Chasmocarcinidae Serene 1964

Family CHASMOCARCINIDAE Serène, 1964

Chasmocarcininae Serène, 1964a: 196; 1964b: 185.— Guinot 1969: 699; 1986: 312.—Davie 2002: 192, 196.

Chasmocarcinidae—Karasawa & Schweitzer 2006: 55.— Števčić 2005: 107; 2013: 186.—Ng et al. 2008: 27, 75, 76, 85.— De Grave et al. 2009: 32 [in list].—Castro et al. 2010: 36, 37, 41, 51 [in key].— Guinot et al. 2013: 42, 82, 111, 113.— Jagt et al. 2015: 879.— Clark & Cuesta 2015: 1011, 1014 [larvae].— Davie et al. 2015c: 1081, 1082. Chasmocarcinoidea—Števčić 2005: 106.

Type genus. Chasmocarcinus Rathbun, 1898 Subfamilies included:

Chasmocarcininae Serène, 1964 Megaesthesiinae Števčić, 2005 Trogloplacinae Guinot, 1986

Diagnosis (modified from Serène 1964a). Carapace subtrapezoidal, subquadrate, quadrate, or subtriangular, high; front almost straight, often bilobed; anteroexternal angle arcuate or straight, usually with minute granules without distinct lobes or teeth. Epistome fused with proepistome (inter-antennular septum), slightly depressed, sometimes reduced; posterior margin delimited, typically with shallow median cleft; proepistome present, narrow, sometimes indistinct, never spiniform. Eye peduncle filling orbit, mobile or immobile (sessile); cornea greatly reduced or normal, with or without pigmentation. Antennule with basal article swollen, tightly fitting in fossa so that antennular flagellum cannot fold completely into fossa (second, third articles not elongated, folding onto swollen basal article [Trogloplacininae]; second, third articles elongated so they cannot fit into the fossa [Chasmocarcininae]; or second, third articles short, swollen with flagellum bearing elongated plumose aesthetascs, not able to fold into fossa [Megaesthesiinae]). Basal antennal article (second article) quadrate to subquadrate, not fused to carapace; flagellum relatively short, usually not extending far beyond lateral edge of carapace. Third maxillipeds completely filling buccal cavern or leaving gap between them when closed; merus varies in shape from subtriangular to quadrate, anteroexternal angle may be auriculiform; ischium varies in shape from elongated to quadrate, same size or longer than merus. Chelipeds subequal in length, dissimilar, heteromorphic in males, propodus of major chela becoming thicker with age in males; anterior portion of meri with large triangular to acute tooth on inner margin. Meri of ambulatory legs smooth or armed. Thoracic sternites 1, 2 completely fused, semicircular or rectangular, proportionally wide, short or narrow; thoracic sternites 3, 4 fused, lateral suture or cleft visible. Thoracic sternites 6–8 visible from posterior view; prominent “supplementary plate” (anterior invagination of sternite 8) present between sternites 7, 8 (Fig. 1 E); sternal sutures 4/5, 5/6, 6/7, 7/8 interrupted; thoracic sternites 7, 8 with poorly defined median line (Figs. 52 D; 56D; also see Guinot et al. 2013: 265, fig. 56I). Sterno-pleonal cavity of male deep, with rounded anterior end. Press-button for pleonal holding as tubercle (2 in one species) just posterior to thoracic sternal suture 4/5 at each edge of sterno-pleonal cavity. Large portions of sternum left exposed by pleon (abdomen). Male pleon with somites 3–5 completely fused but sutures may visible as shallow grooves; lateral margins of fused somites 3–5 slightly convex or nearly straight; telson proportionally long or short, fused thoracic sternites 1, 2 short, never extending deep into buccal cavity. Male gonopore coxal; penis coxo-sternal, lying in a channel between sternites 7, 8, anteriorly covered in part by posterior margin of “supplementary plate” between sternites 7, 8, plate varies in length, width, shape, proximity to other plates, stretching across full extent or most of anterior margin of thoracic sternite 8 on each side of posterior, widest portion of pleon; penis may be partially exposed or covered by narrow plate (also see Guinot et al. 2013: 82, 111, fig. 24). G1 stout, distal segment straight or curved, distal surfaces covered by short and/or long spinules, without long setae. G2 slender, as long as, longer, or shorter than G1. Female pleon triangular, with all somites, telson free, pleopods prolonging beyond pleon margin; telson proportionally long or short. Sterno-pleonal cavity of female shallow, vulvae mesial in flat portion of thoracic sternite 6 (also see Guinot et al. 2013: 42). Vulva wide, occupying most or complete width of thoracic sternite 6 (conspicuously large in Trogloplacinae), round, flat, with conspicuous margins, covered by chitinised membrane without sternal vulvar cover or operculum. Eggs relatively large, indirect development with planktotrophic larvae (direct in Trogloplacinae). Marine (shallow to deep water) or freshwater, including land caves (Trogloplacinae).

Remarks. Many genera of chasmocarcinids were originally described as members of the Rhizopinae Stimpson, 1858 (or Rhizopidae), but the current concept of this taxon is very different from that conceived or recognised until the 1960s. Guinot (1978, 1979) restricted the Rhizopinae s. str. to species in the Pilumnidae, and argued that the “rhizopine” condition is only the broad carapace shape, an adaptation to their habitats in fine sand and mud, a lifestyle termed “luteophilous” by Ng (1990), as well as the coxosternal state of the penis (see Guinot et al. 2013: 112, fig. 24). Ng (1987) reviewed the taxonomy of the Rhizopinae and treated it as a subfamily of Pilumnidae, with many “rhizopines” today classified in a multitude of families (see Ng et al. 2008: 75).

Serène was the first to treat the genera revised in this study as a separate subfamily, Chasmocarcininae (present Chasmocarcinidae), of Goneplacidae. There is a small issue with the authority for Chasmocarcinidae. The name was first used by Serène in two papers, both published in 1964. Serène (1964a) treated material from the Mortensen Expedition in Videnskabelige Meddelelser fra Dansk Naturhistorisk Forening I KØbenhavn and according to the cover on the original paper, it was issued in March 1964. Serène (1964b) redescribed Megaesthesius and established a new subfamily, Chasmocarcininae, in Crustaceana, and according to Holthuis (1986: 221), the part in which it appeared was published on 7 September 1964. Most authors (e.g., Ng et al. 2008) cite Serène (1964b) as the authority of Chasmocarcininae. Serène (1964a: 196), however, in his discussions of the groups, mentioned that a number of genera in the Rhizopinae should be referred to a new subfamily, Chasmocarcininae, although he did not formally diagnose it:

“ Les six genres représentés dans la présente collection ne permettent pas une révision de l'ensemble de la sousfamille; mais l'étude de la collection Gallardo comprendra une clé de séparation de tous les genres des Rhizopinae, complétant et corrigeant celle de TESCH (1918). Je signale en particulier que Chasmocarcinus appartient au groupe des genres dont le flagelle antennulaire ne peut se rétracter dans les fosses correspondantes et non à l'autre groupe, comme il est classé dans la clé de TESCH (1918). Ce caractère est commun à plusieurs genres de la sous-famille qui constituent un groupe distinct comprenant: Hephthopelta, Chasmocarcinus, Chasmocarcinops, Camatopsis, Megaesthesius. Tous ces genres ont d'autres caractères communs (abdomen avec les segments 3 à 5 soudés, pléopode mâle d'un type particulier) qui pourraient servir à définir et séparer des Rhizopinae une autre sous-famille (Chasmocarcininae). De toute manière, dans l'actuelle classification, l'abdomen et les pléopodes mâles fournissent un bon caractère de différenciation générique et de séparation de ce groupe dans la sous-famille. A ces niveaux (générique et sous-familial) le mxp 3 fournit un autre caractère qui isole plus ou moins ces genres des autres. Mais c'est à tort, à mon avis, qu'on lui a accordé de l'importance au niveau familial. ” [The six genera represented in this collection do not allow a revision of the entire subfamily, but the study of the Gallardo collection will include a key to separate all genera of Rhizopinae, completing and correcting the key of Tesch (1918). I note in particular that Chasmocarcinus belongs to the group of genera whose antennular flagellum cannot retract into the corresponding pits and not to the other group, as it is classified in Tesch’s (1918) key. This character is shared by many genera of the subfamily that constitutes a distinct group that comprises: Hephthopelta, Chasmocarcinus, Chasmocarcinops, Camatopsis, Megaesthesius. All these genera have other common characteristics (pleon with segments 3‒5 fused, male pleopod of a particular type) that could be used to identify and separate the Rhizopinae into another subfamily (Chasmocarcininae). In any case, in the present classification, the pleon and the male pleopods provide good characters for generic differentiation and the separation of this group in the subfamily. At these levels (generic and sub-familial) the mxp 3 provides another character that more or less separates these genera from others. But it is wrong, in my opinion, for this group to be granted the importance of a family level.]

Although Serène (1964a) did not “formally” name the Chasmocarcininae as new, the current zoological code (ICZN 1999) allows for this as long as there is a diagnosis, and he clearly states characters that distinguish the group. As such, the authorship and year for the group should be Chasmocarcininae Serène, 1964, as given in Serène (1964a).

In the modern classification of the Brachyura adopted since Guinot (1978), the “ Chasmocarcininae ” of Serène (1964a, b) should clearly now be treated as a family grouping. In particular, it is the only goneplacoid taxon with a unique apomorphy in the “supplementary plate” on the male thoracic sternum. The “supplementary plate” is considered an invagination of thoracic sternite 8 as defined by Guinot et al. (2013: 87, 111; see Guinot 1986: pl. 2, fig. D; Davie & Guinot 1996: fig. 7) (“complete invagination of sternite 8 in the form of two contiguous plates”) (see Fig.1 E). The “supplementary plate” has also been referred to as “intercalated” (Rathbun 1914: 149), “supplementary plate” (Ng et al. 2008), and “accessory plates” (Davie et al. 2015b). While the presence of this plate is now regarded as diagnostic for the Chasmocarcinidae (see Davie & Guinot 1996), Serène’s (1964a, b) original concept of the taxon (as the subfamily Chasmocarcininae) actually did not include anything about the “supplementary plate” and also included Scalopidia Stimpson, 1858. As was discussed by Ng et al. (2008: 85), Scalopidia is now placed in its own family, Scalopidiidae Števčić, 2005, as it does not possess a “supplementary plate” and the penis lies in a deep groove between sternites 7 and 8 (see also Ng & Castro 2013; Guinot et al. 2013: 118). A second character unique to Chasmocarcinidae, which was observed by Serène (1964a, b), is in the morphology of the antennules. The peduncle, or basal article, is so large and swollen that the flagellum cannot retract into the antennular fossa at all, with the subsequent articles short and swollen (Megaesthesiinae) or simply too long (Chasmocarcininae). Even when the articles are relatively short and slender (Trogloplacinae), they cannot retract into the fossa since the peduncle already completely fills it. Among the Heterotremata, the inability of the antennular articles to completely retract into the fossae is a character chasmocarcinids share only with the pseudozioid Christmaplax mirabilis Naruse & Ng, 2014 (Christmaplacidae Naruse & Ng, 2014) from Christmas Island in the Indian Ocean (see Naruse & Ng 2014)

While the characters discussed above easily distinguish male specimens of chasmocarcinids from all confamilials, separating juveniles and females is not always easy. Their close resemblance to rhizopines is also substantial; the easiest character to use is the structure of the antennules, the peduncle and most of the flagellum neatly folding into the antennular fossa in rhizopines.

Ng et al. (2008: 76) briefly discussed the status of Trogloplacinae Guinot, 1986. As a family, Trogloplacidae (see Davie 2002) only contains two genera, Australocarcinus Davie, 1988, and Trogloplax Guinot, 1986. Ng et al. (2008) argued that it should be recognised as a subfamily of the Chasmocarcinidae, noting that the basal antennular article of trogloplacines is normal and not swollen and the G2 is always longer than the G1 (basal article bulbous and swollen, G2 shorter than the G 1 in most chasmocarcinines). This revision of the family shows that the gonopod differences are not effective, with species of Hephthopelta s. str. and Tenagopelta n. gen. also possessing a long G2 that is longer than the G1 (e.g., Fig. 71 E). The structure of the antennules can still separate trogloplacines from chasmocarcinines, with the basal antennular article of trogloplacines rectangular in shape but still too swollen and the articles of the flagellum too long to fit into the fossa (Fig. 94 E, F, J). All chasmocarcinines have a more rounded or quadrate antennular peduncle (e.g., Fig. 21 A). In addition, the vulvae of trogloplacines are substantially larger, extending across much of the median portion of thoracic sternite 6, and bordered by well-developed, thick margins (e.g., Fig. 99 F), in contrast to the smaller vulva of chasmocarcinines, which typically occupies a smaller portion of thoracic sternite 6 and is typically bordered by simple margins (Figs. 89‒92). This is almost certainly associated with large eggs, which develop directly into megalopae (see Davie & Guinot 1996: figs. 2, 6), in contrast to the relatively smaller eggs of chasmocarcinines, which show indirect development with the release of planktotrophic larvae. For these reasons, the recognition of two subfamilies in the Chasmocarcinidae thus seems warranted.

Typhlocarcinodes Alcock, 1900 (type species Typhlocarcinus integrifrons Miers, 1881) and Raoulia Ng, 1987 (type species Raoulia limosa Ng, 1987) were provisionally placed in the Chasmocarcinidae by Ng et al. (2008) although they concluded that the systematics of these two genera needed more study. Raoulia was excluded from Chasmocarcinidae by Guinot et al. (2013: 113). Števčić (2005) had placed both genera in new families, Typhlocarcinodidae Števčić, 2005, and Raouliidae Števčić, 2005, respectively, but without explanation. Ng et al. (2008: 75) provisionally treated Raouliidae and Typhlocarcinodidae as synonyms of Chasmocarcinidae pending documentation of sternal, pleonal and gonopodal characters, features poorly known in most genera.

We examined the types of Typhlocarcinodes and Raoulia, and on the basis of the characters for Chasmocarcinidae discussed above, these two genera cannot be placed in this family. Both are allied to Caecopilumnus Borradaile, 1902, a genus provisionally placed in Pilumnidae Samouelle, 1819, by Ng et al. (2008). Species of Typhlocarcinodes, Raoulia, and Caecopilumnus do not possess the diagnostic “supplementary plate” on the male thoracic sternum (Fig. 1A –C), which is diagnostic for Chasmocarcinidae (see review by Ng & Rahayu 2014). Their male pleons and gonopods are nevertheless similar to those of chasmocarcinids. While genera like Typhlocarcinodes and Raoulia have a narrow plate-like structure between thoracic sternites 7 that superficially resembles the chasmocarcinid “supplementary plate”, we do not think they are homologous. This absence of homology between apparently similar dispositions was stressed by Guinot et al. (2013). These plates are actually part of the penis tube and are not derived from the thoracic sternites. The plate-like structure of Raoulia is conspicuously narrow and small (Fig. 1 D), whereas that of Typhlocarcinodes is relatively large but is attached to the penis and is mobile (Fig. 1A). They are also related to the Scalopidiidae Števčić, 2005, members which have a sternal structure similar (Fig. 1 D) to that of Typhlocarcinodes (see Ng et al. 2008; Ng & Castro 2013). Typhlocarcinodes, Raoulia, and Caecopilumnus are closely related in their coxosternal condition of the penis, and Ng & Rahayu (2014) referred them to the Acidopsidae Števčić, 2005, in a separate family, Raoulinae Števčić, 2005 (see also Ng 2002; Ng & Chen 2004; Ng et al. 2008).

Ovigerous females were rather rare among the chasmocarcinids examined. Chasmocarcinines are known to have relatively large egg, about 0.04 mm in a female of Chasmocarcinus typicus Rathbun, 1898, but their larvae (none which have been described thus far) are probably still planktonic. Although held by pleopods, many eggs are located outside the pleonal cavity (Figs. 10 E; 87A). The same seems to be true for megaesthesiines. Trogloplacines have even larger eggs, and Davie & Guinot (1996) have found megalopae under the female pleon, suggesting direct development.

Knowledge about the sperm structure in the family is restricted to only one species in the Trogloplacinae: Australocarcinus riparius Davie, 1988 (cf. Jamieson & Guinot 1996; Jamieson & Tudge, 2000), and as such, not much can be said.

No chasmocarcinids have been reported from Arctic or Antarctic seas to date; and while they are present in the other oceans, it is noteworthy that none are known as yet from the eastern Atlantic.