Key to the Haplosyllis species

1 Anterior chaetae clearly different from midbody chaetae; some with an incision on MF, unidentate or pseudocom- pound ............................................................................................................................................................................ 2

- Anterior chaetae similar to midbody chaetae, often small, with short MF.................................................................. 5

2 Small worms, 2–3 mm long. Chaetae from first 6 chaetigers, with an incision on MF (Fig. 5 C).... H. anthogorgicola

- Medium to long worms, 6–20 mm long (rarely 4–6 mm). Anterior chaetae without such incision ........................... 3

3 Anterior chaetae with clear traces of fusion between blade and shaft; midbody chaetae completely simple, without such traces (Figs 19 C, 20C–D) ............................................................................................................. H. spongiphila

- All chaetae simple, without such traces of fusion; some with anterior unidentate chaetae, or with small denticles on MF (Figs 18 A–C, 21A)................................................................................................................................................ 4

4 Anterior chaetae bidentate. Midbody chaetae unidentate. Denticles on MF absent (Fig. 21 A–B) … H. streptocephala

- All chaetae bidentate. Anterior ones with small denticles on MF, usually eroded in midbody chaetae (Fig. 18 A–D). ................................................................................................................................................................. H. spongicola

5 Midbody parapodia with 2–3 chaetae differing in size and shape. Smallest chaeta with short proximal tooth; largest chaeta with MJP long and straight (Fig. 23 E, G, H) .................................................................................................... 6

- Midbody parapodia with chaetae all similar (Figs 10 B, 22F) ..................................................................................... 7

6 Pharynx extending 10–11 segments. Largest chaetae of each midbody parapodium with short, almost invisible distal tooth (Fig. 23 A, H). 4–6 posterior aciculae ............................................................................................. H. uncinigera

- Pharynx extending 4–6 segments. Largest chaetae of each midbody parapodium with apical teeth similarly in size, or distal one slightly shorter but clearly visible (Fig. 12 H). 1–2 posterior aciculae............................. H. djiboutiensis

7 Midbody chaetae with proximal tooth long, with pointed tip; distal tooth absent or very short, thin (Fig. 9 B–C) .... 8

- Midbody chaetae with apical teeth of similar length, sometimes proximal tooth sligtly shorter ................................ 9

8 Anterior and midbody chaetae unidentate, or with a very small distal tooth; more hooked and clearly bidentate when posterior-most. Body yellow to violet “in vivo”, with dark violet dorsal marks .................................. H. chamaeleon

- All chaetae bidentate, distal tooth small, often difficult to see. Body uniformly yellowish, without marks.................. ............................................................................................................................................................ H. villogorgicola

9 Chaetae with MJP short and curved. Denticles of US of MF absent (Figs 4 B, 7F, 10B) ......................................... 10

- Chaetae with MJP diagonal (Fig. 2 C) or straight (only in largest chaetae) (Figs 8 D, 13H, 16E). Spines of US of MF present or absent.......................................................................................................................................................... 13

10 Body fragile, slender, small, 1–5 mm long, less than 0.5 mm wide .......................................................................... 11

- Body robust, long, 10–30 mm long, about 1 mm wide .............................................................................................. 12

11 All dorsal cirri slender. Chaetae broad, with short apical teeth, distal tooth shorter than distal one (Figs 6 A, 7F) ...... ................................................................................................................................................................... H. basticola

- Tentacular cirri and first dorsal cirri slender; dorsal cirri often very broad. Chaetae slender, apical teeth long, similar in size (Fig. 10 B) ................................................................................................................................. H. crassicirrata

12 MF resembling a third tooth (Fig. 22 E–F); 3–6 chaetae per parapodia..................................................... H. trifalcata

- MF very short, but not resembling a third tooth (Fig. 4 A–C); 1–2 chaetae per parapodia ........................... H. agelas

13 MJP diagonal. Denticles on US long, reaching apical teeth (Fig. 2 C) ............................................................. H. loboi

- MJP long and straight. Denticles on US never reaching apical teeth ......................................................................... 14

14 Pharynx half as thin as proventricle................................................................................................... H. carmenbritoae

- Pharynx broad, similar to proventricle width or slightly thinner................................................................................ 15

15 Pharynx short, extending 3–4 segments, broader in the middle, reaching the proventricle as a funnel (Fig. 13 A) ....... ........................................................................................................................................................................... H. gula

- All pharynx similarly wide, extending 4–18 segments (Fig. 8 A).............................................................................. 16

16 Mid-sized worms, 4–10 mm long, less than 1 mm wide. All dorsal cirri short......................................................... 17

- Long, robust worms, 25–44 mm long, 1–3 mm wide ............................................................................................... 18

17 Parapodia with 1–2 chaetae, spines on US of MF absent or very short .................................................. H. cephalata

- Parapodia with 2–7 chaetae, long spines on US of MF ............................................................................ H. granulosa

18 Parapodia with 14–20 chaetae, 8–13 aciculae. Trepan absent ............................................................... H. cratericola

- Parapodia with 5–8 chaetae, 3 aciculae. Trepan may be present .................................................................... H. ohma