Solomon’s Gold Mine: Description or redescription of 24 species of Caridina (Crustacea: Decapoda: Atyidae) freshwater shrimps from the Solomon Islands, including 11 new species

Following recent (2014–2017) collections made in the Solomon Islands by the MNHN and the NGO ESSI, we provide a checklist of the species of amphidromous freshwater shrimps of the genus Caridina H. Milne Edwards, 1837 from this region. Using morphological as well as molecular data in an integrative taxonomic perspective, we found a total of 24 species, including 11 new for science, that are described or re-described, illustrated and discussed in relation to their habitat and distribution. Newly described species are Caridina barakoma sp. nov., C. choiseul sp. nov., C. intermedia sp. nov., C. maeana sp. nov., C. nana sp. nov., C. piokerai sp. nov., C. pisuku sp. nov., C. paratypus sp. nov., C. poarae sp. nov., C. sikipozo sp. nov. and C. turipi sp. nov. Caridina gueryi Marquet, Keith & Kalfatak, 2009 is re-validated as a species distinct from C. buehleri Roux, 1934. Lectotypes are designated for C. mertoni Roux, 1911 and C. papuana Nobili, 1905. Diagnoses for 6 informative species groups are provided: C. brevicarpalis group, C. gracilirostris group, C. nilotica group, C. typus group, C. serratirostris group and C. weberi group. A map of the species distribution in the Solomon Islands, as well as the phylogenetic relationships between the species and their relatives, are provided.


Introduction
The freshwater shrimp genus Caridina H. Milne-Edwards, 1837, comprising 311 species (WoRMS database: http://www.marinespecies.org/aphia.php?p=taxdetails&id=240672 as of March 2020) and mostly present in the Indo-Pacific region, is the most diversified genus of the family Atyidae (De Grave et al. 2015) and an important ecological component in the tropical streams (Covich et al. 1999;Pringle et al. 1993). Their high diversity combined with the lack of informative morphological characters have led to a confused taxonomy (Richard & Clark 2009). Indeed, until recently, their taxonomy was mainly based on morphological characters. Some have been proven highly variable within a species (e.g., rostrum shape and indentation (de Mazancourt et al. 2017b) or coloration) and so taxonomically non-informative, making it difficult to establish good species delimitations (von Rintelen & Cai 2009). There is thus a need for an integrative and standardized approach to improve the taxonomy of the group, focusing on informative morphological features and using molecular characters (Page et al. 2005;Page & Hughes 2011;de Mazancourt et al. 2017b). In Indo-Pacific islands, amphidromous shrimps have been grouped into six main complexes: (1) C. nilotica complex (Johnson 1963;Holthuis 1978;Jalihal et al. 1984;Choy 1991;Richard & Clark 2005;), (2) C. weberi complex (Richard & Chandran 1994;Cai & Shokita 2006a), (3) C. gracilirostris complex , (4) C. typus complex Bernardes et al. 2017), (5) C. serratirostris complex (Cai & Shokita 2006b), (6) C. brevicarpalis complex (Short 2009). Little work has been done to differentiate among the species clustered within these complexes, possibly due to the difficulty and costs associated with genetic analyses and the highly similar morphology of shrimps from different populations.
To illustrate this problem, species of the genus Caridina from the Solomon Islands were studied. Before this study, the neighboring islands belonging to the Bismarck and Admiralty archipelagoes in Papua New Guinea (Roux 1934) or those of the Vanuatu archipelago (Keith et al. 2010;de Mazancourt et al. 2017a) were inventoried, but none of the Solomon Islands were surveyed for freshwater shrimps. In order to fill this gap, in the context of the CEPF "Melanesia Hotspot", an inventory of freshwater fauna was done in the Solomon Islands between 2014 and 2017, supported by the French Ichthyological Society. This archipelago, consisting of six major islands and over 900 smaller islands lies in Oceania to the east of Papua New Guinea and northwest of Vanuatu and covers a land area of 28 400 km 2 . Six islands (

Collection of specimens
Specimens from the Solomon Islands were collected by electrofishing (portable Dekka 3000 electric device, Germany) (Lamarque et al. 1975). All material was preserved in 75-95% ethanol and has been deposited in the collections of the Muséum national d'histoire naturelle of Paris (MNHN).

Morphological comparison
The rostrum, the general cephalon, the pereiopods 1-3 and 5 and the abdomen were observed using a stereoscopic microscope. The proportions of the various joints of the appendages were measured using microphotographs and the AnalySIS Works software (Olympus). Drawings were made using the "Digital Inking" method (Coleman 2003(Coleman , 2006 by tracing vectoral paths on high-resolution photographs using Adobe Illustrator. de MAZANCOURT V. et al., List of Caridina freshwater shrimp species from the Solomon Islands MrBayes implemented in the Cyber Infrastructure for Phylogenetic Research (CIPRES) portal v.3.1. (Miller et al. 2010; https://www.phylo.org/). One hundred independent searches, each starting from distinct random trees, were conducted. Robustness of the nodes was assessed using non-parametric bootstrapping (Felsenstein 1985) with 1000 bootstrap replicates. We considered a group to be 'moderately supported' if it had a bootstrap support value (B) between 75 and 89%, and Bayesian posterior probabilities (PP) between 0.8 and 0.95, and 'highly supported' when B ≥ 90% and PP ≥ 0.95. For the analysis, we included 49 specimens collected during the authors' field trips to the Solomon Islands to which were added 18 specimens of species occuring in the Solomon Islands collected by the authors from other Indo-Pacific localities. Five sequences were also retrieved from GenBank (Table 1). Two species of Paratya Miers, 1882 were used as outgroups. Three different species delimitation methods were tested on the dataset. First, ABGD (Puillandre et al. 2012) was implemented on the web server https://bioinfo.mnhn.fr/abi/public/abgd/abgdweb.html, for which a pairwise distances matrix was built from the sequence's alignment using the JC69 Jukes-Cantor model. We used Pmin = 0.001 and Pmax = 0.1 with 10 steps and X = 1.0. A graphical representation of the distribution of the distances showed a barcoding gap between 0.04 and 0.05 divergence. Second, we ran a bPTP analysis (Zhang et al. 2013) from the web server http://species.h-its.org/ptp/ using the ML tree as input. Other parameters were left as default. Finally, we used the GMYC method (Pons et al. 2006) implemented in the web server https://species.h-its.org/gmyc/ using only the single threshold method (phylogeny composed of several species with one coalescent time value), as species studied are phylogenetically close to each other. To run this analysis, an ultrametric tree is required, which was constructed using BEAST ver. 1.10.4 (Suchard et al. 2018). We used a relaxed lognormal clock with a coalescent tree prior, as these have been identified as best prior parameters for GMYC analyses (Esselstyn et al. 2012;Monaghan et al. 2009). Monte Carlo Markov chains (MCMC) ran for 10 000 000 generations, sampling every 2000 generations. Chain convergence was assessed using Tracer ver. 1.6 (Rambaut et al. 2014). The consensus tree (maximum clade credibility tree; 10% burn in; tree not presented) was constructed with TreeAnnotator ver. 1.10.4 (Drummond & Rambaut 2007).

Description
Cephalothorax. Antennal spine below suborbital angle. Pterygostomian margin sub rectangular. Rostrum ( Fig. 3m) long, 0.9-1.2 of cl, passing end of scaphocerite, armed with 12-16 teeth on dorsal margin, without apical teeth, 0-1 of them situated on carapace behind orbital margin, ventral margin with 5-10 teeth. Eyes well developed, anterior end reaching to 0.50-0.60 length of basal segment of antennular peduncle. Antennular peduncle 0.72 (♀) or 0.88 (♂) times as long as carapace. Anterolateral angle reaching 0.20 length of second segment, basal segment of antennular peduncle longer than sum of second and third segment lengths, second segment distinctly longer than third segment. Stylocerite reaching 0.75-0.78 length of basal segment of antennular peduncle.
abdomen. Third abdominal somite with moderarely convex dorsal profile. Sixth abdominal somite 0.58 times cl, 1.6 times as long as fifth somite, shorter than telson.
telson (Fig. 3h). 3.2-3.4 times as long as wide, with four to six pairs of dorsal spinules and one pair of dorsolateral spinules; posterior margin with a median process, it is rounded with 4 intermediate setae shorter than lateral ones. pre-anal Carina (Fig. 3g). High, armed with a spine.

Habitat
In the vegetation at the edge of the rivers in flowing water in the lower course of rivers.

Colour pattern
Unknown.
In the litterature on C. brevicarpalis, we only find 3 drawings: in De Man (1892), Bouvier (1925) and Edmondson (1935). The latter studied specimens from Viti Levu (Fiji) which seem different from our species by their rostrum armed with 18 teeth on the dorsal margin and 7 on the ventral margin (vs respectively 12-16 and 5-10 in C. barakoma sp. nov. and 11-14 and 4-7 in the type specimens of C. brevicarpalis). By their uropodal diaeresis, 10-13, it also seems different from our new species (8-10) and the type specimens (8-9 according Bouvier 1925). Fiji is the easternmost limit of the C. brevicarpalis complex in the Pacific, as no species of this group occur in Futuna, Samoa or French Polynesia (Keith et al. 2013). In this paper we postulate that the C. brevicarpalis group includes several different species allied to C. brevicarpalis like C. barakoma sp. nov., C. endehensis and the Fijian species.

Caridina gracilirostris species group Diagnosis
Slender morphology with a very long and upcurved rostrum (twice the carapace length), armed with few dorsal teeth (fewer than 10), apical teeth present, the antennal spine ventral to the inferior orbital angle, a long antennular peduncle (more than 0.70 times as long as carapace), pterygostomian margin rounded, segments of walking legs slender, a typical dorsal hump over the third abdominal somite, a long sixth abdominal somite (always more than half of carapace length), a small pre-anal carina bearing or not an acute spine, few spinules on the uropodal diaeresis (>10), very few short terminal setae on the telson, endopod of the first male pleopod subtriangular mostly without an appendix, but when it is present, on the subdistal outer margin, reaches beyond distal end of endopod by a short length.
abdomen. Third abdominal somite with moderarely convex dorsal profile. Sixth abdominal somite 0.8 times as long as carapace, 2.0 times as long as fifth somite, as long as telson. telson (Fig. 4i). Very slender, 4.5 times as long as wide, with four pairs of dorsal spinules and one pair of dorsolateral spinules; posterior margin without a median process; distal setae very stout, lateral pair distinctly longer than intermediate setae (mostly one pair). male pleopods. Pl1 (Fig. 4j): endopod subtriangular, 2.1 times as long as wide, 0.21 times as long as exopod, without appendix interna. Pl2 (Fig. 4k): appendix masculina reaching 0.50 times length of endopod; appendix interna reaching 0.50 of appendix masculina.

Habitat
In the lower course of streams under marine influence, very often in brackish water.
Colour pattern (Fig. 25A) Totally translucent body with a rostrum alternating black and red areas.

Distribution
Widely distributed in the tropical and subtropical Indo-Pacific region: Indonesia, the Philippines, India, Australia and the Solomon Islands.

Description
Cephalothorax. Antennal spine short, situated below inferior orbital angle. Pterygostomian margin rectangular round. Rostrum (Fig. 5m): strongly upturned, very long, 1.6-2.2 of cl, reaching far beyond distal end of scaphocerite, armed with 5-9 teeth on dorsal margin, with 1-2 apical teeth, non of them situated on carapace behind orbital margin, ventral margin with 21-27 teeth. Eyes well developed, anterior end reaching to 0.7 times length of basal segment of antennular peduncle. Antennular peduncle 0.69 (♀) -0.88 (♂) times as long as carapace. Anterolateral angle reaching 0.32 length of second segment, second segment distinctly longer than third segment. Stylocerite reaching to 0.86 length of basal segment of antennular peduncle.
abdomen. Third abdominal somite with moderately convex dorsal profile. Sixth abdominal somite 0.80 as long as carapace, 1.76 times as long as fifth somite, slightly shorter than telson.   MNHN-IU-2018-2811 and MNHN-IU-2018-2808. telson (Fig. 5i). 3.4-3.7 times as long as wide, with three or four pairs of dorsal spinules and one pair of dorsolateral spinules; posterior margin with a median process, distal setae short, lateral pair slightly longer than intermediate setae (mostly two pairs). male pleopods. Pl1 (Fig. 5j): endopod subtriangular, 1.5 times as long as wide, 0.17 times as long as exopod, with an appendix interna, on the subdistal outer margin, which reaches beyond distal end of endopod by a short length. Pl2 (Fig. 5k): appendix masculina reaching 0.57 times length of endopod; appendix interna reaching 0.71 of appendix masculina.

Habitat
In the lower course of streams, near the estuary.
Colour pattern (Fig. 25B) Totally translucent body with a characteristic red ventral margin on the rostrum.

Distribution
This species occurs in Indonesia, the Philippines, the Solomon Islands and Vanuatu.
The presence of an appendix interna on the endopod of the male first pleopod (vs no appendix interna) and a longer P1 carpus, 2.3-3.1 times as long as wide (vs 1.5-1.9), and P2 carpus, 5.2-6.4 times as long as wide (vs 3.2-5.1), separates C. neglecta from C. gracilirostris.

Caridina nilotica species group Diagnosis
Slender morphology, with a mostly long rostrum (longer than the antennular peduncle) but variable, the antennal spine ventral to the inferior orbital angle, a long antennular peduncle (subequal to carapace length), segments of walking legs slender, a typical dorsal hump over the third abdominal somite, a long sixth abdominal somite (always more than half of carapace length), a small pre-anal carina sometimes bearing an acute spine, a moderate number of spinules on the uropodal diaeresis (<15), fewer, medium to short and terminal setae on the telson and a subtriangular endopod of the first male pleopod with or without an appendix on the subdistal outer margin or even placed at the distal end. An oblique red band on the cephalothorax is very characteristic. Jalihal & Shenoy, 1998 Figs 2H, 25C

Description
Cephalothorax. Carapace (Fig. 6m) smooth, glabrous, with sharp antennal spine placed at lower orbital angle. Pterygostomial margin subrectangular. Variable length rostrum, 0.5-1.2 of cl, 16-28 dorsal teeth closely set, leaving 0.0-0.35 unarmed distally, 1-4 post-orbital teeth present. 7-12 teeth present on ventral margin extending from proximal end either to tip or with short distal part unarmed. Number of dorsal teeth on rostrum before first ventral tooth 13-19. Eyes well developed, anterior end reaching to 0.72 times length of basal segment of antennular peduncle. Antennular peduncle 0.75 (♀) -0.88 (♂) times as long as carapace. Anterolateral angle reaching 0.30 length of second segment, second segment distinctly longer than third segment. Stylocerite reaching to 0.83 length of basal segment of antennular peduncle.
abdomen. A typical dorsal hump over third abdominal somite. Sixth abdominal somite 0.7 as long as carapace, 1.9 times as long as fifth somite, slightly shorter than telson. telson (Fig. 6i). 2.9-3.2 times as long as wide, with four pairs of dorsal spinules and one pair of dorsolateral spinules; posterior margin with a median process, distal setae short, lateral pair slightly longer than intermediate setae (4-7). pre-anal Carina (Fig. 6g). With a spine or not.

Habitat
This species prefers fresh and well-oxygenated waters. It is found from the lower to the higher course. It is more abundant in the areas situated above waterfalls, where predators are less numerous.
Colour pattern (Fig. 25E) The colour of the body is hyaline with many red dots.

Distribution
As far as we know, this species is known only from Choiseul Island.

Remarks
In this new species, rostrum length is variable (cl 0.5-1.2), being longer in the lower course and shorter in the upper course, as recently found by de Mazancourt et al. (2017b) who highlighted the "Pinocchioshrimp effect" on C. variabilirostris de Mazancourt, Marquet & Keith, 2018 from Pohnpei. When the rostrum is short, the general appearance resembles that of C. mertoni Roux, 1911, whereas when the rostrum is long, the general appearance is similar to that of C. brachydactyla De Man, 1908. Similarly, C. variabilis de Mazancourt, Rogers & Keith, 2018 from Guam and Palau shows a longer or a shorter rostrum depending on the altitude.
Caridina choiseul sp. nov. looks like C. variabilis and C. variabilirostris from Micronesia by the number of teeth and their placement on the rostrum, the proportions between the joints of pereiopods and egg size. However, it can be differentiated by the pre-anal carina that sometimes has a spine (vs always unarmed in C. variabilis and C. variabilirostris) and the P5 dactylus that has more spiniform setae on the flexor margin: 28-41 (vs 13-30 and 18-29, respectively). Caridina choiseul sp. nov. is most similar to C. brachydactyla De Man, 1908 and C. peninsularis Kemp, 1918, but its P2 chela is stouter (2.0-2.7 times as long as wide vs 2.7-3.2 and 2.8-3.0, respectively) and the pre-anal carina which has a spine or not (vs always armed). Caridina choiseul sp. nov. looks like Caridina elongapoda Liang & Yan, 1977, but the pre-anal carina has a spine or not (vs always unarmed) and its P3 dactylus with 5-6 spiniform setae on the flexor margin in addition to the terminal one (vs 6-7). Caridina choiseul sp. nov. looks like C. mertoni Roux, 1911 andC. sundanella Holthuis, 1978, but the pre-anal carina has a spine or not (vs always unarmed) and its rostrum has 7-12 ventral teeth (vs 4-9 and 7-9, respectively).

Description
Cephalothorax. Carapace ( Fig. 7m) smooth, glabrous, with sharp antennal spine placed at lower orbital angle. Pterygostomial margin subrectangular. Long rostrum, 1.1-1.8 of cl, curved up distally, reaching well beyond scaphocerite. 16-26 dorsal teeth closely set, leaving 0.4-1.3 unarmed distally except for one or two subapical teeth, 1-3 post-orbital teeth present. 9-20 teeth present of ventral margin, extending from proximal end either to tip or with short distal part unarmed. Number of dorsal teeth on rostrum before first ventral tooth 11-16. Eyes well developed, anterior end reaching to 0.60-0.63 length of basal segment of antennular peduncle. Antennular peduncle 0.69 (♀) -0.90 (♂) times as long as carapace. Anterolateral angle pointed reaching to 0.37 length of the second segment; second segment distinctly longer than third segment. Stylocerite reaching to 0.83-0.90 length of basal segment of antennular peduncle.
abdomen. Typical dorsal hump over third abdominal somite, sixth abdominal somite 0.74 as long as carapace, 1.9 times as long as fifth somite, 1.0 times as long as telson.
telson (Fig. 7i). 3.8 times as long as wide, with four pairs of dorsal spinules and one pair of dorsolateral spinules; posterior margin with median process, distal setae short, lateral pair slightly longer than intermediate setae (4).

Habitat
All specimens were collected in a typical brackish water environment (brackish water pool or lower part of rivers, near the estuary). In rivers of Solomon Islands, C. intermedia sp. nov. is syntopic with C. brevidactyla and C. appendiculata.
Colour pattern (Fig. 25F) The colour of the body is hyaline with many reds dots.

Distribution
Caridina intermedia sp. nov. is known from the Solomon Islands (Choiseul, Kolombangara, Vella Lavella and Isabel) and Papua New Guinea (New Britain).
Caridina intermedia sp. nov. differs from C. appendiculata by its greater number of teeth on the proximal part of the dorsal margin of the rostrum, 16-26 closely set (vs 12-17 teeth somewhat irregular spaced in C. appendiculata), greater number of dorsal teeth before the first ventral teeth 11-16 (vs 8-12) and a slightly greater number of spiniform setae on the dactylus of the fifth pereiopod 31-59 (vs 33-44).

Habitat
This species prefers fresh and well-oxygenated waters. It is found from the middle to the higher course of rivers. They are more abundant in areas situated above waterfalls where predators are less numerous.
Colour pattern (Fig. 25G) The colour of the body is hyaline with many red dots and oblique red bands.

Etymology
Named after the river Pisuku, in Choiseul Island, the type locality where this new species occurs. The name is used as a noun in apposition.

Habitat
This species is found from the lower to higher course of rivers (5-317 m). It can be found in areas where the current speed is low, rich in vegetation debris, as well as in zones where the current is strong.

Colour pattern
The colour of the body can vary from pink-orange to blackish brown, sometimes with a brown to white stripe on the back.

Distribution
Caridina typus, the type species of the genus, has a very wide distribution in the Indo-West Pacific area, ranging from South Africa, Madagascar and the Seychelles to Japan, Malaysia, Philippines, Australia, Micronesia (Pohnpei), Papua New Guinea, Solomon Islands, New Caledonia, Vanuatu and Fiji. In contrast to what Holthuis (1965) and Chace (1997) stated, C. typus does not occur in Polynesia, but a species allied to C. jeani Cai, 2010 (see below).

Remarks
According to Chace (1997) and Cai et al. (2006) the type locality of C. typus is unknown. Bouvier (1925) ensures that type specimens have been caught on Mauritius Island but were lost following a move in 1918 caused by bombardments of Paris.
Our specimens fit well with the descriptions of this species occurring in Japan by Cai et al. (2006) by its short rostrum reaching near the end to the third segment of the antennular peduncle (vs to end of second segment of antennular peduncle according to Cai et al. (2006)), unarmed dorsally and armed ventrally, with 0-4 teeth (vs 1-4

Holotype
abdomen. Third abdominal somite with moderarely convex dorsal profile. Sixth abdominal somite 0.43 times as long as carapace, 1.3 times as long as fifth somite, slightly shorter than telson. telson (Fig. 11i). 2.3 times as long as wide, with four or five pairs of dorsal spinules and pair of dorsolateral spinules; posterior margin without median process, rounded, with 4-7 long intermediate setae longer than lateral ones. male pleopods. No males.

Habitat
This species is largely rheophile and prefers fresh and well-oxygenated waters from the lower to the higher course of rivers.

Colour pattern
Unknown.

Distribution
As far as we know, this species occurs only in Choiseul Island.
In contrast to previous definitions , we consider that the C. typus group is not characterized by the absence of dorsal teeth on the rostrum. Bouvier (1925) includes many species with an armed rostrum in his "groupe du C. typus"; however, we consider some of them to be part of the C. weberi species group (see below).
According to our study, the length of setae on the telson (namely plumose terminal setae on the telson subequal to lateral ones or slightly longer) is a better criteria to characterize this group that agree with our molecular results (see de Mazancourt et al. 2019a where it appears as a monophyletic group). We thus consider some species like C. turipi or C. sumatrensis that have numerous dorsal teeth to be part of the C. typus group.

Diagnosis
Robust morphology with a straight or bent rostrum, armed or not on the dorsal margin, without apical teeth, the antennal spine fused with the inferior orbital angle, antennular peduncle equal to or more than half of carapace in length, pterygostomian margin rounded, stout walking leg segments, the carpus of the first pereiopod often deeply excavated, a short sixth abdominal somite (around half of carapace length), a high pre-anal carina with no spine or a small one, a great number of spinules on the uropodal diaeresis (>15), long and plumose terminal setae on the telson clearly longer than lateral ones, and a long subrectangular endopod of the first male pleopod with a short appendix on the subdistal outer margin. Roux, 1934 Figs 2W, 12, 26A

Description
Cephalothorax. Antennal spine below suborbital angle. Pterygostomian margin sub rectangular. Rostrum (Fig. 12l): 0.4-0.7 of cl, reaching to distal end of antennular peduncle, armed with 13-16 teeth on dorsal margin, 2-5 of them situated on carapace behind orbital margin, ventral margin with 2-5 teeth. Eyes well developed, anterior end reaching to 0.66 length of basal segment of antennular peduncle. Antennular peduncle 0.50-0.55 (♀) times as long as carapace. Anterolateral angle reaching 0.30 length of second segment, basal segment of antennular peduncle longer than sum of second and third segment lengths, second segment distinctly longer than third segment. Stylocerite sharp, long, reaching to about half of second segment of antennular peduncle.

Habitat
This species lives in a typical brackish water environment (brackishwater pools or lower part of rivers, near the estuary). The species is rarely found over the superior limit of the tide-influenced zone, and was not found in the true freshwater zone.
Colour pattern (Fig. 26A) Body usually dark red to black, with a lighter dorsal band from the rostrum to the telson and sometimes white stripes on each abdominal somite.

Distribution
This species occurs in Solomon Islands (Kolombangara, Vella Lavella), in the Vanuatu archipelago (Malekula and Santo islands) and in Papua New Guinea (New Britain).

Description
Cephalothorax. Antennal spine below suborbital angle. Pterygostomian margin sub rectangular. Rostrum (Fig. 13k): 0.5-1.4 of cl, reaching to or overreaching distal end of scaphocerite, armed with 9-17 teeth on dorsal margin, 3-5 of them situated on carapace behind orbital margin, ventral margin with 2-7 teeth. Eyes well developed, anterior end reaching to 0.63 length of basal segment of antennular peduncle. Antennular peduncle 0.56-0.57 (♀) times as long as carapace. Anterolateral angle reaching 0.33 length of second segment, basal segment of antennular peduncle longer than sum of second and third segment lengths, second segment distinctly longer than third segment. Stylocerite sharp, long, reaching to about half of second segment of antennular peduncle.
abdomen. Third abdominal somite with moderately convex dorsal profile. Sixth abdominal somite 0.48 as long as carapace, 1.5 times as long as fifth somite, shorter than telson. telson (Fig. 13j). 2.5 times as long as wide, with 5-6 pairs of dorsal spinules and one pair of dorsolateral spinules; posterior margin with a median process, broadly convex with 7-12 plumose intermediate setae much longer than lateral. male pleopods. Pl1: endopod subtriangular, 2.9 times as long as wide, reaching 0.37 times length of endopod, with an appendix on subdistal outer margin which reaches beyond distal end of endopod with most of its length. Pl2: appendix masculina reaching 0.57 times length of endopod; appendix interna reaching 0.60 of appendix masculina.

Habitat
This species lives in a typical brackish water environment (brackishwater pools or lower part of rivers, near the estuary). The species is rarely found over the superior limit of the tide-influenced zone, and was not found in the true freshwater zone.
Colour pattern (Fig. 26B) Body usually dark red to black, with a lighter dorsal band from the rostrum to the telson and sometimes white stripes on each abdominal somite.

Distribution
This species occurs in the Solomon Islands (Kolombangara), in the Vanuatu archipelago (Malekula and Santo Islands) and in Indonesia (Sulawesi).

Remarks
Our specimens are distinguished from the holotype of C. buelheri from New Ireland by its long P5 propodus, 12.3-20.5 times as long as wide (vs short P5 propodus 9.4 in the holotype of C. buehleri) and 4.4-6.2 times as long as the dactylus (vs 3.8). In contrast, they fit well with the holotype of C. gueryi from Santo (vs 12.3-15.9 and 4.6-5.2 in the holotype of C. gueryi, respectively). However, if we include other specimens of C. buehleri, the characters overlap, which is the reason why we previously mistakenly placed them in synonymy (de Mazancourt et al. 2017a). These two species can thus be considered to be cryptic.
abdomen. Third abdominal somite with moderately convex dorsal profile. Sixth abdominal somite about half length of carapace, 1.7 times as long as fifth somite, slightly shorter than telson. telson (Fig. 14i). 2.6 times as long as wide, with four or five pairs of dorsal spinules and one pair of dorsolateral spinules; posterior margin without a median process, rounded, with 5-9 very long intermediate setae longer than lateral. male pleopods. Pl1 (Fig. 14j): endopod subrectangular, 3 times as long as wide, reaching 0.43 times length of endopod, with an appendix on subdistal outer margin which reaches beyond distal end of endopod on a short length. Pl2 (Fig. 14k): appendix masculina reaching 0.76 times length of endopod; appendix interna reaching 0.64 of appendix masculina.  (Fig. 14g). High, unarmed.

Habitat
This species is largely rheophile and prefers fresh and well-oxygenated waters from the lower to the higher course of rivers.
Colour pattern (Fig. 26E) Body dark all over, sometimes with a white dorsal band running from the rostrum to the telson.

Distribution
This species occurs in the Solomon Islands (Choiseul) and in Papua New Guinea.

Description
Cephalothorax. Suborbital angle indistinguishably fused with antennal spine. Pterygostomian margin rounded. Rostrum (Fig. 15m): straight, short, 0.4 of cl, reaching to base or near middle of second segment of antennular peduncle, armed with 11-23 teeth on dorsal margin, 0-2 of them situated on carapace behind orbital margin, ventral margin with 2-5 teeth. Eyes well developed, anterior end reaching to 0.7 times length of basal segment of antennular peduncle. Antennular peduncle 0.64 times as long as carapace. Anterolateral angle reaching 0.21 length of second segment, second segment longer than third segment. Stylocerite reaching to 0.75 length of basal segment of antennular peduncle.
abdomen. Third abdominal somite with moderately convex dorsal profile. Sixth abdominal somite about half length of carapace, 1.7 times as long as fifth somite, reaching 0.84 times length of telson. telson (Fig. 15i). 2.6 times as long as wide, with five to seven pairs of dorsal spinules and one pair of dorsolateral spinules; posterior margin with median process, rounded, with 6-11 very long intermediate setae longer than one or two lateral ones. male pleopods. Pl1 (Fig. 15j): endopod of male subrectangular, 2.5 times as long as wide, reaching 0.40 lenght of endopod, with an appendix on distal outer margin which reaches beyond distal end of endopod on a short length. Pl2 (Fig. 15k): appendix masculina reaching 0.57 times length of endopod; appendix interna reaching 0.75 of appendix masculina.

Habitat
This species is largely rheophile and prefers fresh and well-oxygenated waters from the higher course of rivers (148-276 m a.s.l.).
Colour pattern (Fig. 26C) Body covered in small dots, several red stripes, sometimes with a white dorsal band running from the rostrum to the telson.

Distribution
This species occurs in Indonesia (Sumba, Halmahera), Papua New Guinea (New Britain) and the Solomon Islands (Kolombangara and Malaita).

Habitat
This species is largely rheophile and prefers fresh and well-oxygenated waters from the lower course to the higher course of rivers.

Colour pattern
The live general colour of the body is yellowish and slightly translucent. Numerous red spots are visible all over the body. Sometimes, a lighter longitudinal dorsal band is present stretching from the rostrum to the telson (see de Mazancourt et al. 2019b).

Distribution
This species seems to have a wide distribution: Solomon Islands (Malaita and Choiseul), Fiji, Samoa, Cook (Rarotonga) and French Polynesia (Society and Austral archipelagos).

Remarks
Recently de Mazancourt et al. (2019b) have revised the species of the C. weberi group from Polynesia. Contrary to what Edmondson (1935) wrote, C. weberi does not occur in Polynesia but four other species of the C. weberi complex. Among these species, one also occurs in the Solomon Islands: C. tupaia. Curiously, although the dorsal teeth of the rostrum are situated all along its length, with sometimes 1-3 of them situated on the carapace behind the orbital margin in French Polynesia, in Samoa (Upolu) and the Cook Islands, specimens from the Solomon Islands have dorsal teeth situated considerably anterior to the orbital margin with none of them situated on the carapace behind the orbital margin (see Fig. 16).

Etymology
This new species is named after the local name of the type locality, River Maeana. The name is used as an apposition.

Description
Cephalothorax. Suborbital angle indistinguishably fused with antennal spine. Pterygostomian margin rounded. Rostrum (Fig. 17m): bent down, short, 0.3-0.5 of cl, reaching to base or middle of second de MAZANCOURT V. et al., List of Caridina freshwater shrimp species from the Solomon Islands segment of antennular peduncle, armed with 8-13 teeth on dorsal margin, all on rostrum considerably anterior to orbital margin, ventral margin with 2-5 teeth. Eyes well developed, anterior end reaching to 0.75 length of basal segment of antennular peduncle. Antennular peduncle 0.70 times as long as carapace. Anterolateral angle reaching 0.20 length of second segment, second segment longer than third segment. Stylocerite reaching to 0.62 length of basal segment of antennular peduncle.
abdomen. Third abdominal somite with moderately convex dorsal profile. Sixth abdominal somite 0.49 times as long as carapace, 1.6 times as long as fifth somite, slightly shorter than telson. telson (Fig. 17i). 3 times as long as wide, with four or five pairs of dorsal spinules and one pair of dorsolateral spinules; posterior margin with median process, rounded, with 6 very long intermediate setae longer than lateral. male pleopods. Pl1 (Fig. 17j): endopod subrectangular, 2.5 times as long as wide, reaching 0.41 lenght of endopod, with appendix on subdistal outer margin which reaches beyond distal end of endopod on short length. Pl2 (Fig. 17k): appendix masculina reaching 0.70 times length of endopod; appendix interna reaching 0.64 times length of appendix masculina.

Habitat
This new species is largely rheophile and prefers fresh and well-oxygenated waters in the higher course of rivers (279-315 m a.s.l.).
Colour pattern (Fig. 26F) The general colour is blueish overall, reddish on the cephalothorax with many red dots all over the body.

Distribution
This species occurs in the Solomon Islands (Malaita and Choiseul) and Vanuatu (Santo Island).

Etymology
This species is dedicated to Piokera S. Holland (Ecological Solutions Solomon Islands team member) who helped with the sampling in Kolombangara Island.
abdomen. Third abdominal somite with moderately convex dorsal profile. Sixth abdominal somite 0.48 times as long as carapace, 1.4 times as long as fifth somite, 0.88 as long as telson.
telson (Fig. 18i). 3 times as long as wide, with five pairs of dorsal spinules and pair of dorsolateral spinules; posterior margin with median process, rounded, with 5-10 very long intermediate setae longer than lateral ones. male pleopods. No males.

Habitat
This species is largely rheophile and prefers fresh and well-oxygenated waters from the lower course to the higher course of rivers (17-148m a.s.l.).

Colour pattern
Unknown.

Distribution
This new species occurs on several of the Solomon Islands (Choiseul, Kolombangara and Vella Lavella).

Holotype
abdomen. Third abdominal somite with moderately convex dorsal profile. Sixth abdominal somite 0.47 times as long as carapace, 1.5 times as long as fifth somite, 0.91 times as long as telson. telson (Fig. 19i). slender, 4.3 times as long as wide, with four pairs of dorsal spinules and pair of dorsolateral spinules; posterior margin with median process, rounded, with 6-7 very long intermediate setae longer than lateral ones. male pleopods. No males.

Habitat
This species is largely rheophile and prefers fresh and well-oxygenated waters from the higher course of rivers.

Colour pattern
Unknown.

Distribution
As far as we know, this new species occurs only in Vella Lavella (Solomon Islands).
abdomen. Third abdominal somite with moderately convex dorsal profile. Sixth abdominal somite 0.48 times as long as carapace, 1.7 times as long as fifth somite, 0.86 times as long as telson. telson (Fig. 20g). 2.3 times as long as wide, eight pairs of dorsal spinules and pair of dorsolateral spinules; posterior margin without median process, rounded, with 6-9 long intermediate setae longer than lateral ones. male pleopods. Pl1 (Fig. 20h): endopod subrectangular, 3 times as long as wide, reaching 0.53 lenght of endopod, with an appendix on subdistal outer margin which reaches beyond distal end of endopod on a short length. Pl2 (Fig. 20i): appendix masculina reaching 0.72 times length of endopod; appendix interna reaching 0.61 lenght of appendix masculina.

Habitat
This new species is largely rheophile and prefers fresh and well-oxygenated waters in the higher course of rivers.

Colour pattern
Unknown.

Distribution
As far as we know, this new species occurs only on Choiseul (Solomon Islands).

Description
Cephalothorax. Suborbital angle indistinguishably fused with antennal spine. Pterygostomian margin broadly subrectangular. Rostrum (Fig. 21k): straight, short, 0.3 of cl, reaching to near end of first segment of antennular peduncle, unarmed dorsally, ventral margin with 1-3 teeth. Eyes well developed. Antennular peduncle 0.54 times as long as carapace. Anterolateral angle reaching 0.33 length of second segment, second segment little longer than third segment. Stylocerite reaching to 0.83 times length of basal segment of antennular peduncle.
abdomen. Third abdominal somite with moderately convex dorsal profile. Sixth abdominal somite 0.48 times as long as carapace, 1.5 times as long as fifth somite, shorter than telson. telson (Fig. 21i). 2.7 times as long as wide, with four or five pairs of dorsal spinules and pair of dorsolateral spinules, posterior margin with median process, rounded, with 5-8 plumose intermediate setae longer than lateral ones. male pleopods. No males.

Habitat
This new species is rheophile and prefers fresh and well-oxygenated waters from the middle to upper course of rivers.

Colour pattern
Unknown.

Distribution
This species occurs in Solomon Islands (Ranongga) and Vanuatu (Santo).

Remarks
This new species looks like C. typus by its unarmed dorsal rostrum, but it is distinguished by its shorter P3 propodus, 6.7-7.7 times as long as wide (vs 7.9-10.0 in C. typus), and P3 propodus 3.2-3.7 times as long as dactylus, (vs 3. 6-4.6). This new species also looks like C. jeani, by its unarmed dorsal rostrum but it is easily distinguish by its longer P3 dactylus, 4.4-4.6 times as long as wide (vs 1.6 in C. jeani) and its P5 dactylus, 3.9-4.5 times as long as wide (vs 2.7-3.2).

Etymology
This new species is named after its resemblance to C. typus.
abdomen. Third abdominal somite with moderately convex dorsal profile. Sixth abdominal somite 0.46 times as long as carapace, 1.5 times as long as fifth somite, shorter than telson. telson (Fig. 22i). 2.3 times as long as wide, with five or six pairs of dorsal spinules and pair of dorsolateral spinules; posterior margin with median process, rounded, with 10-11 plumose intermediate setae longer than lateral ones. male pleopods. Pl1 (Fig. 22j): endopod subrectangular, 3.2 times as long as wide, reaching 0.54 length of endopod, with appendix on subdistal outer margin which reaches slightly beyond distal end of endopod on short length. Pl2 (Fig. 22k): appendix masculina reaching 0.58 times length of endopod; appendix interna reaching 0.41 times length of appendix masculina.

Habitat
This species is largely rheophile and prefers fresh and well-oxygenated waters from the middle to upper course of rivers (117-250 m a.s.l.).

Colour pattern
Unknown.

Distribution
This species occurs in the Solomon Islands (Malaita) and Vanuatu (Epi).
This new species also looks like C. jeani by its unarmed dorsal rostrum and its telson with 10-11 plumose intermediate setae longer than lateral ones (vs 6-8 in C. jeani), but it is easily distinguished by its longer P3 dactylus, 4.4-4.6 times as long as wide (terminal spiniform seta included), with 7 spiniform setae on the flexor margin in addition to the terminal one (vs 1.6 with 5 spiniform setae), and by its shorter appendix masculine on the second male pleopod, 0.58 of the length ofthe endopod (vs 0.80).
In the same way as for the C. typus group, we include in the C. weberi group species with or without dorsal teeth on their rostrum. The length of setae on the telson (namely long and plumose terminal setae on the telson clearly longer than lateral ones) is a better criterion to characterize this group.

Caridina serratirostris species group Diagnosis
Moderately robust morphology with a moderately short and straight rostrum (reaching end of antennular peduncle), armed with many dorsal teeth, at least 6 of them on the carapace, without apical teeth, the antennal spine ventral to the inferior orbital angle, a long antennular peduncle (about half the carapace length) with an usually long stylocerite (reaching end of first segment of antennular peduncle), pterygostomian margin rounded, segments of walking legs very slender, sixth abdominal somite about half of carapace length, a small pre-anal carina sometimes bearing an acute spine, a great number of spinules on the uropodal diaeresis (>15), numerous, long and plumose terminal setae on the telson and a rounded endopod of the first male pleopod, without appendix interna. De Man, 1892 Figs 2A, 23 Caridina serratirostris De Man, 1892: 382, pl. 23, figs 28a-e (type locality: "Bangkalan" and "Bonea" Rivers, Selayar, Indonesia). Caridina serratirostris var. typica Bouvier, 1925: 218 (partim), 480-486.
abdomen. Third abdominal somite with moderately convex dorsal profile. Sixth abdominal somite 0.56 times as long as carapace, 1.7 times as long as fifth somite, shorter than telson. telson (Fig. 23i). 2.5-3.5 times as long as wide, with five pairs of dorsal spinules and pair of dorsolateral spinules; posterior margin with small median process, rounded, with 8-10 very long intermediate plumose setae longer than lateral ones. male pleopods. Pl1 (Fig. 23j): endopod rounded, 1.4 times as long as wide, reaching 0.22 times length of endopod, no appendix. Pl2 (Fig. 23k): appendix masculina reaching 0.77 times length of endopod; appendix interna reaching 0.75 times length of appendix masculina.

Habitat
Caridina serratirostris lives from the estuarine brackish water to the fresh water in the lower course of the rivers.
Colour pattern (Fig. 26D) The body is yellowish to reddish with many red dots.

Distribution
This species occurs in Indonesia (Selajar), the Solomon Islands, Australia and Papua New Guinea (New Britain).

Description
Cephalothorax. Antennal spine ventral to inferior orbital angle. Pterygostomian margin rounded. Rostrum (Fig. 24k): straight, 0.6-0.7 of cl, reaching to base of third segment of antennular peduncle, armed with 17-22 teeth on dorsal margin, 6-8 of them situated on carapace behind orbital margin, ventral margin with 4-7 teeth. Eyes developed, anterior end reaching to 0.66 times length of basal segment of antennular peduncle. Long antennular peduncle, 0.57-0.75 (♀) times as long as carapace. Anterolateral angle reaching 0.22 times length of second segment, second segment distinctly longer than third segment. Stylocerite reaching just beyond beginning of second segment of antennular peduncle.
abdomen. Third abdominal somite with moderately convex dorsal profile. Sixth abdominal somite 0.58 times as long as carapace, 1.7 times as long as fifth somite, shorter than telson. telson (Fig. 24i). 2.4 times as long as wide, with four or five pairs of dorsal spinules and pair of dorsolateral spinules; posterior margin with median process, rounded, with 9-11 very long intermediate setae longer than lateral ones. male pleopods. No males.

Conclusion
By visiting only five major islands of the Solomon Archipelago, we identified 24 Caridina species including 11 of which are new. As expected for such a wide-ranging genus, regional differences in diversity of Caridina exist in the Indo-Pacific. When focusing on endemic species of Caridina, the maximum diversity occurs around the Coral Triangle (Veron et al. 2009), between the Philippines, Indonesia, and Papua-New Guinea. According to our study, the Solomon Archipelgo belongs to this maximum diversity area. Indeed, among our 24 Solomon Caridina species, 5 appear to be endemic to this archipelago.
Regional patterns appear in the diversity of Caridina in the Indo-Pacific, which are explained by the age, size, and latitude of the islands. Geologically old islands have had more opportunity to be colonized by shrimps, and had time for colonizers to adapt and specialize to the different habitats and form new species (Chen & He 2009). Indeed, the complex geological history of the Solomon Islands may explain the richness of its fauna: the extension of the archipelago, stretching over 1300 km, with many large, high elevation and old islands (2700 m high, 10 049 km 2 and formation in Early Miocene for Bougainville) promoted its great diversity.
The patterns in the diversity of endemic species vary considerably from those of amphidromous Caridina species (de Mazancourt et al. in press). The greatest diversity of amphidromous species is found in Indonesia, in the Solomon Islands and Vanuatu. Indeed, among our 24 species of the Solomon Islands, 12 are shared with Indonesia, 10 with Vanuatu and 6 with Papua New Guinea. On the other hand, only 4 species are shared with Australia and 3 with New Caledonia.
This may also include the Coral Triangle, as the diversity of Caridina from the area remains poorly known. The lowest diversity of amphidromous species is found in Polynesia and at the borders of the distribution area in general. Important areas of endemism, such as Madagascar, are not hotspots of diversity for amphidromous species. Localities with low endemism, like the Polynesian or Micronesian islands, often have a higher proportion of amphidromous species. Patterns in the diversity of amphidromous species may be explained by the recent colonization of the islands by amphidromous shrimps with great dispersal abilities, allowing them to colonize these isolated habitats without becoming reproductively isolated. Furthermore, small islands exhibit unstable conditions in which endemic landlocked populations could not maintain themselves in rivers, in contrast to amphidromous species that maintain a stock of larvae in the ocean, ready to recolonize depopulated habitats (McDowall 2007).
We would like to thank Brendan Ebner, Philippe Gerbeaux, Clara Lord and Robson Hevalo for their help collecting specimens on Choiseul and Kolombangara islands. Part of the study was made possible by a grant given to the French Ichtyological Society in the context of the 'Critical Ecosystem Partnership Fund (CEPF)' (Melanesia hotspot). The Critical Ecosystem Partnership Fund is a joint initiative of l'Agence Française de Développement, Conservation International, the Global Environment Facility, the Government of Japan, the MacArthur Foundation and the World Bank. A fundamental goal is to ensure civil society is engaged in biodiversity conservation. Part of the sampling was made possible by a grant of the Fondation de France in the context of a freshwater fish biodiversity study. We would like to acknowledge the almost 20 customary landowners and tribes of Mount Maetambe in the Kolobangara River Watershed for allowing the expedition team to enter their customary lands. We would also like to thank the Lauru Land Conference of Tribal Community, Choiseul Province, the customary landowners, villages and tribes of Hunda-Kena, Jack Harbour, Poitete, Lodumoe and Vanga who welcomed us on Kolombangara Island as on Vella Lavella and Malaita, and for allowing the expedition team to enter their customary lands, the Solomon Islands Government for the support and facilitation of the legal processes that have allowed the expedition team to conduct the scientific research in Choiseul Province, and of course ESSI (Ecological Solutions Solomon Islands) which organised the trip very efficiently. We would also like to thank all of our partners, institutions, museums, universities and collaborators.