Hansenocaris, Ito, 1985, Ito, 1985

Olesen, Jørgen & Grygier, Mark J., 2024, Taxonomic diversity of marine planktonic ‘ y-larvae’ (Crustacea: Facetotecta) from a coral reef hotspot locality (Japan, Okinawa), with a key to y-nauplii, European Journal of Taxonomy 929 (1), pp. 1-90 : 51-55

Key to lecithotrophic last-stage y-nauplii

An identification key has been constructed for the 34 lecithotrophic naupliar morphospecies of facetotectans described herein from Sesoko Island, Okinawa, Japan. This key is designed for last-stage y-nauplii (LSNs). Primarily making use of external (cuticular) morphology, it is supplemented with information on coloration of the cyprid developing within when such data are helpful. Before identification can take place, it is necessary first to distinguish y-nauplii from the marine planktonic nauplii of other crustaceans, something that is actually very simple even without reference to the characteristic reticulation of the cephalic shield, as is done in the first couplet of two recent keys to crustacean nauplii ( Dahms et al. 2006; Martin et al. 2014b). Y-nauplii differ from all cirriped nauplii in lacking fronto-lateral horns. They differ from also-hornless copepod and ascothoracidan nauplii in lacking furcal setae and in having a long, exposed trunk dorsum, respectively. The metanauplii differ from those of copepods, euphausiaceans and dendrobranchiate shrimp in never displaying thoracopodal limb-buds. After it has been established that the freshly-caught nauplii at hand are y-nauplii, the first question in the key deals with their feeding status. If the nauplius is planktotrophic ( Figs 1C , 2C , 4 ), further identification using this key is not possible. The nine or so tentatively recognized planktotrophic y-naupliar morphospecies from Sesoko Island are not included in the key as their LSNs have not been recognized, but habitus photographs of them ( Fig. 2 ), as well as detailed illustrations of one very common planktotrophic morphospecies (Type A*; Figs 1C , 4 ), are provided to help users through the first couplet.

If the y-nauplius is lecithotrophic, the next step is to determine whether it is a last-stage nauplius—easily recognizable by the presence of three eyes anteriorly (one nauplius eye and the developing cyprid’s two compound eyes)—in which case use of the key can proceed. However, if the sampled y-nauplius is at an earlier stage of development, as was the case for more than 90% of the live-sampled lecithotrophic y-nauplii in 2018/2019 (Olesen et al. 2022) ( Fig. 1A–B ), then identification can proceed after the naupliar specimen has been reared to its last naupliar stage following procedures described above and elsewhere (Olesen et al. 2022). This may take up to ca 10 days depending on morphospecies. If the nauplius under study is at an earlier stage of development, the key may still be used if the nauplius has a particularly distinctive body shape, labral form or caudal-spine array, but the extent of this potential utility is as yet untested. Since the key is based mainly on external morphology, it should be applicable to preserved nauplii if these are LSNs. Once a nauplius has been tentatively identified to a particular morphospecies using this key, additional confirmatory information about living specimens, such as their coloration, should be checked in the respective descriptions and figures. It is likely, of course, that lecithotrophic nauplii will be found that fail to match any morphospecies in this key, especially in samples taken from distant locations and different habitats. Since this manuscript was written, we have found a few additional lecithotrophic morphospecies even at Sesoko Island (unpublished data from October 2023).

1. Second antennae and mandibles with feeding spines (either weakly or well developed); body with ovoid outline in dorsal view and dorso-ventrally flattened; trunk with rows of distinct postero-lateral spines (except Types J and S; Figs 1C , 2C , 4 ) .............................................................. ......................Planktotrophic y-nauplius (further identification not possible here but see Fig. 2C )

– Second antennae and mandibles without feeding spines; with yolk present internally and therefore often swollen; body generally lacking distinct postero-lateral spines (except Types U* and V; Fig. 12 )...........................................................................2, Lecithotrophic y-nauplius (Note: if not already an LSN (with three anterior eyes), the nauplius must be reared to this stage to proceed further)

2. Dorso-caudal spine>3 times as long as basal width........................................................................3

– Dorso-caudal spine shorter than this, or low-conical......................................................................21

3. Labral midline with keel bearing row of at least 3 spines; keel sometimes much extended ventrally or posteriorly.....................................................................................................................4

– Labral midline with no row of spines.........................................................................................6

4. Labrum extended ventrally into huge crest-like extension with apical row of spines ..................... ...................................................... Hansenocaris cristalabri Olesen & Grygier, 2022 ( Fig. 9A–C )

– Labrum with midline spines in simple row....................................................................................5

5. Trunk with 4 distinct dorsal rows of spines; body about 400 µm long excluding dorso-caudal spine ................................................... Hansenocaris aquila Olesen & Grygier, 2022 ( Fig. 9D–E )

– Trunk lacking any dorsal rows of spines; body about 270 µm long excluding dorso-caudal spine .........................................................................................Y-nauplius Type AC ( Fig. 9F–H )

6. Labrum with median spine on posterior margin.............................................................................7

– Labrum with no median spine on posterior margin........................................................................11

7. Dorso-caudal spine relatively short (ca 1/8 of body length) ....Y-nauplius Type AD ( Fig. 11H–M )

– Dorso-caudal spine relatively long (> 1/6 of body length)..................................................................8

8. Labrum with rounded-quadrangular outline with nearly parallel lateral margins ......................... .....................................................................................................Y-nauplius Type AG* ( Fig. 10G–K )

– Labrum with differently shaped outline.............................................................................................9

9. Labrum lacking free, rounded posterior margin; labral outline pentagonal ............................... ........................................................................................................Y-nauplius Type E* ( Fig. 10A–F )

– Labrum with free, rounded posterior margin; labral outline not pentagonal................................10

10. Posterior margin of labrum extended as median spine; keel absent on labral midline ...................... .......................................................................................................Y-nauplius Type AH* ( Fig. 9I–N )

– Posterior margin of labrum not extended as median spine; distinct keel present on labral midline .........................................................................................................Y-nauplius Type W ( Fig. 11A–G )

11. Furcal spines ca 3 times as long as wide, curved........................................................................12

– Furcal spines shorter or vestigial...................................................................................................13

12. Cephalic shield much wider than half body length; with weak but distinct discontinuity in body outline at posterior end of cephalic shield leading into trunk ....Y-nauplius Type AJ ( Fig. 11G–J )

– Cephalic shield about half as wide as body length; lateral body margins tapering gradually in ventral view, with no interruption at posterior end of cephalic shield ........Y-nauplius Type D* ( Fig. 7A–G )

13. Furcal spines present as double-spines ..........................................Y-nauplius Type AI ( Fig. 14G–L )

– Furcal spines present as single spines or vestigial...........................................................................14

14. Body ca 1.3 times as long as wide ........................................................................Y-nauplius Type U*

– Body ≥5 times as long as wide.....................................................................................................15

15. Dorso-caudal spine with broad base, almost continuous with lateral margins of caudal end of body ..............................................................................................Y-nauplius Type X ( Fig. 13A–E )

– Dorso-caudal spine with narrower basis, less continuous with lateral margins of caudal end of body.................................................................................................................................................16

16. Cephalic shield large, rounded and set off from rest of body .......Y-nauplius Type AM ( Fig. 19H )

– Cephalic shield otherwise...............................................................................................................17

17. Body ca 2.5 times as long as wide ............................................Y-nauplius Type AO ( Fig. 19F–G )

– Body ≤2 times as long as wide...................................................................................................18

18. Labrum produced as sub-triangular elevation................................................................................19

– Labrum produced as sub-quadrangular elevation.........................................................................20

19. Cyprid (within last-stage nauplius) completely brown ..................Y-nauplius Type L ( Fig. 15A–F )

– Cyprid (within last-stage nauplius) only with brown telson .........Y-nauplius Type M ( Fig. 14A–F )

20. Length of body 320–350 µm; lateral margins of body with slight discontinuity at posterior end of cephalic shield in dorsal view; dorso-caudal spine 2–3 times as long as its basal width .........................................................................................................Y-nauplius Type AN ( Fig. 19I–K )

– Length of body 360–400 µm; lateral margins of body tapering gradually in dorsal view, without discontinuity at posterior end of cephalic shield; dorso-caudal spine 4–5 times as long as its basal width .................................................................................................Y-nauplius Type Y ( Fig. 16A–E )

21. Body significantly bent, angle between longitudinal axes of cephalic shield and trunk>45°...22

– Body not significantly bent, angle between longitudinal axes of cephalic shield and trunk <40°...30

22. Furcal spines>3 times as long as basal width and curved................................................................23

– Furcal spines approximately equal in length to basal width and uncurved.....................................26

23. Dorso-caudal spine placed relatively far dorsally, not conical ......Y-nauplius Type B ( Fig. 7H–M )

– Dorso-caudal spine not placed particularly far dorsally, conical...................................................24

24. Furcal spines shorter than width of caudal region, trunk axis downturned much less than 90° with respect to cephalic axis .........................................................Y-nauplius Type F ( Fig. 8A–D )

– Furcal spines longer than width of caudal region, trunk axis downturned ca 90° with respect to cephalic axis .................................................................................................................................... 25

25. Furcal spines ca 5 times as long as basal width, each with 1 small subsidiary spine at base; dorso-caudal spine conical, robust ...............................................................Y-nauplius Type H ( Fig. 8H–J )

– Furcal spines ca 3 times as long as basal width, each with 2 small subsidiary spines at base; dorso-caudal spine conical, small ................................................................Y-nauplius Type G ( Fig. 8E–G )

26. Dorso-caudal spine not conical, instead thin and small ......Y-nauplius Type K ( Figs 5D–F , 13F–J )

– Dorso-caudal spine conical, sometimes very short........................................................................27

27. Conical dorso-caudal spine very short, about as long as basal width ............................... .......................................................................................................Y-nauplius Type AB ( Fig. 15G–M )

– Conical dorso-caudal spine 2–3 times as long as basal width...................................................28

28. Labrum without spine(s); furcal spines closely adjacent to base of dorso-caudal spine ............. ........................................................................................................Y-nauplius Type N* ( Fig. 17F–K )

– Labrum with spine(s); furcal spines distant from base of dorso-caudal spine........................29

29. Labrum extending into needle-like spine .................................Y-nauplius Type AF ( Fig. 17A–E )

– Labrum bearing row of 3 large spines ..........................................Y-nauplius Type AP ( Fig. 17L )

30. Body length ca 3 times its width .... Hansenocaris demodex Olesen et al., 2022 ( Figs 5A–B , 6A–F )

– Body length ca 2 times its width.....................................................................................................31

31. Caudal end of body lined laterally with rows of distinct spines ....Y-nauplius Type V ( Fig. 12E–G )

– Caudal end of body lacking any such rows of distinct spines.........................................................32

32. Caudal spines all of about same length..........................................................................................33

– Caudal spines unequal in length.....................................................................................................34

33. Caudal spines ca 3 times as long as wide .................................Y-nauplius Type AK ( Fig. 19A–E )

– Caudal spines about as long as wide ..............................................Y-nauplius Type C ( Fig. 6G–L )

34. Body sub-ovoidal ........................................................................Y-nauplius Type AL ( Fig. 18A–F )

– Body sub-triangular .....................................................................Y-nauplius Type O* ( Fig. 16F–J )

Figures

Fig. 1. Field work to collect y-larvae (Facetotecta) at Sesoko Island (Okinawa, Japan) in 2018 and 2019. A. Example of newly collected y-nauplii (19 specimens) and y-cyprids (1 specimen). B. Seven lecithotrophic y-nauplii showing internal yolk and pigment, too early in development to be identified except for a specimen of Type AM and possible specimens of Types K and AG. C. Two common morphospecies of planktotrophic nauplii, Types A* and I*. D. Tray with code-labelled petri dishes used for rearing. E. Sesoko Island. F. Pier of Sesoko Station. G. Sorting and photography of y-larvae in the lab. A–C from Olesen et al. (2002).

Fig. 2. Y-naupliar (Facetotecta) diversity at Sesoko Island (Okinawa, Japan). A. Schematic life cycle of y-larvae (modified from Itô 1991; Glenner et al. 2008). B. Overview of 34 morphospecies of lecithotrophic last-stage y-nauplii obtained by laboratory rearing of earlier-stage nauplii collected in the plankton; three of them represent formally described species while 31 remain undescribed. C. Nine types of planktotrophic y-larvae, of which only Type A* is treated in detail in this paper. All photos in B and C are to the same scale. Figure also used in Olesen (2004). Examples of live video of most of the y-naupliar morphospecies can be seen at https://youtu.be/er0mYLswV-c and are also deposited at Figshare.com: https://doi.org/10.6084/m9.figshare.24953568.v1.

Fig. 4. Planktotrophic (feeding) y-nauplius of morphospecies Type A*. A. Ventral view. B. Lateral view. C. Naupliar feeding apparatus of right side. D. Postero-lateral spines of left side, dorso-caudal organ, dorso-caudal spine and furcal spines. The naupliar feeding apparatus, postero-lateral spines, dorsocaudal organ and dorso-ventrally flattened body (especially the first of these) are the key characters for separating planktotrophic y-nauplii from lecithotrophic y-nauplii. Abbreviations: Bas = basis; Cox = coxa; En = endopod; 1–3 = postero-lateral spine rows on trunk.

Fig. 12. Last-stage nauplii of two different morphospecies of y-larvae (Facetotecta) from Sesoko Island (Okinawa, Japan). A–D. Y-nauplius Type U*. E–G. Y-nauplius Type V. Shown either in life (A, E–G) or as slide-mounted exuviae (B–D). Abbreviations: A1 = first antenna; A2 = second antenna; Md = mandible.

Fig. 9. Last-stage nauplii of four different morphospecies of y-larvae (Facetotecta) from Sesoko Island (Okinawa, Japan). A–C. Hansenocaris cristalabri Olesen & Grygier, 2022. D–E. Hansenocaris aquila Olesen & Grygier, 2022. F–H. Y-nauplius Type AC. I–N. Y-nauplius Type AH*. Shown either in life (A–B, F–H, I–J), as slide-mounted exuviae (D–E, K–N) or in SEM (C). Abbreviations: A1 = first antenna; A2 = second antenna; Md = mandible. A–E from Olesen & Grygier (2022).

Fig. 11. Last-stage nauplii of two different morphospecies of y-larvae (Facetotecta) from Sesoko Island (Okinawa, Japan). A–G. Y-nauplius Type W. H–M. Y-nauplius Type AD. Shown either in life (A–C, H–J) or as slide-mounted exuviae (D–G, K–M). Abbreviations: A1 = first antenna; A2 = second antenna; Md = mandible.

Fig. 10. Last-stage nauplii of two different morphospecies of y-larvae (Facetotecta) from Sesoko Island (Okinawa, Japan). A–F. Y-nauplius Type E*. G–K. Y-nauplius Type AG*. Shown either in life (A–B, G–H) or as slide-mounted exuviae (C–F, I–K). Abbreviations: A1 = first antenna; A2 = second antenna; Md = mandible.

Fig. 7. Last-stage nauplii of two different morphospecies of y-larvae (Facetotecta) from Sesoko Island (Okinawa, Japan). A–G. Type D* (“Big brown”). H–M. Type B. Shown either in life (A–B, H–J) or as slide-mounted exuviae (C–G, K–M). Abbreviations: A1 = first antenna; A2 = second antenna; Md = mandible.

Fig. 14. Last-stage nauplii of two different morphospecies of y-larvae (Facetotecta) from Sesoko Island (Okinawa, Japan). A–F. Y-nauplius Type M. G–L. Y-nauplius Type AI. Shown either in life (A–B, G–I) or as slide-mounted exuviae (C–F, J–L). Abbreviations: A1 = first antenna; A2 = second antenna; Md = mandible.

Fig. 13. Last-stage nauplii of two different morphospecies of y-larvae (Facetotecta) from Sesoko Island (Okinawa, Japan). A–E. Y-nauplius Type X. F–J. Y-nauplius Type K. Shown either in life (A–B, F–H) or as slide-mounted exuviae (C–E, I–J). Abbreviations: A1 = first antenna; A2 = second antenna; Md = mandible.

Fig. 19. Last-stage nauplii of four different morphospecies of y-larvae (Facetotecta) from Sesoko Island (Okinawa, Japan). A–E. Y-nauplius Type AK. F–G. Y-nauplius Type AO. H. Y-nauplius Type AM. I–K. Y-nauplius Type AN. Shown either in life (A–B, H) or as slide-mounted exuviae (C–G, I–K). Abbreviations: A1 = first antenna; A2 = second antenna; Md = mandible.

Fig. 15. Last-stage nauplii of two different morphospecies of y-larvae (Facetotecta) from Sesoko Island (Okinawa, Japan). A–F. Y-nauplius Type L. G–M. Y-nauplius Type AB. Shown either in life (A–C, G–J) or as slide-mounted exuviae (D–F, K–M). Abbreviations: A1 = first antenna; A2 = second antenna; Md = mandible.

Fig. 16. Last-stage nauplii of two different morphospecies of y-larvae (Facetotecta) from Sesoko Island (Okinawa, Japan). A–E. Y-nauplius Type Y. F–J. Y-nauplius Type O*. Shown either in life (A, F–G) or as slide-mounted exuviae (B–E, H–J). Abbreviations: A1 = first antenna; A2 = second antenna; Md = mandible.

Fig. 8. Last-stage nauplii of three different morphospecies of y-larvae (Facetotecta) from Sesoko Island (Okinawa, Japan). A–D. Type F. E–G. Type G. H–J. Type H. Shown either in life (A–B, E–F, H–I) or as slide-mounted exuviae (C–D, G, J). Abbreviations: A1 = first antenna; A2 = second antenna; Md = mandible.

Fig. 5. Scanning electron micrographs of three significantly different kinds of last-stage lecithotrophic (non-feeding) y-nauplii (Facetotecta) illustrating key characters (general body size and shape, and morphology of labrum and caudal spines) used herein to separate and describe 34 y-naupliar morphospecies, with various measurements explained directly on the figure. A–B. Hansenocaris demodex Olesenet al., 2022, ventral view with detail of labral area.C. Hansenocaris cristalabri Olesen & Grygier, 2022 (holotype, NHMD), lateral view. D–F. Type K, lateral view with details of labrum and caudal spines. A–B from Olesen et al. (2022); C from Olesen & Grygier (2022).

Fig. 17. Last-stage nauplii of three different morphospecies of y-larvae (Facetotecta) from Sesoko Island (Okinawa, Japan). A–E. Y-nauplius Type AF. F–K. Y-nauplius Type N*. L. Y-nauplius Type AP. Shown either in life (A–C, F–G) or as slide-mounted exuviae (D–E, H–L). Abbreviations: A1 = first antenna; A2 = second antenna; Md = mandible. L from Grygier et al. (2019).

Fig. 6. Last-stage nauplii of two different (morpho)species of y-larvae (Facetotecta) from Sesoko Island (Okinawa, Japan). A–F. Hansenocaris demodex Olesen et al., 2022 (all paratypes). G–L. Type C (‘Bumblebee’). Shown either in life (A–B, G–H) or as slide-mounted exuviae (C–F, I –L).Abbreviations: A1 = first antenna; A2 = second antenna; Md = mandible.

Fig. 18. Last-stage nauplii of two different morphospecies of y-larvae (Facetotecta) from Sesoko Island (Okinawa, Japan). A–F. Y-nauplius Type AL. G–J. Y-nauplius Type AJ. Shown either in life (A–C, G–J) or as slide-mounted exuviae (D–F). Abbreviations: A1 = first antenna; A2 = second antenna; Md = mandible.