In function of their feeding and behavioural interactions, crustaceans may have specialized claws as a first pair of pereopods (Mariappan et al. 2000). Hartnoll (1974, 2012) mentions a chelar difference (heterochely) in most decapod crustaceans, generally more pronounced in males, which use it in agonistic interactions related to territorial defence or during competition for sexual partners. During post-larval development, decapods can display body changes with the onset of the secondary sexual characters, generating two phases (juvenile and adult) better described by a power function (Y=aX^b) and separated by a critical moult (puberty moult) (Huxley 1950, Hartnoll 1974). According to Hartnoll (1982), when a dependent variable (e.g. claw size or abdominal article) is related to an independent variable (e.g. body size), growth rate can be obtained by allometric coefficient (‘b’ constant), generating an isometry (b=1) or allometry (positive b>1 and negative b<1). Such growth patterns are specific to each species and vary as a function of development phase of each sex (juvenile and adult), generating two discontinuous phases of growth and consequently two different levels of allometric coefficients that define the moment of the morphological sexual maturity in decapods (Pinheiro and Fransozo 1998, Pardal-Souza and Pinheiro 2013).

Heterochely, here defined as different-sized chelipeds, is a common attribute in most decapods (e.g. Aegla franca Schmitt, 1942: Bueno and Shimizu 2009; Alpheus brasileiro Anker, 2012: Pescinelli et al. 2018; Platyxanthus crenulatus [Milne-Edwards, 1879]: Laitano et al. 2013), including members of the family Callianassidae Dana, 1852 (collectively known as ghost shrimps). Adult males of callianassids invariably develop one hypertrophied cheliped (heterochely), while females and juveniles have subequal or homochelous chelipeds (Hernáez 2018 and references therein). The handedness or laterality, i.e., the tendency for the major cheliped to appear on a particular side of the body, has been poorly studied in these species. A recent study in the callianassid shrimp Callichirus seilacheri (Bott, 1955) demonstrated that the larger chelipeds in adult males develop independently on either the right or left side of the body (Hernáez and João 2018).

The intertidal ghost shrimp Callichirus major (Say, 1818) is a conspicuous species on many sandy beaches of the western Atlantic (Felder 2001). Its geographical distribution covers from North Carolina, USA, to Santa Catarina, Brazil (Hay and Shore 1918, Melo 1999). These animals build burrows in sediments, normally inhabited by only one shrimp (Rodrigues 1976). In the adult phase, the males of C. major are considered heterochelic due to an expressive claw difference in size/shape (Rodrigues 1971, 1985, Manning and Felder 1986), but in juveniles and females the chelipeds are “subtly unequal” or considered homochelic (Rodrigues 1971). Chelar types in C. major have not been investigated and reported in detail.

Rodrigues (1985) first examined the chelar growth pattern of C. major from the Bay of Santos, in the southeastern region of Brazil, based on the relationship between propodus length (PL) and carapace length (CL), finding a negative allometry in juveniles of both sexes and adult females, with a dimorphic pattern and positive allometry in adult males. Subsequently, Alves-Júnior et al. (2014) reported a positive allometry through this species’s ontogeny with this same relationship, confirming a higher chelar growth in males than in females for a population of C. major from Recife, in the northeastern region of Brazil, a fact explained by the thermal regime promoted by latitudinal influence. Both of these studies considered the growth of a chelar article that reaches a smaller size than the carpus, a noteworthy characteristic in species of the genus Callichirus Stimpson, 1866 (Manning and Felder 1986, Hernáez et al. 2015, Hernáez and João 2018). Another recent contribution by Alves-Júnior et al. (2018) indicated that the whole cheliped in C. major grows at a higher rate than the carapace, especially for males. It is important to highlight that Hartnoll (1974, 1978) recommends the use of biometric variables resulting from a more prominent chelar article (dependent variable) to estimate the morphological maturity size in males, generally represented by chelar propodus variables.

Furthermore, considering the intense harvest suffered by C. major as bait for recreational fishing in several Brazilian regions (Rodrigues and Shimizu 1997, Souza and Borzone 2003, Hernáez et al. 2019), the population of this species often fails to maintain its natural structure and a better representation in size classes. We therefore estimated the chelar relative growth and morphological maturity sizes in C. major, using a more reliable biometric variable (carpus size) and based on specimens captured in a stable and well-balanced population protected by law (see Ordinance #850/1992) in Santos Municipality, Brazil. The patterns of handedness in this species were also determined in function of sex through the ontogeny.

Females of C. major attained a larger body size than males, thus confirming the general trend reported in other callianassid shrimps (e.g. Biffarius filholi [A. Milne-Edwards, 1878], Devine 1966; Callichirus garthi [Retamal, 1975], Hernáez and Wehrtmann 2007; Lepidophthalmus siriboia Felder and Rodrigues, 1993, Rosa-Filho et al. 2013; for exceptions, Trypaea australiensis Dana, 1852, Hailstone and Stephenson 1961). One of the few experimental studies about behaviour of ghost shrimps indicated that mating success does not depend on male size, because in these species the male does not guard the female during the sexual encounter (Somiya and Tamaki 2017). Therefore, there is no sexual selection for a large body in males of callianassid shrimps, as is likely to be the case in C. major. Moreover, small body size and one hypertrophied cheliped in males are obviously advantageous for reproduction in C. major given the considerable number of ovigerous females (>40%) observed by us during several sampling months.

Aggressive interactions are common in C. major when two male individuals encounter each other within a gallery, so the development of larger chelipeds in species of this genus might give comparative advantages during territory defense or also during competition for sexual partners (Rodrigues and Höld 1990). In males of C. major, the growth of the cheliped carpus showed positive allometry in individuals larger than 15 mm CL (Figs 3A, 4A), resulting in larger ghost shrimps likely having stronger chelipeds. However, relative growth of the cheliped length in males had a positive allometric pattern in both ontogenetic phases, independent of the article considered (carpus or propodus). This pattern differed from C. garthi, in which the PL grew more in adults (Hernáez and Wehrtmann 2007), a fact that was unfortunately not evaluated by us or other studies on C. major (Alves-Júnior et al. 2014, 2018, Rosa et al. 2018). In females of C. major, the relative growth pattern did not differ between biometric relationships involving the propodus or carpus of the cheliped as a dependent variable. This fact was also reported in Lepidophthalmus louisianensis (Schmitt, 1935) and Lepidophthalmus sinuensis Lemaitre and Rodrigues, 1991 (Table 3), and has been associated with the fact that female callianassids usually invest less energy in chelar growth than males (Hernáez and João 2018), because the reproductive success of females depends on reaching a larger body size and not larger chelipeds (see Bauer 2004).

Biometric relationships involving variables measured from cheliped articles with higher growth can be used with success in maturity size estimates. This fact was verified in our study with chelar carpus variables of C. major, when compared with studies in which PL was used. A longer carpus than propodus seems to be a common character in heterochelic chelipeds of the Callichirus species previously studied (see Fig. 6). In contrast, other genera in Callianassidae have a chelar propodus that is more developed than the carpus (Manning and Felder 1991). Moreover, according to Hyzny and Muller (2010), even in other genera with an elongated carpus (e.g. Trypaea Dana, 1852 and Podocallichirus Sakai, 1999), this article has never been so developed as in the Callichirus species.

The inflection point between two straight lines represents the size of puberty moult (morphological maturity), from which juvenile and adult phases can be potentially separated (Hartnoll 1974, 1978). Our results revealed maturity sizes at 15 mm CL (males) and 16 mm CL (females), which are greater than those found for C. major by Rodrigues (1985), in which PL was used as a dependent variable (11 to 14 mm CL, respectively) and by Alves-Júnior et al. 2018 (11.2 mm for males and 11.04 mm for females, based on major cheliped propodus length). Alternatively, we detected that male heterochely of C. major started from 10 mm CL, reinforcing the information reported previously by these authors. Furthermore, the greater difference between b-values (see Table 3: A-J =–0.60 to –1.15) was obtained with the biometric relationships involving the carpus of the cheliped, independent of the variable considered (length or height). These values were negative and smaller than those found for other callianassid species (see Table 3), which can be related to the morphological structure of Callichirus chelipeds, as observed in Figure 6.

Heterochelous males were common in C. major (87%) from Gonzaga beach, as occurs with other species of the genus Callichirus (Manning and Felder 1986, Hernáez and Wehrtmann 2007, Hernáez et al. 2015). On the other hand, most females of C. major were homochelous (59.3%). This information is also original for this species, because Manning and Felder (1986, 1991) indicated similar cheliped sizes in juveniles and adult females, the latter previously considered ‘almost homochelous’ or having a ‘subtly unequal size’. Females of C. major had a greater mean size (CL) than males, but showed chelipeds with smaller sizes and slightly heterochelic. Furthermore, both sexes of C. major did not show laterality of the chelipeds in relation to the carpus length, which is a peculiar characteristic in Callianassidae species: e.g. Lepidophthalmus louisianensis (=Callianassa louisianensis), studied by Felder and Lovett (1989); Biffarius filholi studied by Berkenbusch and Rowden (1998); Pestarella tyrrhena (Petagna, 1792) (=Callianassa tyrrhena), according to Dworschak (1998); and Lepidophthalmus sinuensis, reported by Nates and Felder (1999).

According to Rodrigues and Höld (1990), the hypertrophied chelipeds in males of C. major are often used to defend galleries against invasion from other shrimps from the same or opposite sex. This information and the presence of a high frequency of heterochelous males in relation to heterochelous females suggest an intense male-male competition for sexual partners in C. major, and therefore offers indirect evidence of polygamy in this species. Alternatively, a low frequency of adult males in C. major were homochelous, which might be related to male feminization caused by hormonal production of the ovarian part of the testis. According to Souza et al. (2017), the ovarian section of the testis in adult males of C. major does not appear to be functional, because there are no ducts for releasing oocytes. However, the same authors mention the existence of mature oocytes in the male gonad, so it is totally possible that the ovarian section is producing hormones that feminize the individual.

Summarizing, in C. major the chelar growth differed according to sex and ontogenetic phases (juvenile and adults), changing with the onset of morphological maturity (15-16 mm CL), when an evident sexual dimorphism was detected (e.g. higher chelar size in males, mainly in adults). Analysis of biometric variables in callianassids, based on the carpus of the cheliped (length and height), can sometimes be more effective during the estimation of morphologic maturity size. Two morphotypes were described in this species according to their chelar sizes (homo- and heterochelous), an attribute observed in both sexes, although heterochely was more frequent (and evident) in adult males. The individuals of each sex did not show laterality of the major cheliped (handedness).