout the waters of the eastern North Pacific Ocean from at least northern Baja California, Mexico, through to eastern Russia and northern Japan in the western Pacific [48,147,148]. Their distribution extends to the Okhotsk Sea and Arctic Ocean (e.g. Chukchi Sea; figure 1 ). While they are most commonly observed over the continental shelf and in inshore waters, they may also occur in oceanic waters beyond the continental shelf edge [147].

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.