Dichotomous key for genusses of the western Indian Ocean

• Exopod of uropod with distinct distal suture (see fig.)    > 2
  
  
• Exopod of uropod undivided (see fig.)     > 4
  
  

• Endopod of uropod with distinct distal suture and a strong spine present on the ventral surface close to the statocyst (see fig.). Marsupium consists of three pairs of lamellae (Sub-Family RHOPALOPHTHALMINAE)     >Rhopalophthalmus
  
  
• Endopod of uropod undivided, eyes normally developped (Sub-Family SIRIELLINAE)     > 3
• Endopod of uropod undivided, eyes reduced, no apical plumose setae on the telson; statocyst absent     > Spelaeomysis

• Third pair of thoracic legs normal and similar to the more posterior legs     > Siriella
• Third pair of thoracic legs extremely elongated, almost twice as long as more posterior legs     > Hemisiriella

• One to many spines but no setae on the outer margin of the exopod of the uropod (see fig.) Marsupium consists of two pairs of lamellae (Sub-Family GASTROSACCINAE)     >5
  
  
• Outer margin of exopod of uropod with setae but no spines     >8

• Outer margin of exopod of uropod naked with one terminal spine (see fig.)    > Pseudanchialina
  
  
• Outer margin of exopod of uropod with more than ten spines     >6

• In male, exopod of pleopod III slightly elongated, endopod with many segments (see fig.)     > Anchialina
  
  
• In male exopod of pleopod III greatly elongated, without copulatory organ, and with the endopod unsegmented (see fig.)     > Haplostylus
  
  
• In male, exopod of pleopod III greatly elongated, endopod with many segments (see fig.)     >Gastrosaccus
  
  
• Pleopod of other form     >7

• In females, pleopods I-V uniramous     > Anchialina
• In females, pleopod I biramous, pleopods II-V uniramous     > Gastrosaccus     > Haplostylus

• Endopods of third to eighth thoracic legs with undivided carpus marked off from propodus by an oblique articulation (see fig.) Antennal scale with outer margin non-setose nearly always with a pronounced external spine (scale sometimes absent or represented by a spine). Pleopods II-V of male well developed and biramous telson entire (Tribe Erythropini)     >9
  
  
• Eyes well developed, not divided into distinct regions, with normally functioning visual elements. Last thoracic segment not elongated. Antennal scale with outer margin naked. Pleopods II-V of male usually well developed and biramous. Antennal scale setose all around, telson variabIe (Tribe Leptomysini)     >19
• Eyes well developed, not divided into distinct regions, with normally functioning visual elements. Last thoracic segment not elongated. Antennal scale with outer margin naked.At least pleopod II of male is rudimentary and uniramous, exopod of pleopod IV elongated and modified. Antennal scale and telson very variable (Tribe Mysini)     >13

• Eyes reniform or markedly flattened dorso-ventrally     >10
• Eyes more or less globular     >11

• Telson shorter than broad, its apex armed with four strong spines, the lateral margins unarmed (see fig.) or their distal regions serrulated (see fig.)     > Erythrops (in part)
  
  
• Telson longer than broad, its apex truncate, armed with six spines, the innermost pair being minute, the outer pairs strong; the lateral margins entirely or partially armed with spines (see fig.)     > Hypererythrops
  
  

• Lateral margins of telson unarmed. Apical region of telson armed with two pairs of spines of other proportions     >12
• Lateral margins of telson unarmed. Apical region of telson armed with three pairs of spines     > Parerythrops (in part)

• Eyes sub-globular, oval; telson of form shown in the figure.     > Erythrops (in part)
  
  
• Eyes globular. Constriction present between thorax and abdomen.     > Pleurerythrops

• Antennal scale with outer margin naked (proximal part), no distal spine (see fig. 1), telson triangular, unarmed (see fig.2).     > Idiomysis
  
  
• Antennal scale setose all round     >14

• Eyes of complex form or markedly reduced, with or without pigment.     >15
• Eyes normally developed; telson without cleft or with cleft without plumose setae present     >16

• Eyes divided as in Fig. 1; telson of forms shown in Fig 2 and 3     > Anisomysis (in part)
  
  
• Eyes as in Fig. 7.12; cleft of telson armed with two plumose setae and no spines (Fig. 8.25)     > Kainomatomysis

• exopod of pleopod IV of male with one or two segments     >17
• exopod of pleopod IV of male with three, four or five segments     >18

• Exopod of pleopod IV unsegmented (Fig. 1); telson of form in Fig. 2; antennal scale as in Fig. 3; pleopod V elongated (Fig. 4)     > Indomysis
  
  
• Exopod of pleopod IV unsegmented (Fig. 5); telson and endopod of uropod as in Fig. 6; antennal scale of form in Fig. 7     > Mesacanthomysis
  
  
• Exopod of pleopod IV of two segments (Fig. 8); telson without cleft, variable in shape but of general form in Fig. 9; Distal end of antennal scale rounded (Fig. 10)     > Acanthomysis
  
  

• Exopod of pleopod IV of three segments (Fig. 1) ; telson of form in Fig. 2; antennal scale of form in Fig. 3; pleopod III biramous with unsegmented endopod and an exopod of two segments (Fig. 4)     > Mesopodopsis
  
  
• Exopod of pleopod IV of four segments (Fig. 5); telson variable, examples being shown in Figs 6 - 10 antennal scale of general form in Fig. 11     > Anisomysis (in part)
  
  

• Antennal scale setose all around; pleopods of male well developed, biramous; Telson without cleft; endopod of uropod armed with spines     >20
• Antennal scale setose all around; pleopods of male well developed, biramous; Telson with cleft, or at least apical incision     >22

• Telson of form shown in Fig. 1; endopod of first thoracic leg as in Fig. 2     > Metamysidopsis
  
  
• Telson of other form     > 21

• Endopod of first thoracic leg as in fig. 1     > Mysidopsis (in part)
• Endopod of first thoracic leg as in fig. 2     > Leptomysis
  
  

• Eyes normal size     >23
• Eyes with accessory eye on dorsal lateral side (see fig.)     > Dioptromysis
  
  

• Telson with unarmed apical incision (fig. 1); endopod of first thoracic leg as fig.2     > Mysidopsis (in part)
  
  
• Telson with an armed cleft cleft; apically plumose setae on telson     >24

• Distal segment of maxilla not expanded but rectangular (fig. 1, 2) ; telson of form in figs 3 and 4     > Tenagomysis
  
  
• Distal segment of maxilla expanded, crescent shaped (fig. 5); telson of form in fig. 6     > Afromysis (in part)
  
  
• Distal segment of maxilla triangular (see fig.7) ; telson as in figs 8 or 9     > Doxomysis
  
  
• Distal segment of maxilla as in fig. 10 ; telson as in fig. 11     > Hyperiimysis
  
  
• Distal segment of maxilla as in Fig. 12 ; telson as in Fig. 13     > Nouvelia