Hypostomus cochliodon

Jonathan W. Armbruster, 2003, The species of the Hypostomus cochliodon group (Siluriformes: Loricariidae)., Zootaxa 249, pp. 1-60 : 54-57

[[ Hypostomus cochliodon Group ]]

Discussion

Delimiting the species of the Hypostomus cochliodon group is made difficult by the fact that the species vary little in morphometrics or meristics, and the species have a well developed ability to alter color pattern. However, the species can be divided into four groups and from those groups it is possible to distinguish the species. The four groups are the intermediate group ( H. hemicochliodon and H. sculpodon ), H. cochliodon with a unique color pattern, the odontodeless opercle group ( H. ericius , H. levis , H. oculeus , H. pyrineusi , and H. taphorni ), and an undifferentiated northern group ( H. hondae , H. pagei , and H. plecostomoides ). The odontodeless opercle group can be further subdivided into a highly keeled group ( H. ericius and H. oculeus ) and a weakly to non-keeled group ( H. levis , H. pyrineusi , and H. taphorni ). Once in these groups, it is fairly simple to separate the species. Principal components analysis is provided for each of the groups although H. levis is excluded from the analysis because it lacks many of the measurements because of its absence of an adipose fin.

The PCA of the intermediate group showed a 100% separation of the species on principal component 2 (Fig. 10A). PC2 is most strongly and negatively effected by dorsal-anal length and anal-fin width and most strongly and positively effected by anal-fin length and folded dorsal-fin length. A ratio of dorsal-anal length to anal-fin length completely separates the two species from one another: average = 111.1 ± 8.4%, 92.5-132.6% in Hypostomus hemicochliodon and average = 79.2 ± 7.6%, 69.6-89.7% in H. sculpodon ).

The PCA of the odontodeless groups showed two trends, a complete or almost complete separation of H. taphorni from H. ericius , H. oculeus , and H. pyrineusi and a complete separation of H. ericius and H. oculeus on a graph of PC2 vs. PC3 (Fig. 10B). PC2 is most strongly and negatively effected by adipose-caudal length and mouth width and most strongly and positively effected by interdorsal length, eye-nare length and orbit length. PC3 is most strongly and negatively effected by base of anal-fin length, anal-fin length, and adipose-spine length and most strongly and positively effected by interdorsal length, adipose-caudal length, and postanal length. Although there is no morphometric evidence suggesting that H. ericius and H. oculeus are distinct from H. pyrineusi there are morphological differences (well-developed keels in H. ericius and H. oculeus vs. no or weak keels in H. pyrineusi ).

The PCA of the northern group showed some general trends separating H. hondae and H. pagei from H. plecostomoides and a complete separation of H. hondae from H. pagei on PC2 vs. PC3 (Fig. 10C). PC2 is most strongly and negatively effected by eye-nare length and interorbital width and most strongly and positively effected by anal-fin length and interdorsal length. PC3 is most strongly and negatively effected by anal-fin length and folded dorsal-fin length and most strongly and positively effected by adipose-caudal length and interdorsal length.

For the most part, the species of the Hypostomus cochliodon group are allopatric (Figs. 8, 11, 17). It is only in the Amazon basin that several species coexist. Panaque , the other wood-eating genus of loricariid catfishes, also reaches its peak of diversity in the Amazon basin (Schaefer & Stewart 1993; Schaefer & Stewart 2002). In the upper Río Marañon drainage of Peru, there are at least nine species of wood-eaters: H. ericius , H. hemicochliodon , H. oculeus , H. pyrineusi , P. albopunctatus , P. dentex , P. gnomus , P. nocturnus , and probably at least one species of the P. nigrolineatus clade (distributions based on this study and Schaefer & Stewart 1993). How these species subdivide the wood-eating niche is unknown, and ecological studies on wood-eating by fishes in the upper Río Marañon would represent a very interesting study.

Weber and Montoya-Burgos (2002) recently described Hypostomus fonchii and suggested that the species was derived from the H. cochliodon group. The teeth described for H. fonchii are elongate and unicuspid and it is suggested that the unicuspid teeth represent a synapomorphy for H. fonchii and the H. cochliodon group. This is problematic in that I have not observed truly unicuspid teeth in any members of the H. cochliodon group. In all specimens of the H. cochliodon group that I have examined, the medial cusp remains present, but is generally fused with the lateral cusp. The mesial cusp remains visible as a slightly darker, thicker ridge on the medial side of the tooth. I have not examined any specimens of H. fonchii , and I cannot determine whether or not this is the case with this species. The only truly unicuspid teeth that have been reported for the Hypostominae are the teeth found in nuptial male Aphanotorulus (Armbruster & Page 1996). I do not consider H. fonchii as a member of the H. cochliodon group as its body shape is that of a generalized, fast-water dwelling Hypostomus and not the derived shape of H. cochliodon . Exact placement of H. fonchii awaits an analysis of its diet, osteology, and external anatomy for the synapomorphies of the H. cochliodon group.

Recent expeditions into poorly collected regions where no members of the Hypostomus cochliodon group have been reported from in the past are finding more species of the H. cochliodon group. Two undescribed species are now known from the Essequibo and Takutu River drainages of Guyana (pers. obs.) and one species from the Rio Tocantins in Brazil (Reis, pers. comm.). These species and a revised key will be presented in future publications.

Phylogeny

Character 1: Teeth - 0: elongate (Fig. 1A); 1: intermediate throughout life (Fig. 1B); 2: spoon-shaped at least in adults (Fig. 1C).

Character 2: Maxilla - 0: straight to moderately curved (Fig. 3A); 1: greatly curved, almost forming right angle (Fig. 3B).

Character 3: Odontodes on opercle - 0: 11+ (Fig. 2A); 1: 0-10 (Fig. 2B).

Character 4: Longitudinal ridge formed from bone and slightly enlarged odontodes on pterotic-supracleithrum - 0: present (Fig. 5A); 1: absent (Fig. 5B).

Character 5: Nuptial body odontodes - 0: absent (Fig. 2A); 1: present (Fig. 2B).

Character 6: Notch between the hyomandibula and metapterygoid - 0: present (Fig. 3A); 1: absent (Fig. 3B).

Character 7: Buccal papilla - 0: present (Fig. 4A); 1: absent or extremely small (Fig. 4B).

Character 8: Dentary angle - 0: averaging greater than 90°; 1: averaging less than 80°.

Character 9: Sharp keel odontodes - 0: present; 1: absent.

Character 10: Body shape deep and narrow with the head taller than wide - 0: absent; 1: present.

Very few morphological characters are useful in elucidating the relationships of the species of the Hypostomus cochliodon group. Based on the ten characters found, a phylogeny is produced for the species of the H. cochliodon group (Fig. 23). The single tree found has 12 steps and CI = 1.00. Character state data is in Table 8. No skeletal material is available for H. sculpodon or H. ericius ; however, external evidence does provide information as to the potential relationships of these species with other members of the H. cochliodon group. Considering there are so few characters available for analysis, the phylogeny should be considered tentative.

Also providing support for the basal position of Hypostomus hemicochliodon and H. sculpodon is diet. In all of the specimens of the H. cochliodon group examined, almost the entire intestine is filled with small wood chips suggesting that the fishes consume little other than wood. In H. hemicochliodon and H. sculpodon the intestine mostly contains wood, but other organic matter makes up a major portion of the diet (not greater than 50%). It is apparent that H. hemicochliodon and H. sculpodon are wood eaters but not wood specialists.