Mitochondrial and nuclear DNA sequences were successfully obtained from an unidentified dead baleen whale, 7.7 m in length that was found near Lake Point in Marsh Island Refuge, Iberia Parish, Louisiana, USA (29.55°, −91.72°; Figs. 1 & 2, Additional file 1: Table S1) in 2013.
Phylogenetic analyses of 344 base pairs (bp) of the mitochondrial DNA (mtDNA) control region [GenBank Accession KU215790] placed the unknown stranded whale from the northern Gulf of Mexico firmly in the B. bonaerensis clade (Fig. 3) using both MrBayes and DNA Surveillance (posterior probability = 1.0, bootstrap value =100 %). Application of DNA Surveillance to the cytb sequence [GenBank Accession KU215791] returned the same result (data not shown).
A total of 1963 bp, 2278 bp, and 2277 bp of DNA sequence spanning four nuclear loci was obtained for the unidentified baleen whale, and for B. acutorostrata and B. edeni samples, respectively [GenBank Accessions KU215771-KU215789]. The total sequence length was shorter for the unidentified whale due to an indel in one allele of the CAT locus, rendering downstream sequence unreadable. After the addition of available GenBank sequences (Additional file 1: Table S2; B. bonaerensis sequences were not available in Genbank for INT), each alignment was truncated to the shortest sequence length resulting in final alignments of 430 bp, 294 bp, 508 bp, and 663 bp for AMBN exon 13, CAT, INT and LALBA, respectively. We identified a total of 58 variable sites and four 1 bp indels and one 21 bp indel within the truncated, aligned 1895 bp of nuclear data when comparing the three balaenopterid species (Fig. 4). For CAT, the unknown baleen whale sequence was identical to seven GenBank entries for known B. bonaerensis samples from Jackson et al. (Jackson et al 2009) and no fixed differences or indels were present in comparison to the other available Antarctic minke whale sequences (Fig. 4). On the other hand, there were three fixed differences and a single base pair indel between the unknown whale and the 13 B. acutorostrata samples at this locus. This animal exhibited from 0 (INT) to 12 (AMBN exon 13) fixed differences from common minke whales and a 21 bp deletion in INT further distinguishing it from common minke whales and Gulf of Mexico Bryde’s whales. Overall, in comparison to the common minke whale and the Gulf of Mexico Bryde’s whale, the unknown stranded whale exhibited a total of 16 and 19 fixed differences, respectively, across the four loci (Fig. 4).
In combination, the mitochondrial and nuclear DNA sequence data indicate that the whale that stranded in the northern Gulf of Mexico was indeed an Antarctic minke whale and further that it was not a hybrid of the two minke whale species. Multiple species-specific nucleotide differences and indels were found between the unknown stranded whale and B. acutorostrata and B. edeni nuclear sequences. The strongest evidence comes from CAT where the unknown whale exhibited sequence identical to seven Antarctic minke whales available in GenBank and from INT where the unknown whale had a 21 bp deletion when compared to the common minke whale (14 individuals). If the stranded animal were a hybrid with a common minke whale, it should have exhibited the longer and shorter alleles, which would have rendered the sequence unreadable at the point of the indel. This was not the case.
This stranding represents the first record for an Antarctic minke whale in U.S. waters of the western North Atlantic and GOMx. In general, common minke whales are uncommon in the GOMx. (Jefferson and Schiro 1997) present 10 records they deemed reliable after thoroughly examining stranding data and observer data. All records are ascribed to the common minke whale, all are strandings and the majority are from winter months. Jefferson and Schiro 1997) and Mitchell (1991) suggested that the winter pattern of strandings is indicative of strays during northward migration from lower-latitude breeding grounds. This explanation cannot account for the presence of this Antarctic minke whale in the Northern Hemisphere as during February, the austral summer, this species is found on its feeding grounds in the Antarctic southward of 60°S latitude. The stranded whale was a female and at 7.7 m in length is slightly smaller than expected for a sexually mature female of either the Antarctic minke whale or the common minke whale in the North Atlantic, and slightly longer than expected for a sexually mature female common minke whale from the Southern Hemisphere (Deméré 2014). Photos of the skull also support that the stranded whale was a minke whale (Fig. 5).
Although a very surprising result, this animal is not the first record for Antarctic minke whale in the Northern Hemisphere (Fig. 1). Rice 1998) reported a specimen from the Atlantic coast of South America in Suriname. More recently, Glover et al. (2010) identified a male Antarctic minke whale north of the Arctic Circle in the eastern North Atlantic using genetic analysis. Finally, a relatively small (~6 m) whale was entangled in purse seine net and brought to shore in the Gulf of Guinea, Togo where it was identified as a juvenile Antarctic minke whale based on morphological characteristics (Segniagbeto et al. 2014).
Recently, minke whale hybrids have been detected in the eastern North Atlantic based on genetic analysis (Glover et al. 2010; Glover et al. 2013). Interestingly, the direction of hybridization was different in the two cases. One female whale captured during the Norwegian commercial harvest in 2007 was the product of a female Antarctic minke whale and male B. acutorostrata, most likely of the North Atlantic subspecies B. acutorostrata acutorostrata, but which subspecies was not fully resolved (Glover et al. 2010). The second whale was also a female, taken in 2010. This whale was identified as the product of a female B. acutorostrata acutorostrata and male Antarctic minke whale; it was also pregnant, suggesting these hybrids are capable of reproducing, and the fetus was determined to be a backcross into B. acutorostrata acutorostrata (Glover et al. 2013).
Photos taken of the stranded whale from the Gulf of Mexico indicate a solid white ventral side with throat grooves that do not extend to the umbilicus. However, much of the epidermis of the animal was abraded away and therefore the complete color pattern is difficult to deduce. No photos of the baleen, a diagnostic characteristic used to separate the two minke whale species, were available. Photos of the pectoral fin, a feature that can be useful for distinguishing among minke whale species and subspecies, do not show evidence for the bright white flipper patch distinctive of the common minke whale in the North Atlantic (Deméré 2014), further supporting the diagnosis of an Antarctic minke whale. The pure Antarctic minke whale from the Northeast Atlantic identified by Glover et al. 2010) also lacked the flipper patch as expected, but at least one of the 2 hybrids did not differ morphologically from that expected from B. acutorostrata acutorostrata, i.e., it did exhibit the flipper patch (Glover et al. 2010). The ventral side of the flipper of the stranded whale appears to be solid white. The dorsal side is dark with a white trailing edge. White also appears on the body above the insertion of the pectoral fin, a characteristic reminiscent of the dwarf minke whale, an unnamed subspecies that is also restricted to the Southern Hemisphere.
One unanswered question across these sightings is why Antarctic minke whales are so recently being found in the Northern Hemisphere. Perhaps there has always been a low level of ‘leakage’ of Antarctic minke whales into the Northern Hemisphere. Increased interest and effort to recover and identify stranded and bycaught animals coupled with advances in molecular genetic techniques for species identification may simply now be better able to reveal the presence of the species in the Northern Hemisphere. Alternatively, could climate change and the coincident changes in the Antarctic ecosystem play a role, as postulated by Glover et al. (2013)? Whatever the cause, the increase in documented cases of inter-ocean movements has relevance for understanding and tracking future changes in distributions and habitat usage, as well as tracing of disease outbreaks in cetaceans. Cross-hemisphere movements of whales have generally been thought to be a rare event and transfer of a disease epidemic across the equator rarely considered. Continued examination of recovered whale carcasses coupled with genetic analysis will play an important role in documenting whether these events are rare or represent a real shift in large whale movements in the world’s oceans.