Although sample size is limited in our collection, it is apparent that in both species some males are markedly larger than most females (Figs 1–2, 3A–B). The size-frequency distributions of both species appear bimodal but only the intermediate and larger size classes were sampled by the collection (Fig. 1). Individuals of Cinetorhynchus sp. B reach a much larger size than those of C. sp. A (Fig. 1A–B). Sexual dimorphism in cheliped weaponry is pronounced in both species. Males bear elongated, heavy major first chelipeds (pereopod 1) with elongated propodi in the larger males (Figs 2A–C, 3A, 4A–B). Additionally, chelipeds of larger males end in a wrench-like subchela (Figs 2A–C, 3A). Major chelipeds of females by comparison are small (Figs 2D–E, 3B, 4A–B) and terminate in a typical chela with equal fingers (Fig. 3B–D). In C. sp. A, some of the larger males have proportionately much larger chelipeds than the smaller ones (Fig. 4A), while in C. sp. B, the growth is more linear although with considerable scatter in the data (Fig. 4B). Female first chelipeds (pereopods 1) are shorter and not markedly robust (Figs 2D–E, 3B, 4A–B). As in all known Rhynchocinetidae (e.g., Holthuis 1993: p. 19, Okuno 1996), the chela fingers of both female pereopods 1-2, as well as male pereopods 2, terminate in several, stout, and black (highly sclerotized) setae (Figs 3C–D, 7A) which are reduced (Fig. 7B–C) or lacking in the major chelipeds of larger males (Fig. 7D–E).

Figure 1. 

Size frequency diagrams of male and female Cinetorhynchus species. A Cinetorhynchus sp. A B Cinetorhynchus sp. B.

Figure 2. 

Photographs of living Cinetorhynchus species from Coconut Island, Hawaii. A Cinetorhynchus sp. B male with subchelate first chelipeds (pereopod 1) B Cinetorhynchus sp. B male with cheliped intermediate between chelate and subchelate C Cinetorhynchus sp. A male with subchelate chelipeds D Cinetorhynchus sp. B female E Cinetorhynchus sp. A female. C1 cheliped 1; C2 cheliped 2; M3 third maxilliped. Scale bars represent 10 mm.

Figure 3. 

Sexual dimorphism in C. sp. B. A Large male (10.8 mm CL) B Female (8.9 mm CL). Scale bars in A and B represent 10 mm C Distal end of chela 1 of a female (11.2 mm CL) showing blackened corneous setae on chela fingers; scale bar represents 0.5 mm D Illustration of tip of chela 1, Rhynchocinetes albatrossae (from Chace 1997, no scale given), showing form of blackened corneous setae (similar to those shown in Figure 3C) typical of chelipeds 1 and 2 of rhynchocinetids except in large males (e.g., as in this study). C1 cheliped 1; cs corneous setae; dactyl (movable finger); M3 third maxilliped; p propodus; pf propodal (fixed) finger.

Figure 4. 

Variation of cheliped size in Cinetorhynchus species. Cheliped size (measured as propodal length) is plotted against body size (CL) in male and females. A Cinetorhynchus sp. A B Cinetorhynchus sp. B.

Two standard measures showing possible importance of the third maxillipeds as weapons (Correa et al. 2000, Thiel et al. 2010, Bauer and Thiel 2011) were compared between males and females of both species (Fig. 5A–B). The third maxillipeds of males in C. sp. B (Figs 2A–B, 3A, 5A) are proportionately but not markedly larger than those of females (Figs 2D–E, 3B, 5A), which is also the case in C. sp. A (Figs 2C, E, 5A), although the sample size of females in the latter is small (N = 6). In both species, there is considerable overlap between the sexes in the number of maxilliped 3 terminal spines (Fig. 5B).

Figure 5. 

Comparison of third maxillipeds in male and female Cinetorhynchus species. A Maxilliped 3 size (measured as length of terminal article) plotted against body size (carapace length, CL) B Number of corneous spines on the terminal article of maxilliped 3 plotted against body size (CL). In B number of observations is the same as A, except for C. sp. B males (n=45) and C. sp. B females (n=13) because spinous portions of the terminal article of some individuals were damaged.

With increasing size, males of both species show a major change in the shape of the chelae of the major cheliped (pereopod 1) while females do not (Fig. 6A–B). These changes were quantified by comparison of the chela’s propodal finger length vs cheliped 1 size, as measured herein by total propodal length (Fig. 6A–B). In males, there is a gradual change in the shape of the major chela from a true chela (with distinct upper movable dactylar and lower fixed propodal fingers that close together as in females, Figs 3B–D, 7A) to a more prehensile chela (Fig. 7B–D) to a subchela (Fig. 7E) in the largest robustus-type males. In females, chela 1 has equal propodal and dactylar fingers of a typical chela (Figs 3C–D, 7A), and is tipped with several black sclerotized (= corneus) setae (Figs 3C–D, 7A). In intermediate-sized males, the propodal finger is somewhat reduced compared to the dactylar finger (Figs 6A–B, 7B–D), and the setation becomes reduced in both fingers to a single black seta at the tip (Fig. 7B–C), which is lost with increasing size (Fig. 7D–E). In addition to decreasing propodal finger size with increasing chela 1 size, the distal end of the propodus also increases in height (Fig. 7B–E). The dactylar finger is flexed (Fig. 7E) against the expanded distal end of the propodus instead of closing against with a propodal finger as in females (Figs 3C–D, 7A), thus forming a subchela.

Figure 6. 

Ontogeny of major chela (pereopod 1) structure from a typical chela to a subchela in Cinetorhynchus species A and B. The length of the propodal finger relative to propodal length (cheliped size) is plotted as the measure of chela structure. With growth, the relative propodal (fixed) finger length decreases in larger males but not in females as male first chelipeds change from chelate to subchelate (see Figure 7). A Cinetorhynchus sp. A B Cinetorhynchus sp. B.

Figure 7. 

Variation in the major chela and chela finger shape with increasing size in males of Cinetorhynchus species B. A female 11.3 mm CL B male 9.4 mm CL C male 10.1 mm CL D male 10.6 mm CL E male 11.4 mm CL. cs corneous (black) seta(e); d dactyl (movable finger); p propodus; pf propodal finger. Unlabeled arrows in D and E show lack of the black corneous setae seen in A–C. Scale bars represent 10 mm.

The major chelipeds of a number of males showed injuries. These injuries took the form of puncture wounds (Fig. 8A–C) to the elongated propodal portion (“palm”) of the major chelipeds. Previous possible injury, i.e., complete loss, of the cheliped may be indicated by regeneration of the cheliped (Fig. 8D). Puncture wounds were recognized by roughly circular perforations in the cuticle surrounded by melanized exoskeleton, a sign of scabbing and healing. Regenerating chelipeds were recognized by a combination of characteristics: much smaller than the other member of the pair (if present), a poorly sclerotized (whitish in preservative) cuticle, and a reduced number of poorly formed articles (Figure 8D). Of 38 males (both species) with one or both chelipeds, 10 individuals (26%) showed signs of injury (5 with puncture wounds, 5 with regenerating cheliped). By species, C. sp. B males (N = 27), 3 (11.1%) had puncture wounds, while 4 (14.8%) had a regenerating major cheliped. In C. sp. A males (N = 11), 2 (18.2%) showed puncture wounds and 1 (9.1%) had a regenerating cheliped. None of the females (C. sp. A, N = 6; C. sp. B, N = 14) showed any obvious sign of injury to their major chelipeds.

Figure 8. 

Puncture wounds (unmarked arrows) on the propodi of the major chelipeds of three large Cinetorhynchus males. A Cinetorhynchus sp. A B and C Cinetorhynchus sp. B D Regenerating major cheliped of a male Cinetorhynchus sp. B; only two articles plus a rudimentary cheliped with underdeveloped propodus and dactyl have formed. d dactyl (movable finger); p propodus. Scale bars represent 3 mm.

Cinetorhynchus sp. A had 484–798 embryos (x = 601 ±128, N = 6), while females of C. sp. B carried from 304 to 3786 embryos per brood, (x = 1491 ± 104, N = 13). Linear regressions on log₁₀-transformed variables (Fig. 9) indicated positive allometry in brood size (b > 3) in C. sp. B (b = 3.983; 95% c.l.: 3.013–4.952). In C. sp. A, b = 2.471, a number which could indicate negative allometry as b < 3. However, the 95% limits on b in C. sp. A are so broad (-0.1469–5.090), probably due to small sample size (N = 6), that a definite statement on allometry in this species cannot be made. Females of C. sp. A had slightly smaller embryos than females of C. sp. B (Table 1) but the sample sizes are too small for a meaningful statistical comparison.

Figure 9. 

Fecundity in Cinetorhynchus species A and B. The log₁₀ number of embryos per brood are plotted against log₁₀ female size (carapace length, mm).

To detect possible male to female sex change, two female breeding-dress characters were measured and compared between males and females: the height of the second pleuron (Fig. 10A) and the width of the basipod flange of the second pleopod (Fig. 10B). In both species the second pleuron is higher and the flange is wider in females than in males (Fig. 10A–B) throughout their size range, typical of gonochoric caridean species.

Figure 10. 

Variation in female breeding dress characters with increasing size in females in Cinetorhynchus sp. A and Cinetorhynchus sp. B. compared with the same structures in males. A Height of the second abdominal pleuron B Width of the basipod flange of the second pleopod.

The sexual dimorphism in size and weaponry of the two Pacific species studied allows, for the first time, the formulation of hypotheses about the sexual and mating systems in the genus Cinetorhynchus. The population structure, composed of smaller females and larger males in the two Cinetorhynchus species, along with the presence of “robustus” males, i.e., large males with hypertrophied appendage weaponry, strongly suggests that the mating system involves some type of male-mate guarding. This type of mating system might vary from temporary (short-term) to extended (socially monogamous) mate guarding (Wickler and Seibt 1981, Bauer 2004) to perhaps the “neighborhoods of dominance” mating system found in species such as R. typus and Macrobrachium rosenbergii (Correa and Thiel 2003a and references therein).

The male weaponry of the Cinetorhynchus species studied consists of elongated, hypertrophied major (first pereopod) chelipeds bearing chelae whose form in larger males is more like a subchela than a chela. In a subchela, the movable (dactylar) finger folds against a vertically expanded distal end of the propodus (Bauer 2004). Males of at least 3 of 11 described Cinetorhynchus species, C. hawaiiensis (see Okuno and Hoover 1998), C. hendersoni and C. reticulatus (see Okuno 1997a) also have hypertrophied major chelipeds. In females, the first chela is a “true” chela, i.e., the movable finger closes against a fixed (propodal) finger of similar length as in all rhynchocinetid species, which might be useful in autogrooming or grazing epibiota of hard surfaces. Rhynchocinetes typus is a known grazer and predator on fouling organisms of rocky and other surfaces (Dumont et al. 2011a,b). In males of the two Cinetorhynchus species, the second chelipeds are much like those of the females. In smaller males, the first chelae are somewhat female-like, but with increasing cheliped size, the sclerotized setae at the tip are lost, the distal end of the propodus expands vertically. The fixed (propodal) finger becomes proportionately shorter with an increase in cheliped size, so that in the larger “robustus” males, the first chelipeds bear a typical subchela. The loss of the possible grooming/grazing setae of the first chelipeds of robustus males may represent a cost associated with increased fighting ability. The considerable variation in cheliped-1 finger size in larger males might be due to loss of the cheliped in fighting and thus represents variation in the stage of regeneration and subsequent growth of the cheliped.

The size and form of the male first chelipeds, as well as injuries to the first chelipeds (puncture wounds on the chela 1 propodus, regenerating chelipeds) in some males clearly suggests fighting between males. Presumably, such combat would be for access to or defense of females or resources attractive to them, as in many animals with hypertrophied weaponry (Emlen and Oring 1977, Emlen 2008). Rypien and Palmer (2007) found puncture wounds on the major chelipeds of a porcellanid crab, but these injuries occurred in both sexes and were more frequent in high density populations in wave-swept habitats than in low density and protected sites. The authors interpreted these data as evidence of both abiotic physical injuries and fighting among individuals of both sexes. Our observations on major male chelipeds injuries in the two Cinetorhynchus species are very similar to those of Rojas et al. (2012), who observed puncture wounds on the major chelae of males in the river shrimp Cryphiops caementarius (Palaemonidae) and reviewed examples from other decapods in which injuries are attributed to intramale fighting. Similar observations on damage in large males of R. brucei were also interpreted as being caused by intramale fighting (Thiel et al. 2010). In the Cinetorhynchus species studied, we found no puncture wounds nor regenerating chelipeds in females, supporting the hypothesis that male injuries were a result of intramale combat. The hypothesis that the puncture wounds and regenerating first chelipeds observed result from fighting and not some other form of injury (predators, or physical injury by, for example, wave action in a coral-rubble habitat on elongated and exposed appendages) requires testing.

Hypertrophied weaponry is usually assumed and often has been shown to have evolved in many animals either because of intrasexual competition (fighting ability) among males for females and/or by female selection of males as potential mates based on their size and weaponry or other attractive characteristics (Darwin 1871, Emlen 2008). In R. typus, robustus males do assess the fighting potential of other males when competing for females (Correa et al. 2003). In this species, males display their major chelipeds in the presence of receptive males by raising and lowering the major chelipeds. Furthermore, females are attracted to and chose among male suitors based on sexually dimorphic characters and behavior (Diaz and Thiel 2003, 2004). Sexual dimorphism in body size and weaponry in the two Cinetorhynchus species studied herein may similarly function in female selection of males.

The form of the subchela and the types of injuries found in the Cinetorhynchus species studied suggests the hypothesis that males may use their major chelae to grasp the long propodus of their opponent’s chelae, perhaps flipping or pushing them away from the female or refuge in which they are found. In R. typus, if cheliped displays between similarly matched robustus males fail to resolve a contest over a receptive female, males fight by grasping each other’s major chelae and jabbing at each other with hypertrophied and spear-like third maxillipeds, often injuring each other (Correa et al. 2003). Although the third maxillipeds of the Cinetorhynchus species in this study were proportionately slightly larger than in the females, these appendages were similar in size and appeareance between the sexes, i.e., typical carideans third maxillipeds (see Bauer 2004). There was not the pronounced sexual dimorphism found in R. typus and R. brucei (Correa et al. 2000, Thiel et al. 2010). We propose that third maxillipeds of males in the two studied Cinetorhynchus species are not significant weapons as in R. typus and R. brucei. Such hypotheses in these two Cinetorhynchus species must await evaluation of larger samples over a longer temporal period, and especially for observations and experiments on living shrimps.

Fecundity comparisons of these two Cinetorhynchus species with species of Rhynchocinetes (R. uritai and R. typus) suggest that fecundity is similar when body size differences are taken into account (Bauer and Thiel 2011, Vásquez and Castilla 1982, respectively). Our limited results suggest positive allometry in brood size (number of embryos with female size) in C. sp. B and perhaps negative allometry in C. sp. A. Early stage embryos were twice as large in size (an order of magnitude larger in embryo volume) than those reported for R. uritai. Differences in embryo size and volume may indicate different fecundity/incubation period/larval development strategies among rhynchocinetid species that warrant further and more comprehensive study.

Males of these two species are as large or larger than females, a size frequency distribution which does not indicate protandric (male to female) sex change. However, given the finding of protandry in another rhynchocinetid species, R. uritai (Bauer and Thiel 2011), other morphological characters indicating male to female sex change were analyzed. Key characters that indicate sex change are those involved in the reproductive female spawning/incubation chamber below the abdomen, i.e., the “breeding dress” (Höglund 1943). The height of the second pleuron and the width of the basipod flange of the second pleopod are two such characters (Bauer 1986, Bauer and Thiel 2011). In the Cinetorhynchus species examined here, there was no morphological shift in males to a female-like condition with an increase in size as one would expect in a sex-changing species. Thus, transitional individuals (sex changers) of intermediate external sexual morphology were not observed. The evidence presented here shows that these two Cinetorhynchus species, with large males bearing hypertrophied chelipeds, are gonochoric (separate sexes), as in two other such “large male” species investigated, R. typus (Correa and Thiel 2003b) and R. brucei (Thiel et al. 2010).

In both Rhynchocinetes (e.g., R. typus, R. brucei) and Cinetorhynchus (this study), the two current genera of the Rhynchocinetidae, there are species with large male size and hypertrophied male weaponry. A preliminary survey of the taxonomic literature on rhynchocinetids (J. Okuno, pers. obs.) indicates that there are also a number of species in the family in which males are on average smaller than females and without hypertrophied weaponry (authors pers. obs.). At least one species, R. uritai, has a protandric sexual system (Bauer and Thiel 2011). Thus, this family may serve as an excellent model for testing hypotheses on the evolution of sexual and mating systems as well as sexual selection. The costs (e.g., higher exposure to injuries by predation, intramale combat, physical injury, energetic costs of growth) and benefits (access and/or attractiveness to potential mates) of hypertrophied sexual weaponry have been and continue to be a topic of considerable interest to evolutionary biologists (e.g., Darwin 1871, West-Eberhard 1979, 1983, Emlen 1983, Emlen 2008, Tobias et al. 2012). We suggest the family Rhynchocinetidae is an excellent system for such research. We will continue to expand our research on this family and invite others to join us in this venture.