Lanternfish

Lanternfishes (Myctophidae) are a family of small, bioluminescent mesopelagic fishes in the order Myctophiformes, characterized by their species-specific patterns of photophores that produce glowing blue-green lights for camouflage, communication, and predator evasion.^[1] With approximately 252 valid species across 34 genera, they represent one of the most diverse and abundant vertebrate groups in the ocean, comprising over 60% of the biomass of deep-sea fishes.^[2] These fishes typically measure 2–15 cm in length, though some reach up to 30 cm, and possess large eyes adapted for low-light conditions, along with slender bodies and adipose fins.^[1][3] Found throughout the world's oceans from the Arctic to Antarctic regions, lanternfishes primarily inhabit the mesopelagic or "twilight" zone at depths of 200–1,000 meters during the day, where sunlight barely penetrates.^[3] They exhibit pronounced diurnal vertical migration, ascending to the surface at night to feed on zooplankton and returning to deeper waters by dawn to avoid predators, a behavior facilitated by gas-filled swim bladders for buoyancy control.^[1] This migration, observable via sonar as a "false bottom" layer, underscores their massive scale, with global biomass estimates ranging from 600 million to 1 billion metric tons.^[4][5] Ecologically, lanternfishes play a pivotal role as a foundational prey species in marine food webs, supporting predators such as tunas, sharks, squid, and marine mammals, while also contributing to nutrient cycling through their migrations that transport organic matter vertically.^[1] Their bioluminescence, generated by luciferin-based reactions in specialized photocytes within photophores, enables counter-illumination to match downwelling light and evade detection, as well as rapid flashes from caudal organs to disorient pursuers.^[3] Despite their ubiquity and ecological significance, lanternfishes remain understudied due to the challenges of deep-sea sampling, though ongoing research highlights their vulnerability to ocean changes like warming and deoxygenation.^[6]

Taxonomy and Classification

Family Overview

Lanternfish, also known as myctophids, constitute the family Myctophidae within the order Myctophiformes, which encompasses two families: Myctophidae and the less diverse Neoscopelidae.^[7] This classification places them in the superorder Scopelomorpha of the class Teleostei, reflecting their position as small, specialized mesopelagic fishes.^[8] The family name Myctophidae derives from the Greek words myktēr (nose) and ophis (serpent), alluding to the elongated snout observed in certain species.^[9] Key diagnostic traits of Myctophidae include their small-bodied, elongate, and compressed form as teleosts, typically ranging from 2 to 30 cm in length, with a conspicuous adipose fin located behind the dorsal fin.^[10] They possess cycloid scales (ctenoid in a few species), a swim bladder in most members, and 28-45 vertebrae, along with a subocular shelf and a cartilaginous plate beneath the adipose fin.^[9] The anal fin originates below or slightly behind the dorsal fin base, and many species feature distinct patterns of photophores—bioluminescent organs—on the head and body, though one species lacks them entirely.^[10] These adaptations suit their mesopelagic lifestyle, enabling diurnal vertical migrations between depths of 300-1200 m during the day and 10-100 m at night.^[9] As of November 2025, the family Myctophidae comprises 253 valid species across 34 genera, a figure informed by ongoing taxonomic revisions integrating molecular phylogenetics and morphological analyses.^[11] These updates have refined subfamilial divisions, including Myctophinae, Lampanyctinae, Diaphinae, Gymnoscopelinae, and Notolychninae, based on osteological and genetic data, enhancing understanding of their evolutionary relationships within the order.^[12]

Genera and Species Diversity

The family Myctophidae exhibits substantial taxonomic diversity, comprising 34 genera and 253 species as recognized in recent phylogenetic analyses.^[11] This richness underscores the family's prominence among mesopelagic fishes, with species distributed across all major ocean basins. Key genera include Diaphus, the most speciose with 79 species characterized by varied photophore patterns and body proportions; Lampanyctus, encompassing 39 species noted for their elongated forms and widespread occurrence; and Myctophum, with 11 species featuring distinctive spotted or metallic appearances.^[13][14][15] Species diversity within Myctophidae is greatest in tropical and temperate oceanic regions, reflecting adaptations to stable mesopelagic conditions in these areas.^[16] Recent additions to the known diversity have emerged from DNA barcoding efforts, including the revelation of cryptic speciation in Benthosema species complexes through COI gene analysis in Indian Ocean surveys.^[17] For instance, molecular studies have identified previously unrecognized lineages in Benthosema fibulatum, contributing to updated species counts in regional faunas as of 2024.^[18] Endemism patterns highlight the role of isolated habitats in driving Myctophid diversification, with elevated levels observed in seamount ecosystems and polar waters; for example, Diaphus balanovi is restricted to the Emperor Seamount chain.^[19] In polar regions, species like Gymnoscopelus opisthopterus demonstrate localized distributions tied to Antarctic convergence zones.^[20] These patterns are influenced by limited gene flow across bathymetric and oceanographic barriers. Ongoing taxonomic revisions are necessitated by the prevalence of cryptic species, particularly those distinguished via COI gene sequencing, which has uncovered hidden diversity in genera like Ceratoscopelus and prompted re-evaluations of morphological boundaries across the family.^[21] Such molecular approaches reveal that traditional classifications based on photophore arrangements and meristics often overlook genetic discontinuities, leading to dynamic updates in species delineations.^[17]

Physical Characteristics

Morphology and Anatomy

Lanternfish (family Myctophidae) possess an elongated, fusiform body shape that facilitates streamlined movement through the water column in their mesopelagic habitat.^[22] This body form is characterized by a terminal mouth equipped with small, non-fang-like teeth extending to or beyond the posterior margin of the eye, moderate to large eyes positioned high on the head, a single dorsal fin with a short base, an adipose fin, and abdominal pelvic fins.^[22] The anal fin typically originates under or slightly behind the dorsal fin, while the caudal fin is deeply forked, aiding in agile navigation at depths up to 1000 m.^[1] Scales cover the body, contributing to a silvery or black coloration that provides camouflage in low-light conditions.^[22] In terms of size, lanternfish are generally small, ranging from 2 to 30 cm in total length, though most species measure under 15 cm.^[1] Photophores, the bioluminescent organs integral to their morphology, are arranged in species-specific patterns on the body and, in some genera such as certain species of Lampanyctus, absent on the head.^[23] Sexual dimorphism is evident, with females typically larger than males at maturity, though differences in fin ray counts are less pronounced and vary by species.^[3] Internally, lanternfish exhibit adaptations suited to high-pressure, low-oxygen deep-sea environments, including poorly calcified bones that reduce density and enhance buoyancy alongside high lipid content in tissues.^[3] The swim bladder is often reduced, atrophied, or absent in adults, with buoyancy primarily maintained through fat-invested structures or residual gas in juveniles; for example, species like Stenobrachius leucopsarus rely on lipid sacs as they mature, allowing specific gravity close to that of seawater (1.026–1.030) for vertical migrations.^[24] In species such as Lampanyctus regalis and Tarletonbeania crenularis, the swim bladder becomes highly reduced, increasing specific gravity to around 1.040–1.088 and necessitating alternative buoyancy mechanisms.^[24] Sensory structures are highly specialized for dim-light conditions, featuring large eyes with pure rod retinas that achieve exceptionally high rod densities—the highest recorded among vertebrates—for maximal photon capture.^[25] These retinas include thin photoreceptors and specializations like a ventro-temporal area for detecting downwelling light, with visual acuity ranging from 1.6 to 4.8 cycles per degree.^[25] The lateral line system, while not uniquely enhanced beyond typical teleost configurations, supports detection of water movements in the stable deep-sea environment.^[26]

Bioluminescence and Adaptations

Lanternfish possess specialized photophore organs distributed across their ventral, lateral, and head regions, which generate blue-green bioluminescent light through the oxidation of coelenterazine luciferin by a luciferase enzyme in the presence of oxygen.^[2] These photophores, numbering up to over 100 per individual in some species, consist of photogenic cells (photocytes) housed within a pigmented cup often backed by a reflective guanine layer to direct and intensify the emission.^[27] The light produced peaks at wavelengths around 454 nm, facilitating effective penetration through the deep-sea water column where red light is scarce.^[28] The bioluminescent system in lanternfish is intrinsic and non-symbiotic, relying on luciferin acquired through diet rather than bacterial partnerships, though the exact luciferase enzyme remains under investigation with candidates like 14-3-3 proteins proposed in recent analyses.^[2][29] Neural innervation from spinal and sympathetic fibers enables precise control over light intensity and timing, allowing rapid adjustments in response to environmental cues.^[3][30] These adaptations serve key functions, including counter-illumination, where ventral photophores emit light matching the intensity and spectrum of downwelling surface light to erase the fish's silhouette against the brighter waters above, thereby evading predators below.^[28] Species-specific photophore patterns also aid in conspecific recognition, with sexual dimorphism in arrangements facilitating mate attraction, as evidenced by studies showing divergent patterns between males and females that promote speciation in the dark mesopelagic.^[31] In genera like Lampanyctus, patterns tend toward symmetrical ventral arrangements, contrasting with the more asymmetrical and complex configurations in Diaphus, which include proliferated headlight structures; recent 2025 comparative analyses confirm these variations enhance reproductive isolation.^[32][33]

Habitat and Distribution

Global Range and Environments

Lanternfishes (family Myctophidae) exhibit a cosmopolitan distribution across all major ocean basins, including the Atlantic, Pacific, Indian, and Southern Oceans, though they are largely absent from permanently ice-covered polar regions such as the central Arctic.^[34] Their highest abundances occur in the epipelagic to mesopelagic zones, spanning depths from approximately 0 to 1000 meters, with primary residency in the mesopelagic layer (200–1000 m) where they form a substantial portion of the global fish biomass.^[35] This wide-ranging presence is facilitated by their adaptability to open-ocean pelagic environments, though populations are notably scarce in coastal and shelf waters (typically within the 200 m isobath).^[16] Environmental preferences of lanternfishes favor temperate to tropical waters, with tolerance for temperatures ranging from 4°C to 29°C and oxygen levels between 2.5 and 4.4 ml/L, allowing them to inhabit vertically stratified zones influenced by thermoclines and oxygen minimum layers.^[34] These conditions are prevalent in mid-latitude and equatorial regions, where oceanographic features like currents and fronts shape assemblage structures; for instance, in the Southwestern Atlantic, distributions align with water masses such as the Brazil and Malvinas Currents.^[16] Larval stages typically occupy surface waters (0–100 m), while adults show vertical stratification tied to these physicochemical gradients.^[35] Regionally, lanternfishes dominate mesopelagic biomass in the Pacific Ocean, which accounts for the largest share of global oceanic volume.^[36] Biodiversity hotspots are concentrated in the Indo-Pacific, particularly the Southern Central Pacific ecoregion, where species richness peaks due to favorable thermal and productivity regimes.^[37] In the Southern Ocean, modeling indicates a projected expansion of subtropical lanternfish distributions driven by climate-induced warming that shifts temperature barriers like the Subtropical Front, leading to a circumpolar increase in subtropical species range by up to 15.5 million km² under high-emission scenarios by 2081–2100.^[38] As of 2025, ongoing ocean deoxygenation is compressing suitable habitats in oxygen minimum zones, potentially reducing vertical range for some species.^[39] Habitat types vary between vast open-ocean expanses, where most species thrive as pelagic migrants, and seamount-associated populations that aggregate around topographic features for enhanced productivity.^[34] These seamount habitats support localized high abundances, contrasting with the diffuse distributions in oligotrophic open waters, though all populations remain fundamentally oceanic rather than neritic.^[16]

Vertical Migration Patterns

Lanternfish, members of the family Myctophidae, predominantly exhibit diel vertical migration, ascending from mesopelagic depths of 200–1000 m during the day to the epipelagic zone (0–200 m) at night, with the reverse descent occurring around dawn.^[40] This pattern is observed across global oceans and is highly synchronous, with upward migrations typically beginning 17 ± 23 minutes after sunset and completing within 30–120 minutes of full darkness, while downward migrations initiate about 21 ± 20 minutes before sunrise and finish 10–50 minutes after dawn. Velocities during these migrations average 6.5 cm/s upward and 7.6 cm/s downward, reflecting a slightly faster descent. The migrations are primarily synchronized by light cycles, with phototaxis—positive at night and negative during the day—serving as a key proximate mechanism, often modulated by changes in irradiance at dawn and dusk.^[41] Endogenous circadian rhythms also contribute, maintaining periodicity even under constant conditions, though external light cues dominate initiation.^[41] Depth preferences vary by species; for instance, Myctophum species often migrate to shallower epipelagic depths (0–100 m at night) compared to Benthosema species, which remain below 200 m nocturnally and descend to 500–750 m diurnally. Most lanternfish species participate in these migrations, with at least 73% observed to do so in a Southwestern Atlantic study, forming a substantial portion of vertically migrating mesopelagic biomass.^[42] Variations in migration patterns occur seasonally and regionally, influenced by lunar cycles, temperature gradients, and prey availability. Lunar illumination can limit ascent depths during full moons, reducing upward excursions by up to 50 m in some populations to minimize visibility. Temperature differences across layers (up to 20°C between day and night depths) and seasonal shifts in prey distribution alter migration amplitudes; for example, in the North Atlantic, Benthosema glaciale shows normal diel vertical migration in spring and summer but switches to inverse or no migration in autumn and winter when prey concentrates mid-water.^[43] Acoustic surveys from 2017 in the South China Sea confirm these dynamics, with 53.5% of migrating mesopelagic species being lanternfish and regional biomass participation ranging from 25–58% across seasons.^[44] While most species migrate, some deep-water taxa, such as certain Sternoptyx congeners, exhibit no diel movement, residing below 300 m year-round.^[44]

Ecology and Behavior

Feeding and Diet

Lanternfish primarily consume zooplankton, with diets dominated by crustaceans such as copepods, euphausiids, amphipods, and ostracods, alongside occasional small fish and gelatinous organisms.^[45][46] Larval lanternfish exhibit ontogenetic shifts in feeding, initially targeting smaller prey like protists, copepod nauplii, and ostracods before transitioning to larger crustaceans as they mature into juveniles and adults.^[47] While some species incorporate minor amounts of phytoplankton, zooplankton forms the bulk of their intake across life stages. As gape-limited predators, lanternfish employ suction feeding mechanisms to capture prey, expanding their mouths rapidly to create a vacuum that draws in small, mobile zooplankton.^[48] Their foraging is enhanced by acute visual adaptations, including eyes tuned to detect bioluminescent signals, which aid in locating prey during nocturnal vertical migrations to surface waters.^[25] This opportunistic strategy allows them to exploit abundant zooplankton patches in the epipelagic zone at night while minimizing competition and predation risk.^[49] Stable isotope analyses position lanternfish as mid-level carnivores in pelagic food webs, with nitrogen isotope ratios (δ¹⁵N) typically ranging from 8 to 12‰, reflecting a trophic level of approximately 2.5 to 3.0 above primary producers and herbivores.^[50][51] These values confirm their role as secondary consumers reliant on herbivorous zooplankton, with regional variations influenced by baseline isotopic signatures in different ocean basins.^[52] Seasonal variations in lanternfish diets involve prey switching in response to zooplankton blooms, such as increased consumption of copepods during summer upwelling periods and a shift toward euphausiids in winter.^[47][53] This adaptability supports higher lipid storage in winter, enabling energy reserves for overwintering and pre-spawning periods when prey availability declines.^[54]

Reproduction and Life Cycle

Lanternfish (family Myctophidae) are oviparous, releasing buoyant eggs into the water column where external fertilization occurs.^[10] Spawning typically takes place in surface waters during batch events, with females capable of multiple spawning cycles per year, often aligned with seasonal productivity peaks.^[55] This reproductive strategy supports high larval survival in nutrient-rich epipelagic zones, though overall fecundity remains low compared to other marine fishes.^[10] The life cycle begins with pelagic eggs that hatch into larvae, which remain in the upper water layers to feed on plankton. Metamorphosis from larva to juvenile occurs at standard lengths of 10-20 mm, marked by the development of photophores and fin rays.^[56] Juveniles gradually descend to mesopelagic depths as they grow, adopting the diel vertical migration patterns of adults.^[3] Adults typically live 1-5 years, with longevity varying by species and environmental conditions.^[10] Fecundity in female lanternfish ranges from 500 to 2000 eggs per batch, influenced by body size and nutritional status. Recent studies using DNA barcoding have identified 38 lanternfish species from larval samples in the Indian Ocean, indicating regional larval diversity.^[18] Mating behaviors often involve species-specific bioluminescent displays from sexual dimorphic photophores, aiding mate attraction in the dim deep sea.^[57] Some deep-water genera exhibit hermaphroditism, potentially enhancing reproductive success in low-density populations.^[58]

Ecological Significance

Formation of the Deep Scattering Layer

The deep scattering layer (DSL) is a prominent acoustic feature in the ocean, consisting of a sound-reflecting zone in the water column typically located between 200 and 500 meters depth during the day. This layer arises from the collective backscattering of sound waves by dense aggregations of marine organisms, primarily those possessing gas-filled swim bladders that act as resonant scatterers.^[59][60] Lanternfish (family Myctophidae) form the dominant component of the DSL in most oceanic regions, contributing the majority of the scattering biomass due to their immense abundance and schooling behavior. These small mesopelagic fish, equipped with swim bladders, reflect acoustic signals effectively, with estimates indicating they account for over 50% of midwater fish biomass globally and up to 65% of deep-sea fish biomass in sampled areas. While other micronekton such as euphausiids, cephalopods, and siphonophores contribute to the layer, Myctophidae's prevalence underscores their primary role in DSL formation across diverse environments.^[61][62] The diurnal vertical migrations of lanternfish induce dynamic shifts in DSL depth, with the layer descending to 300–500 meters in daylight to avoid predators and ascending to shallower depths (often above 200 meters) at night to feed. This migration pattern, observed in approximately 50% of mesopelagic backscatter on average (ranging 20–90% regionally), results in a predictable oscillation of the layer's position, influencing acoustic profiles throughout the day-night cycle.^[59][61] Detection of the DSL relies on active acoustic technologies, including multibeam sonar and scientific echosounders operating at frequencies like 38 kHz, which capture the layer as distinct echoes on echograms. Recent acoustic surveys, such as those conducted in 2023–2024 in regions like the Indian Ocean and Eastern Tropical Pacific, have quantified DSL biomass by integrating backscatter data with trawl validations, revealing mesopelagic fish contributions (dominated by lanternfish) at 2–16 gigatons wet weight globally—representing the largest component of oceanic fish biomass. These methods highlight the DSL's scale, with lanternfish alone estimated at around 600 million tons in 2025 assessments.^{[59][63][64][6]}

Role in Food Webs and Ecosystems

Lanternfish (family Myctophidae) occupy a central position in marine food webs as one of the most abundant prey species in the open ocean. They serve as a primary food source for a wide array of predators, including commercially important tunas, cephalopods such as squids, seabirds, and marine mammals like whales, thereby linking lower trophic levels to higher predators. This role supports the foraging needs of these apex consumers across global oceans, facilitating energy transfer from zooplankton to larger vertebrates.^[3][65] With an estimated global biomass of 600 million tonnes, lanternfish dominate the mesopelagic realm, comprising up to 65% of all mesopelagic fish biomass.^[39] This immense abundance underscores their foundational importance in oceanic ecosystems, where they sustain vast predator populations and contribute to overall marine productivity. Their high biomass also positions them as a critical buffer in food web dynamics, potentially influencing the resilience of dependent species to environmental changes.^[39] Through diel vertical migrations, lanternfish actively transport organic carbon and nutrients from surface waters to the deep sea, enhancing the efficiency of the biological carbon pump. By feeding on plankton in the epipelagic zone at night and descending to mesopelagic depths during the day, they facilitate the sequestration of carbon in deeper layers, where it can be stored for extended periods. This process not only cycles essential nutrients like nitrogen and phosphorus but also helps regulate oceanic carbon dioxide levels, indirectly supporting oxygen dynamics in stratified waters.^[39][66] Lanternfish exhibit sensitivity to climate-driven ocean warming, with recent biogeographical analyses revealing projected poleward distributional shifts in response to rising temperatures. In the Southern Ocean, for instance, models under high-emission scenarios forecast expansions of subtropical lanternfish regions and contractions of southern ones, driven by thermal barriers around 8–9°C. These shifts, observed through regional species assemblages, highlight potential disruptions to existing food webs and underscore the vulnerability of lanternfish to ongoing climate change. In October 2025, the International Union for Conservation of Nature (IUCN) added the mesopelagic zone to its list of key biodiversity areas, emphasizing the need to protect lanternfish habitats and the broader twilight zone ecosystem.^[67][68]

Evolutionary History

Origins and Fossil Record

Lanternfish, belonging to the family Myctophidae within the order Myctophiformes, trace their phylogenetic origins to the diversification of stem-group teleosts during the Cretaceous period, approximately 100 million years ago (Ma). This early radiation occurred amid the broader expansion of neoteleostean fishes in marine environments, setting the stage for the emergence of mesopelagic lineages. The order Myctophiformes diverged around 80 Ma in the Late Cretaceous (Campanian stage), as indicated by time-calibrated molecular phylogenies that place the crown age of the order in the Upper Cretaceous. These estimates highlight an ancestral connection to early teleost clades adapting to open-ocean habitats, though direct fossil evidence for myctophiforms remains limited prior to the Paleogene. While molecular estimates suggest a crown age in the Upper Cretaceous, the fossil record is sparse until the Paleogene, likely due to preservational biases in deep-sea environments.^[69] The fossil record of myctophids is sparse, primarily preserved as otoliths due to the challenges of deep-sea sedimentation and the fragility of their small, lightly ossified skeletons, which rarely fossilize in mesopelagic settings. The earliest unambiguous myctophid fossils appear in late Paleocene deposits around 55-56 Ma, such as the species Eokrefftia prediaphus from South Australia, marking the family's initial post-Cretaceous-Paleogene (K-Pg) boundary diversification. By the early Eocene (approximately 55 Ma), otoliths of early myctophids, including forms assigned to genera like Eomyctophum, become more common in shallow to upper-slope marine sediments across regions such as Antarctica and the eastern USA, reflecting a transitional phase from neritic to deeper-water niches.^[70][71] This scarcity underscores the bias toward otolith-based records, as complete skeletal fossils are exceptional before the Oligocene. Key evolutionary adaptations, such as bioluminescent photophores, are first evidenced in Oligocene fossils around 34-23 Ma, indicating the development of this trait during the family's early radiation into bathyal environments. For instance, the Oligocene species Oligophus moravicus from the Carpathian Mountains preserves a basic myctophid photophore pattern on its circumorbital bones, suggesting that ventral and lateral light organs evolved to facilitate communication and predator avoidance in dim-lit depths.^[72] Molecular clock analyses date the crown age of Myctophiformes, including Myctophidae, to approximately 83 Ma in the Upper Cretaceous (95% HPD: 62-106 Ma), supporting a radiation that predates the earliest fossils but accelerated post-K-Pg.^[69]

Rise to Modern Dominance

The diversification of lanternfishes (family Myctophidae) accelerated markedly during the Miocene epoch (approximately 23–5 million years ago), coinciding with global ocean cooling and the expansion of upwelling systems that enhanced nutrient availability in mesopelagic environments.^[73] Fossil evidence from otolith assemblages reveals a proliferation of genera, such as Diaphus, particularly in the late Miocene, reflecting a shift from marginal shelf habitats to widespread bathyal distributions as myctophids adapted to cooler, more productive deep waters.^[73] This expansion is evidenced by increased otolith abundance in sedimentary records from various ocean basins, indicating a rapid genus-level radiation driven by the Miocene "biogenic bloom" associated with heightened diatom productivity. Key innovations in bioluminescence, facilitated by specialized photophores, and enhanced vertical migration behaviors enabled lanternfishes to outcompete earlier mesopelagic dominants like stomiids, securing ecological advantages in resource partitioning and predator avoidance.^[73] Recent global phylogeographic studies highlight vicariance events in ocean basins, such as those linked to the closure of the Isthmus of Panama around 3–4 million years ago, which contributed to lineage splits and further diversification during the late Miocene to Pliocene transition. These patterns, analyzed through molecular phylogenies of multiple species pairs, underscore how basin-scale barriers promoted allopatric speciation alongside prevalent sympatric processes in the deep sea. Paleobiological analyses indicate that lanternfishes rose from comprising 30-55% of mesopelagic fish assemblages in the early Oligocene to over 50% by the Pliocene, marking their ascent to dominance through elevated speciation rates and niche expansion.^[73] This shift is corroborated by otolith-based reconstructions showing accelerated diversification in the late Miocene and Pliocene, with myctophids achieving peak relative abundance amid cooling oceans and intensified circulation. Subsequent Quaternary glaciations amplified lanternfish diversity, particularly in polar frontal zones, by altering ocean circulation and creating new vicariance opportunities through ice sheet advances and enhanced upwelling at high latitudes. The onset of Northern Hemisphere glaciation around 2.7 million years ago further boosted speciation rates, as evidenced by phylogenetic rate shifts correlating with polar cooling and habitat fragmentation.