Tetralobistes, Ayón-Parente, Manuel & Hendrickx, Michel E., 2010

Ayón-Parente, Manuel & Hendrickx, Michel E., 2010, A new genus and new species of hermit crab (Crustacea: Anomura: Paguroidea: Diogenidae) from the eastern tropical Pacific, Zootaxa 2677, pp. 49-59 : 50-52

Tetralobistes gen. nov.

( Fig. 2 )

Diagnosis. Thirteen pairs of biserial gills; shield well calcified; rostrum short, subtriangular. Ocular acicles well developed, terminating in strong multifid projection, mesial margins contiguous. Antennal peduncles with supernumerary segmentation; acicles short, spinose. Antennal flagella of medium length, longer than carapace, with long setae ventrally. Mandible with unarmed mesial margin, palp 3-segmented. Endopod of maxillule ( Fig. 2 A) with well developed, recurved external lobe. Maxilla ( Fig. 2 B) with elongate endopod. First maxilliped ( Fig. 2 C) with well developed epipod. Second maxilliped ( Fig. 2 D) without distinguishing characters. Third maxilliped ( Fig. 2 E) with basal segments approximate; crista dentata well developed; no accessory tooth.

Chelipeds subequal, right usually slightly longer than left, similar in armature; fingers opening in horizontal plane, fingertips acuminate. Fourth pereopods simple, without preungual process at base of claw. Fifth pereopods chelate.

Males with paired gonopores; paired pleopods modified as gonopods on first pleonal somite; no pleopods on the second somite; unpaired, well developed, uniramous left pleopods on somites 3–5.

Females with paired gonopores; first pleonal somite without modified pleopods; following 3 pleomeres with unequally biramous left pleopods; fifth pleopod uniramous; brood pouch represented by row of setae.

Uropods asymmetrical. Telson asymmetrical, with lateral indentation; posterior margin divided into four lobes (tetralobular); external lobes unequal, larger than symmetrical, inner lobes.

Type species. Tetralobistes bicentenarius n. sp., by present designation.

Etymology. The name of the genus refer to the posterior margin of the telson which is divided into four lobes (tetralobular), and is a combination of the Greek words tetra (four) and lobos (lobe), and the Greek suffix – istes. Gender, masculine.

Distribution. Matanchen Bay, Nayarit, Mexico; 0.5–1.5 m.

Remarks. Diogenidae have long been considered as evolutionarily more primitive than Paguridae . In her review of characters used to define affinities among hermit crabs, de Saint Laurent (1970) recognised the importance of the presence and number of pleurobranchiae on pereopods, the modification of pleopods into gonopods in males, the presence or absence of first (males and females) and second (males only) pleopods, and relative lengths of the endo- and exopodites of the pleopods. Additionally, the presence, reduction or absence of the brood pouch in females has also been considered an important phylogenetic character ( McLaughlin & Hogarth 1998).

Considering the most significant characters that separate the genera of Diogenidae , Tetralobistes gen. nov. is more closely related to Areopaguristes , Paguristes and Pseudopaguristes than to other genera. For comparative purposes, 12 characters were selected and analysed (see Tab. 1). In Tetralobistes gen. nov., some of these characters are similar to one, two or all these three other genera, but other characters are clearly distinct or even unique. In Tetralobistes gen. nov., for example, the presence of 13 pairs of phyllobranchiate biserial gills is unique among these four genera.

In her review of Paguristes sensu lato Rahayu (2005) considered reduction of the number of gills (branchiae) as an evolutionary trend. On this basis, she suggested splitting Paguristes sensu lato into three genera: Paguristes sensu stricto, with 13 pairs of gills, Areopaguristes with 12 pairs, and Pseudopaguristes with only 8 pairs. This is also supported by Lemaitre et al. (2009), who considered Paguropsis Henderson, 1888 , and Paguristes (both with 13 pairs of gills) among the most primitive of diogenids. Species of these two genera have first and second pairs of males pleopods modified as gonopods, a character considered by McLaughlin & Lemaitre (1997) to be a plesiomorphic condition that evolved into a more advanced, apomorphic condition in which reduction and ultimate loss are eventually observed. The first and second pairs of pleopods (modified as gonopods) are also conserved in Areopaguristes . However, this is not true in some species of Pseudopaguristes [e.g., P. h i a n s ( Henderson, 1888), P. asper Rahayu, 2005 ] and in Tetralobistes gen. nov. Rahayu (2005) concluded that Areopaguristes and Pseudopaguristes (12 and 8 pairs of gills, respectivelly; first and second pair of gonopods usually present in males) are more derived than Paguristes and also suggested that a well developed brood pouch, as is found in Paguristes , denotes a primitive state.

Although in Tetralobistes gen. nov. the absence of the female brood pouch (reduced to a tufts of setae) is a distinctive character, the latter can be occasionally absent or also reduced to tufts of setae in some species of Areopaguristes [e.g., A. tuberculatus ( Rahayu, 2005) and A. mclaughlinae ( Ayón-Parente & Hendrickx, 2006) ], Paguristes (e.g., P. anomalus Bouvier, 1918 and P. fagei Forest, 1954 ), and Pseudopaguristes [ P. janetkae ( Rahayu, 2005) ] (see also McLaughlin 2002, Rahayu 2005). Female first pleopods are always present and modified as gonopods in Paguristes and Pseudopaguristes , but in Areopaguristes these are either present or absent [e.g., Areopaguristes hummi ( Wass, 1955) and Areopaguristes mclaughlinae ( Ayón-Parente & Hendrickx 2006) ], as in Tetralobistes gen. nov. With 13 pairs of gills, Tetralobistes gen. nov. would appear to be more primitive than Areopaguristes and Pseudopaguristes , but more derived than Paguristes and Paguropsis . This would also be supported by the loss of the males’ second pair and females’ first pair of pleopods, and of the brood pouch in Tetralobistes gen. nov.. These characters would set Tetralobistes gen. nov. in an intermediate position between these two pairs of genera.

The size, armature and distance separating the ocular acicles in the hermit crabs had been considered a generic diagnostic character. In Tetralobistes gen. nov., ocular acicles are contiguous, as in some species of Areopaguristes , but are well, or more or less separated in Paguristes and Pseudopaguristes .

De Saint Laurent (1970) and McLaughlin (1974) mentioned that the preungual process is an important diagnostic character at genus level. The presence or absence of this preungal process in the fourth pereopod appears to be a highly variable character in Paguristes , Areopaguristes and Pseudopaguristes ; in Tetralobistes gen. nov., so far a monotypic genus, the value of this character is difficult to assess.

The shape of the telson, including the depth of lateral constriction and posteromedian cleft, the type of armature, and the pilosity of posterior lobes, are important characters used to define relationships among hermit crab genera (de Saint Laurent 1970). Perhaps the most conspicuous character separating Tetralobistes gen. nov. from the other three genera of Table 1 is the shape of its telson: posterior margin typically divided into four lobes, the two inner lobes much smaller than the two outer lobes. Actually, no genus of Diogenidae features a four-lobed posterior margin. In the case of Paguristes frontalis H. Milne Edwards, 1836 and P. purpureantennatus Morgan, 1987 , the telson is four-lobed, but these correspond to anterior (two weakly marked lobes) and posterior (two lobes) sections of the telson, not to the posterior margin. As seen here, the combination of several characters set Tetralobistes gen. nov. apart from the three other genera, and supports the recognition of a distinct, new genus.

A comprehensive revision of the “ Paguristes ” (sensu lato) species occurring in the western Atlantlic might reveal an amphi-American distribution of Tetralobistes gen. nov., as it is known to occur with many decapod crustacean genera (see Boschi 2000). A careful review of species of Paguristes , Areopaguristes and Pseudopaguristes from other regions (including both coasts of Central and South America) that share some apomorphic characters with Tetralobistes gen. nov. (e.g., absence of paired second male and female first pleopods; absence of brood pouch; presence or absence of epipod on first maxilliped) is also needed to define more clearly the affinities of the later genus with the other three.

Figures

FIGURE 2. Tetralobistes bicentenarius gen. nov. and sp. nov., A – E, holotype female (SL, 2.10 mm) (EMU- 8730). Left mouthparts, inner view. A, maxillule; B, maxilla; C, first maxilliped; D, second maxilliped; E, third maxilliped.