Nagpur Diatom Identification and Reference System (NDIRS)


Department of Forensic Biology
Government Institute of Forensic Science, Nagpur
Project under: BEEJ (Building Excellence and Exploration in Junior Research)

About Diatoms

Diatoms are unicellular, photosynthetic microalgae belonging primarily to the class Bacillariophyceae. They possess silica-based cell walls known as frustules, which exhibit species-specific morphological features. Due to their structural durability and ecological specificity, diatoms serve as important bioindicators and valuable forensic markers in drowning investigations. Morphological characteristics such as valve symmetry, shape, length, and width assist in taxonomic identification and development of region-specific reference databases.

Futala Lake Diatoms

Diploneis

  • Diploneis frustules are elliptic to panduriform with bluntly rounded apices, commonly observed in benthic samples from Futala Lake sediments.
  • Each valve contains two longitudinal canals embedded in the silica wall, opening externally but not internally.
  • The frustules are heavily silicified, giving them a robust and resistant structure in freshwater environments.
  • Areolae are complex and loculate, contributing to distinctive microscopic ornamentation.
  • Most species are epipelic and commonly occur in lake-bottom mud habitats.
Category: Symmetric Biraphid
Class: Bacillariophyceae
Order: Naviculales
Valve: Elliptic to panduriform
Morphometry: Length 28μm | Width 6μm

Rhopalodia

  • Rhopalodia shows strong dorsiventral symmetry, a key identifying feature in Futala Lake diatom samples.
  • The raphe lies dorsally within a shallow keel and is clearly visible under light microscopy.
  • Valve faces are nearly planar and resemble half of an orange in cross-section.
  • The central raphe endings are expanded and easy to recognize in cleaned frustules.
  • Internally, the raphe is supported by costae, giving structural rigidity.
Category: Epithemioid
Class: Bacillariophyceae
Order: Rhopalodiales
Family: Rhopalodiaceae
Morphometry: Length 47-118μm | Width 18-28μm

Gomphonema

  • Gomphonema valves are symmetric to the apical axis but asymmetric to the transapical axis, giving a clavate outline.
  • In girdle view, frustules are wedge-shaped and attach to substrates via mucilage stalks in lake habitats.
  • A stigmoid may be present near the central area on one side of the valve.
  • An apical pore field is present and bisected by the terminal raphe.
  • Striae are uniseriate and evenly spaced along the valve surface.
Category: Asymmetric Biraphid
Class: Bacillariophyceae
Order: Cymbellales
Family: Gomphonemataceae
Morphometry: Length 52μm | Width 12μm

Stephanocyclus

  • Stephanocyclus is identified by its fultoportulae and rimoportulae, important for classification in centric diatoms.
  • Central fultoportulae are present and surrounded by three satellite pores.
  • Marginal rimoportulae occur on a costa between valve face and mantle.
  • Rimoportulae are located within a ring of marginal fultoportulae, a diagnostic feature.
  • These structures are important for distinguishing species in plankton samples.
Category: Centric
Class: Coscinodiscophyceae
Order: Thalassiosirales
Family: Stephanodiscaceae
Morphometry: Diameter 19μm | Area 302μm²

Cymbella

  • Cymbella valves are asymmetric to the apical axis but symmetric to the transapical axis.
  • The terminal raphe fissures are deflected toward the dorsal side, a key diagnostic character.
  • Apical pore fields are present at both poles and visible in cleaned specimens.
  • Stigmata are located on the ventral side of the central area when present.
  • Internally, stigmata have convoluted occlusions and proximal raphe may be continuous.
Category: Asymmetric Biraphid
Class: Bacillariophyceae
Order: Cymbellales
Family: Cymbellaceae
Morphometry: Length 136μm | Width 29μm

Bacillaria

  • Bacillaria has a centrally positioned raphe within a raised keel, unlike eccentric raphe genera.
  • Striae are coarse and distinct, visible even under moderate magnification.
  • Living cells form sliding colonies joined by silica hooks near the keel.
  • This genus is common in nutrient-rich and high-conductivity waters.
Category: Nitzschioid
Class: Bacillariophyceae
Order: Bacillariales
Family: Bacillariaceae

Pleurosigma

  • Pleurosigma valves are elongate and sigmoid.
  • Striae form three intersecting patterns: transapical and two oblique rows.
  • The raphe is strongly sigmoid with a narrow axial area.
  • Distal raphe ends are hooked and deflected onto the mantle.
  • Cells contain ribbon-like plastids extending from apex to apex.
Category: Symmetric Biraphid
Class: Bacillariophyceae
Order: Naviculales
Morphometry: Length 99μm | Width 14μm

Ambazari Lake Diatoms

Aulacoseira

  • Aulacoseira is a freshwater centric diatom forming long filamentous chains.
  • Frustules are linked by spines and have deep ornamented mantles.
  • Rimoportulae occur on the mantle, often associated with a ringleiste.
  • Girdle bands are split rings with ligulae, aiding flexibility.
  • Produces thick resting spores that accumulate in lake sediments.
Category: Centric
Class: Coscinodiscophyceae
Order: Aulacoseirales
Family: Aulacoseiraceae
Morphometry: Length 24μm | Width 7μm

Nitzschia

  • Nitzschia possesses an eccentric raphe positioned within a keel supported by fibulae.
  • Valves lack a sternum and vary greatly in size and shape.
  • The genus is abundant in Ambazari Lake water samples.
  • Species are often difficult to distinguish due to high variability.
  • Common in nutrient-rich and organic waters.
Category: Nitzschioid
Class: Bacillariophyceae
Order: Bacillariales
Family: Bacillariaceae
Morphometry: Length 28-41μm | Width 4-5μm

Navicula

  • Navicula valves are elliptic to broadly lanceolate with varied apex shapes.
  • The central sternum is thickened and may show slight asymmetry.
  • The raphe is straight and filiform with slightly deflected proximal ends.
  • The central area may be expanded but not thickened into a fascia.
  • Areolae are lineate, forming characteristic lineolae visible under microscope.
Category: Symmetric Biraphid
Class: Bacillariophyceae
Order: Naviculales
Family: Naviculaceae
Morphometry: Length 24-30μm | Width 4-8μm

Gomphonema

  • Gomphonema valves are symmetric to the apical axis but asymmetric to the transapical axis, giving a clavate outline.
  • In girdle view, frustules are wedge-shaped and attach to substrates via mucilage stalks in lake habitats.
  • A stigmoid may be present near the central area on one side of the valve.
  • An apical pore field is present and bisected by the terminal raphe.
  • Striae are uniseriate and evenly spaced along the valve surface.
Category: Asymmetric Biraphid
Class: Bacillariophyceae
Order: Cymbellales
Family: Gomphonemataceae
Morphometry: Length 44-52μm | Width 9-12μm

Bacillaria

  • Bacillaria has a centrally positioned raphe within a raised keel, unlike eccentric raphe genera.
  • Striae are coarse and distinct, visible even under moderate magnification.
  • Living cells form sliding colonies joined by silica hooks near the keel.
  • Colonies show coordinated gliding movement in Ambazari Lake plankton.
  • This genus is common in nutrient-rich and high-conductivity waters.
Category: Nitzschioid
Class: Bacillariophyceae
Order: Bacillariales
Family: Bacillariaceae

Stephanocyclus

  • Stephanocyclus is identified by its fultoportulae and rimoportulae, important for classification in centric diatoms.
  • Central fultoportulae are present and surrounded by three satellite pores.
  • Marginal rimoportulae occur on a costa between valve face and mantle.
  • Rimoportulae are located within a ring of marginal fultoportulae, a diagnostic feature.
  • These structures are important for distinguishing species in plankton samples.
Category: Centric
Class: Coscinodiscophyceae
Order: Thalassiosirales
Family: Stephanodiscaceae
Morphometry: Diameter 15μm | Area 180μm²

Tabularia

  • Tabularia species are robust and needle-like in appearance.
  • The sternum is broad and centrally positioned.
  • Striae are broad and composed of complex areolae.
  • An apical pore field is present at each pole.
  • A single rimoportula is located near one pole.
Category: Araphid
Class: Fragilariophyceae
Order: Fragilariales
Family: Fragilariaceae
Morphometry: Length 100-110μm | Width 3-7μm

Bacillaria

  • Bacillaria has a centrally positioned raphe within a raised keel, unlike eccentric raphe genera.
  • Striae are coarse and distinct, visible even under moderate magnification.
  • Living cells form sliding colonies joined by silica hooks near the keel.
  • Colonies show coordinated gliding movement in Ambazari Lake plankton.
  • This genus is common in nutrient-rich and high-conductivity waters.
Category: Nitzschioid
Class: Bacillariophyceae
Order: Bacillariales
Family: Bacillariaceae

Gomphonella

  • Gomphonella valves are wedge-shaped and resemble Gomphonema in outline.
  • Striae are biseriate to triseriate with small round areolae.
  • Areolae are unoccluded and clearly visible under light microscopy.
  • This genus lacks stigmoids or stigmata entirely.
  • The apical pore field is undifferentiated and bisected by the raphe.
Category: Asymmetric Biraphid
Class: Bacillariophyceae
Order: Cymbellales
Morphometry: Length 19-23μm | Width 3-5μm
Contributor: Mr. Avinash Tanaji Gavade M.sc II year (Department of Forensic Biology), Govt. Inst. of Forensic Science, Nagpur.
©Diatom Morphological Database