Walters Shoal Research Articles

The Vetigastropoda (Mollusca) of Walters Shoal, with descriptions of two new genera and thirty new species

The vetigastropod material collected on Walters Shoal during Cruise MD208 of the Tropical Deep-Sea Benthos programme is documented. In total, 50 species were obtained, 30 of which are new and apparently endemic to the seamount. Of the other 20 species, eight are regionally endemic to the south-western Indian Ocean, 11 are more widely distributed in the Indo-West Pacific and one is possibly of deep-water Atlantic origin. The primary affinities of the fauna are with warm temperate South Africa and the tropical western Indian Ocean, but one species is potentially a seamount endemic of southern affinity. A new pseudococculinid genus living on decomposing bird feathers is described, a biogenic substrate association previously unknown in the Mollusca. The following new genera are described: Imbricoscelis gen. nov. and Pterodacna gen. nov. The following new species are described: Akritogyra crenulata sp. nov., Bathymophila williamsae sp. nov., Benthobrookula araneum sp. nov., Be. galeneae sp. nov., Be. laticostata sp. nov., Be. scalaroides sp. nov., Be. semisculpta sp. nov., Bruceina areneformis sp. nov., Calliostoma pantopunctatum sp. nov., Cantrainea herosae sp. nov., Carinastele achrosta sp. nov., Cornisepta marshalli sp. nov., Emarginula lentiginosa sp. nov., E. nodulicostata sp. nov., E. retrogyra sp. nov., E. salebrosa sp. nov., Fluxinella dufresneae sp. nov., Gibbula roseosticta sp. nov., Hadroconus scobina sp. nov., Kaiparathina monticola sp. nov., Lissotesta wareni sp. nov., Microcollonia miniata sp. nov., Mikro crassus sp. nov., Parviturbo cicatricosus sp. nov., Phragmomphalina candida sp. nov., Pterodacna boucheti gen. et sp. nov., Solariella asaphea sp. nov., Spinicalliotropis lepidota sp. nov., Stomatella multilirata sp. nov. and Trenchia mcleani sp. nov. The following new combinations are proposed: Brookula coronis Barnard, 1963 is transferred to Imbricoscelis gen. nov., Cantharidus nolfi Poppe, Tagaro & H. Dekker, 2006 is transferred to Kaiparathina Laws, 1941 and Solariella incisura Melvill, 1909 is transferred to Phragmomphalina Herbert & Williams, 2020. The following new synonyms are proposed: Carinastele wareni Vilvens, 2014 is a synonym of Bruceina cognata (Marshall, 1988); Fluxinella stellaris Bozzetti, 2008 is a synonym of Agagus stellamaris Herbert, 1991.

An overview of the specific diversity in the genus Cystiscus Stimpson, 1865 and the description of 27 new Cystiscus species (Muricoidea: Cystiscidae) from the Walters Shoal, southwestern Indian Ocean

The Cystiscus fauna collected on the Walters Shoal by the 2017 MNHN/IRD Expedition is revised on the ground of shell morphology. Twenty-nine different species are recognized, 27 of them being formally described as Cystiscus subcylindricus, sp. nov., C. triplicatus sp. nov., C. herberti sp. nov., C. inflatus sp. nov., C. stimpsoni sp. nov., C. pyriformis sp. nov., C. harpaeformis sp. nov., C. laurae sp. nov., C. robustus sp. nov., C. minimus sp. nov., C. ovoides sp. nov., C. mamillatus sp. nov., C. angustus sp. nov., C. profundus sp. nov., C. granum sp. nov., C. dentiferus sp. nov., C. alatus sp. nov., C. apertus sp. nov., C. contortus sp. nov., C. sinuosus sp. nov., C. pupoides sp. nov., C. lussii sp. nov., C. smithi sp. nov., C. serratus sp. nov., C. lachrymaformis sp. nov., and C. quinqueplicatus sp. nov. Among the 27 described species, 25 of them are only known from their holotype, one paratype from the type station is reported for C. subcylindricus, and two conspecific specimens collected from a non-type station are reported for C. lussii. Eight morphs represented by one or two shells are tentatively ascribed to some of the 27 recognized species. Among the 29 recognized species, 28 of them are found at bathyal levels, and only one of them (C. quinqueplicatus) was collected at a reef level, so the Cystiscus fauna of the Walters Shoal is characterized as bathyal fauna. From the fact that any other Cystiscus assemblage was ever studied from the bathyal of the Indo-Pacific Province, no direct comparison is made with external items, and the Cystiscus species of the Walters Shoal are only compared to each other.

Assessing marine mammal diversity in remote Indian Ocean regions, using an acoustic glider

Many observations collected from whaling logbooks or more recent satellite tags and acoustic surveys report that the Indian Ocean is a very important place for large baleen whales. They undergo long seasonal migrations from Southern feeding grounds to tropical and subtropical mating and breeding grounds. However, whether and where they stop to rest or feed during their long travels are poorly known. The Indian Ocean is also home to many odontocete species such as sperm whales, killer whales and multiple delphinid species. In this paper, we analyze passive acoustic data collected by an electric glider around two steep bathymetric features located in the Western sub-tropical Indian Ocean (Walters Shoal) and in the mid sub-tropical Indian Ocean (St. Paul and Amsterdam islands), both included in Important Marine Mammal Areas (IMMAs). The acoustic data were manually reviewed and annotated by two analysts. The aim of this experiment was to improve the knowledge on marine mammal presence in these little studied IMMAs. We found that bioacoustic activity was quite high in both monitored areas with 40% of the records containing marine mammal sounds in Walters Shoal and 70% in St. Paul and Amsterdam islands. Calls from Antarctic blue whales, Southwestern and Southeastern Indian Ocean pygmy blue whales, fin whales and an unidentified baleen whale were detected at one or both sites. Odontocete clicks and whistles were also recorded at both sites. The discussion puts these marine mammal acoustic detections back into the context of their seasonal and geographical presence already described by other studies in the Indian Ocean and makes hypotheses about the role of the two studied areas for marine mammals.

New Latrunculiidae genus (Porifera, Poecilosclerida) from the Madagascar Ridge.

Family Latrunculiidae Topsent, 1922 comprises seven genera: Latrunculia Barboza du Bocage, 1869; Sceptrella Schmidt, 1870; Strongylodesma Lvi, 1969; Tsitsikamma Samaai Kelly, 2002; Cyclacanthia Samaai Kelly in Samaai et al. (2004); Bomba and Latrunclava Kelly, Reiswig Samaai in Kelly et al. (2016) (Samaai Kelly 2002; Kelly et al. 2016; Samaai et al. 2020), with 83 valid species predominantly in the Southern Hemisphere (de Voogd et al. 2021). Latrunculiidae are differentiated primarily on the form and ornamentation of the diagnostic discorhabd microscleres (see Fig. 1) which may be anisodiscorhabds (Latrunculia, Bomba), isospinodiscorhabds (Cyclacanthia), and isochiadiscorhabds (Tsitsikamma). These may be accompanied by an additional longer microsclere, the amphiclad sceptre and anisoconicorhabd, in Sceptrella and Latrunclava, respectively. Here we describe a new latrunculid genus and species, Biverticillus tenuissimus gen. et sp. nov., from Walters Shoal on the Madagascar Ridge south of Madagascar in the Western Indian Ocean (Fig. 1A), the diagnostic microscleres of which are anisospinodiscorhabds, with two centrally located equidiametral whorls, equally spaced between each other and the apical whorl and manubrium. The microscleres are aniso- in their form because the apical and basal substructures differ slightly in the angle of repose of the spines.

Shallow seamounts represent speciation islands for circumglobal yellowtail Seriola lalandi

Phenotypic plasticity in life-history traits in response to heterogeneous environments has been observed in a number of fishes. Conversely, genetic structure has recently been detected in even the most wide ranging pelagic teleost fish and shark species with massive dispersal potential, putting into question previous expectations of panmixia. Shallow oceanic seamounts are known aggregation sites for pelagic species, but their role in genetic structuring of widely distributed species remains poorly understood. The yellowtail kingfish (Seriola lalandi), a commercially valuable, circumglobal, epipelagic fish species occurs in two genetically distinct Southern Hemisphere populations (South Pacific and southern Africa) with low levels of gene-flow between the regions. Two shallow oceanic seamounts exist in the ocean basins around southern Africa; Vema and Walters Shoal in the Atlantic and Indian oceans, respectively. We analysed rare samples from these remote locations and from the South African continental shelf to assess genetic structure and population connectivity in S. lalandi and investigated life-history traits by comparing diet, age, growth and maturation among the three sites. The results suggest that yellowtail from South Africa and the two seamounts are genetically and phenotypically distinct. Rather than mere feeding oases, we postulate that these seamounts represent islands of breeding populations with site-specific adaptations.

Thin-sectioned otoliths reveal extended longevity of southern boarfish (Pentaceros richardsoni) and are used to investigate inter-oceanic differences in length and age structure and growth

The age and growth of southern boarfish (Pentaceros richardsoni) was determined from samples collected by deep-sea trawlers over the Lord Howe Rise region in the Tasman Sea and Walters Shoal region in the south-west Indian Ocean. The number of growth (opaque) zones counted in whole otoliths and in thick sections (450 μm) of otoliths were lower than the counts of opaque zones in thin-sections (300 μm) of otoliths. Visual analyses and modelling of the trends in marginal increments on thin-sections of otoliths show that opaque zones are formed annually. The maximum ages determined for female and male P. richardsoni of 41 and 60 years respectively, greatly exceed the estimate of 14 years previously determined for this species based on counts of opaque zones in whole otoliths and the 2–8 years for its congener Pentaceros wheeleri from the North Pacific Ocean (NPO). The high maximum age for P. richardsoni and corresponding low natural mortality rates parallel those of Pentacerotids in coastal waters of southern Australia. Trends in the growth of P. richardsoni from the Tasman Sea demonstrate that this species exhibits rapid growth in the first 4–5 years of life with little or no subsequent growth throughout the remainder of their protracted life, which is analogous to other Pentacerotids. The truncation of the lengths and ages of P. richardsoni from the Walters Shoal suggests that the population of this species in these waters has experienced greater fishing mortality than the populations over the Lord Howe Rise region in the Tasman Sea, which is also confirmed by the far higher total mortality estimates in the former region. These findings for P. richardsoni, and the knowledge that P. wheeleri populations in the NPO have still not recovered from sustained, high fishing mortalities in the late 1960s to mid-1970s should inform the management strategies for P. richardsoni in the Southern Hemisphere.

The MADRidge project: Bio-physical coupling around three shallow seamounts in the South West Indian Ocean

Compared with other ocean basins, little is known scientifically about the seamounts in the Indian Ocean. Nonetheless, fishers have plundered these fragile ecosystems for decades, and now mining is becoming a reality. We introduce a multidisciplinary project referred to as MADRidge that recently focused on three shallow seamounts in the South West Indian Ocean between 19°S and 34°S. The larger Walters Shoal (summit at 18 m) discovered in 1963 occupies the southern part of the Madagascar Ridge and has long received attention from the fishing industry, and only recently by scientists. In contrast, nothing is known of the northern region of the ridge, which is characterised by a prominent, steep-sided seamount that has a flat circular summit at 240 m and width of ~20 km. This seamount is some 200 km south of Madagascar and unnamed; it is referred to here as the MAD-Ridge seamount. MAD-Ridge is the shallowest of a constellation of five deeper (>1200 m) seamounts on that part of the ridge, all within the EEZ of Madagascar. It lies in a highly dynamic region at the end of the East Madagascar Current, where mesoscale eddies are produced continuously, typically as dipoles. The Madagascar Ridge appears to be an area of great productivity, as suggested by the foraging behaviour of some tropical seabirds during chick-rearing and a longline fishery that operates there. The third seamount, La Pérouse, is located between Réunion Island and Madagascar. With a summit 60 m below the sea surface, La Pérouse is distinct from MAD-Ridge and Walters Shoal; it is a solitary pinnacle surrounded by deep abyssal plains and positioned in an oligotrophic region with low mesoscale activities. The overall aim of the MADRidge project was to examine the flow structures induced by the abrupt topographies, and to evaluate whether biological responses could be detected that better explain the observed increased in fish and top predator biomasses. The MADRidge project comprised a multidisciplinary team of senior and early career scientists, along with postgraduate students from France, South Africa, Mauritius and Madagascar. The investigation was based around three cruises using the French vessels RV Antea (35 m) and RV Marion Dufresne (120 m) in September 2016 (La Pérouse), November-December 2016 (MAD-Ridge) and May 2017 (Walters Shoal). This manuscript presents the rationale for the MADRidge project, the background, a description of the research approach including the cruises, and a synopsis of the results gathered in the papers published in this Special Issue.

Ichthyoplankton assemblages at three shallow seamounts in the South West Indian Ocean

The composition and spatial variability of ichthyoplankton assemblages were investigated at three shallow seamounts between latitudes 19°S and 33°S in the South West Indian Ocean (SWIO) – La Pérouse (60 m), an unnamed pinnacle south of Madagascar, referred to hereafter as MAD-Ridge (240 m), and the Walters Shoal seamount (18 m). In all, 299 larvae (23 families, 54 species) were present at La Pérouse, 964 larvae (58 families and 127 species) at MAD-Ridge, and 129 larvae (9 families, 24 species) at the Walters Shoal. Larvae of mesopelagic fish in the families Myctophidae and Gonostomatidae were the most dominant at all three seamounts. All developmental stages were present at each seamount, suggesting the larval pelagic phase of certain species occurs at the seamounts. A ‘seamount effect’ was detected only at MAD-Ridge where larval fish densities were significantly higher at summit stations. Overall, MAD-Ridge had much higher densities of fish larvae (157.0 larvae 100 m−3) than La Pérouse (31.1 larvae 100 m−3) and the Walters Shoal (9.6 larvae 100 m−3). Our study demonstrates that ichthyoplankton communities at shallow seamounts in the SWIO are more influenced by their location relative to a landmass, and to oceanographic features such as currents, mesoscale eddies and water masses than the seamount latitude and topography itself.

Satellite observations of phytoplankton enrichments around seamounts in the South West Indian Ocean with a special focus on the Walters Shoal

Spatial anomalies associated with seamounts and islands have been detected in the South West Indian Ocean using a new surface chlorophyll-a based enrichment index (EI). Calculated from daily L3 4-km MODIS chl-a data from 2003 to 2018, the EI was used to quantify recurrent (seasonal) surface enrichment associated with these features. Most notable were the shallow Walters Shoal (18 m) and nearby deeper WS-2 seamount (480 m), both located on the southern Madagascar Ridge, which showed high EI values of 40% and 15% local enhancement. Tromelin Island, east of Madagascar, exhibited more moderate values of 15% local increase. Other shallower seamounts including La Pérouse (60 m) and MAD-Ridge (240 m) exhibited sporadic or no measurable surface maxima. Regions of strong mesoscale activity such as south of Madagascar where the South East Madagascar Current detaches from the continent also revealed high EI values. A marked seasonality of the EI was observed over the Walters Shoal, with higher values during the oligotrophic season than in the austral winter. The seasonal variability of the MLD in combination with the seamount bathymetry appears to induce the thermal and biological anomalies observed there, possibly favoured by the presence of a Taylor cap at the summit. Ship-collected in situ measurements showed the satellite observed enrichment to be associated with shallowing of the deep chlorophyll maximum, as well as a shift of phytoplankton groups towards diatoms and small flagellates. No net increase of integrated chl-a was measured but a potential doubling in primary productivity is expected. The EI developed in this study, designed to reveal sporadic, small localised chl-a maxima, is likely applicable to other areas in the ocean where there is local enrichments.

Ocean currents and gradients of surface layer properties in the vicinity of the Madagascar Ridge (including seamounts) in the South West Indian Ocean

This work is part of the MADRidge Project special issue which aims to describe pelagic ecosystems in the vicinity of three prominent shallow seamounts in the South West Indian Ocean: one here named MAD-Ridge (240 m below the surface) plus Walters Shoal (18 m) on the Madagascar Ridge, and La Pérouse (60 m) on the abyssal plain east of Madagascar. The three span latitudes 20°S and 33°S, some 1500 km. The study provides the background oceanography for the once-off, multidisciplinary snapshot cruise studies around the seamounts. As life on seamounts is determined by factors such as summit depth, proximity to the light layers of the ocean, and the ambient circulation, a first description of regional spatial-field climatologies (16–22 years) and monthly along-ridge gradients of surface wind (driving force), water column properties of sea surface temperature, mixed layer depth, chlorophyll-a and eddy kinetic energy, plus ocean currents is provided. Being relevant to many applications in the study domain, these properties in particular reveal contrasting environments along the Madagascar Ridge and between the three seamounts that should drive biological differences. Relative to the other two seamounts, MAD-Ridge is in the more extreme situation, being at the end of the East Madagascar Current, where it experiences sturdy, albeit variable, currents and the frequent passing of mesoscale eddies.

Micronektonic fish species over three seamounts in the southwestern Indian Ocean

Taxonomic composition, abundance and biological features of micronektonic fish were investigated using pelagic trawls conducted near and over the summits of three seamounts located in the western Indian Ocean (La Pérouse, MAD-Ridge and Walters Shoal). Mesopelagic fish from three families accounted for 80% by number of the total catch (5714 specimens, 121 taxa), namely myctophids (59%), gonostomatids (12%) and sternoptychids (9%). Whereas the gonostomatid Sigmops elongatus was the most abundant species around La Pérouse seamount, myctophids were the most diverse and dominant group by number in all three studied areas. Most myctophids were high-oceanic species, which included the numerically dominant Benthosema suborbitale, Ceratoscopelus warmingii, Diaphus perspicillatus, Hygophum hygomii, and Lobianchia dofleini. The few remaining myctophids (Diaphus suborbitalis being the most abundant) were pseudoceanic fish, highlighting the association with landmasses. The study adds one myctophid species new to the Indian Ocean (Diaphus bertelseni), and a second record in the literature of the recently described sternoptychid Argyripnus hulleyi.

Connectivity between seamounts and coastal ecosystems in the Southwestern Indian Ocean

Understanding larval connectivity patterns is critical for marine spatial planning, particularly for designing marine protected areas and managing fisheries. Patterns of larval dispersal and connectivity can be inferred from numerical transport models at large spatial and temporal scales. We assess model-based connectivity patterns between seamounts of the Southwestern Indian Ocean (SWIO) and the coastal ecosystems of Mauritius, La Réunion, Madagascar, Mozambique and South Africa, with emphasis on three shallow seamounts (La Pérouse [LP], MAD-Ridge [MR] and Walters Shoal [WS]). Using drifter trajectory and a Lagrangian model of ichthyoplankton dispersal, we show that larvae can undertake very long dispersion, with larval distances increasing with pelagic larval duration (PLD). There are three groups of greater connectivity: the region between the eastern coast of Madagascar, Mauritius and La Réunion islands; the seamounts of the South West Indian Ridge; and the pair formed by WS and a nearby un-named seamount. Connectivity between these three groups is evident only for the longest PLD examined (360 d). Connectivity from seamounts to coastal ecosystems is weak, with a maximum of 2% of larvae originating from seamounts reaching coastal ecosystems. Local retention at the three focal seamounts (LP, MR and WS) peaks at about 11% for the shortest PLD considered (15 d) at the most retentive seamount (WS) and decreases sharply with increasing PLD. Information on PLD and age of larvae collected at MR and LP are used to assess their putative origin. These larvae are likely self-recruits but it is also plausible that they immigrate from nearby coastal sites, i.e. the southern coast of Madagascar for MR and the islands of La Réunion and Mauritius for LP.

Comparison of mesozooplankton communities at three shallow seamounts in the South West Indian Ocean

Seamounts are recognised as hotspots of biodiversity, attracting large numbers of top predators, but the underlying mechanisms are still unclear. We studied mesozooplankton abundance and size distribution at three shallow seamounts (60 m, 240 m and 18 m deep) in the South West Indian Ocean, along a latitudinal gradient (19°S, 27°S and 33°S). Samples were analysed using a ZooScan, allowing the use of a size-based approach. Differences were observed between seamount areas, but overall zooplankton communities did not seem to be affected by the changes in topography. Only in the lee of La Pérouse seamount was the zooplankton community slightly more concentrated than upstream, suggesting that zooplankton were flushed downstream of the seamount. The southernmost and shallowest seamount, Walters Shoal, had low abundance and its size spectrum differed greatly from the two other seamounts further north. These differences were attributed to seasonality and mesozooplankton population dynamics, whereas the other two seamounts exhibited a more “typical” oligotrophic pelagic ecosystem, at equilibrium and dominated by small organisms. At the time of sampling, the unnamed seamount south of Madagascar was influenced by a mesoscale dipole that impacted the zooplankton distribution, potentially masking any seamount effect. The normalised biomass size spectrum approach contributed to a better understanding of the ecosystem dynamics (i.e. equilibrium vs. non-steady state) but revealed little variability within a stable oligotrophic environment.

Seamounts, plateaus and governance issues in the southwestern Indian Ocean, with emphasis on fisheries management and marine conservation, using the Walters Shoal as a case study for implementing a protection framework

There is a growing interest in the management of seamounts of the Southwestern Indian Ocean (SWIO) both in waters under national jurisdictions and in the Areas Beyond National Jurisdiction (ABNJ). New scientific knowledge has been gathered through various oceanographic cruises during the past decade, and new agreements are under consideration globally to promote conservation and sustainable use of the biodiversity in the ABNJ, where the deep sea ecosystems associated with seamounts are a growing matter of concern. SWIO seamounts have attracted the interests of fishers since the 1960s, and contracts for mining exploration have been granted recently. Seamounts are known to shelter rich, fragile and poorly resilient ecosystems whose important ecological functions are threatened by various anthropogenic pressures. Whereas many seamounts and shoals are located in national waters, many others fall in the ABNJ, with no current legal status per se. To ensure conservation of their habitats and biodiversity, it is essential that protection measures are instigated under an internationally recognized legal and institutional framework. In this paper, we review the current state of such a framework relevant to seamounts, with emphasis on fisheries and conservation in the SWIO. An emblematic seamount, the Walters Shoal, is selected as a case study to discuss how it could become a fully-protected space in the ABNJ. As a large part of the SWIO is under the mandate of the Nairobi Convention (as a Regional Sea under the auspices of UNEP), guidelines are proposed to encourage dedicated seamount governance within the framework of this Convention.

Re-description and new records of a poorly known leucosiid crab, Merocryptus boletisculpta Zarenkov, 1994 (Crustacea, Decapoda, Brachyura)

Sixty-nine specimens of the rare leucosiid crab Merocryptus boletisculpta are reported from Walters Shoal, an isolated seamount in the southwestern Indian Ocean. The species has previously only been known from the type series collected during the R/V Vityaz 1988-1989 expedition. The species is re-described, illustrated, and colour photographs are provided.

New genera, species and occurrence records of Goniasteridae (Asteroidea; Echinodermata) from the Indian Ocean.

Modern goniasterids are the most numerous of living asteroids in terms of described genera and species and they have important ecological roles from shallow to deep-water marine habitats. Recent MNHN expeditions and historical collections in the USNM have resulted in the discovery of 18 new species, three new genera and multiple new occurrence records from the western Indian Ocean region including Madagascar, Glorioso and Mayotte islands, Walters Shoal, South Africa, and Somalia. This report provides the first significant contribution to knowledge of deep-sea Asteroidea from the Indian Ocean since the late 20th Century. Several deep-sea species, previously known from the North Pacific are now reported from the western Indian Ocean. Gut contents from Stellaster and Ogmaster indicate deposit feeding. Feeding modes of this and other deep-sea species are discussed. Comments are made on fossil members of included taxa. A checklist of Indian Ocean Goniasteridae is also included.

New species of Chimaera (Chondrichthyes: Holocephali: Chimaeriformes: Chimaeridae) from the Southwestern Indian Ocean

Three new species of Chimaera are described from the Southwestern Indian Ocean. Chimaera willwatchi sp. nov. is distinguished by its large body, blocky head with square snout, well-defined suborbital ridges, and a strong dorsal spine exceeding first dorsal apex; body brown-purple in color, slight iridescent sheen, heavily mottled around mouth and ventral surface, posterior edge of pelvic fins and first dorsal fin with prominent white margin, and anterior edge of second dorsal fin with white margin. Chimaera didierae sp. nov. can be distinguished by its light tan body color, slender body, short trunk, long tail, relatively robust spine, very small unpaired fins, and extremely deciduous skin. Chimaera buccanigella sp. nov. is distinguished by its light tan body color, stocky body, short trunk, tapering rapidly into a long tail, long, very straight spine, skin not deciduous. The three Chimaera species were collected from seamounts of Walters Shoal, the Madagascar Ridge, and the Southwestern Indian Ridge in an area within 34o30’S–44o05’E and 39o30’S–58o15’E. This is the first record of the genus in the Southwestern Indian Ocean, and brings the global total to 19 species. The new species presented here are distinguishable from their congeners by a combination of coloration, morphology, meristic, and structure of the mitochondrial NADH2 gene.