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Topsent, 1892b

Species Overview

Acarnus tortilis Topsent (1892b) is an orange encrusting species, often agglutinating substrate particles or encrusting Laminaria holdfasts or other sponges. The consistency is very soft. A Mediterranean species recorded a few times from the shallow sublittoral on the coast of NW Spain. It is easily recognized if examined microscopically, because of the presence of characteristic cladotylotes.

Taxonomic Description

Colour: Orange alive, brown in alcohol.
Shape, size, surface and consistency: Massively to thinly encrusting, often agglutinating substrate particles or algae. Size not recorded, but generally small. Surface smooth except where large megascleres pierce the dermis. Oscules inconspicuous. Consistency soft. Sticky.
Spicules: Megascleres: Microspined tylotes with swollen tyles: 250-400 x 3-6 µm. Styles straight, with microspined (occasionally smooth) apices: 295-550 x 5-16 µm. Cladotylotes, two categories: large ones, with cladome provided with 4 (sometimes 5) clads, with heavy spines along the shaft, and often with rather characteristic blunt spines on the tyles, which adhere closely to the shaft: 113-220 x 3-6 µm (Topsent, 1904: up to 270, but examination of his slide could not confirm this); small ones, similar to the large ones, tyles often irregular: 55-110 x 1-3.5 µm.
Microscleres: palmate isochelae: 11-22 µm. Toxas, two categories: oxhorn toxas (toxa I) and thin deep-curved toxas (II) not differentiated, intergrading completely: 20-190 x 0.5-5 µm; the characteristic accolada toxas (III) found in most species of Acarnus are absent, or more precisely: they are modified in the form of thin straight toxa, lacking the usual sharp-angled central curve and recurved wings: 170-600 µm.
Skeleton: Ectosomal: a feltwork of tylotes producing a smooth detachable "skin". Choanosomal: plumose columns of styles echinated by cladotylotes rise up from a basal plate of spongin; smaller cladotylotes echinate lower parts and basal plate, larger cladotylotes echinate upper parts of the columns.
Ecology: On calcareous algae, Laminaria holdfasts, and on other sponges. Exposed at low tide down to 12 m in NW Spain (Solórzano, 1991), elsewhere to 91 m.
Distribution: Atlantic coast of north Spain; Western Mediterranean, Azores, Cape Verde Islands.
Etymology: tortilis (Latin) = twisted, possibly referring to the condition of the toxas.
Type specimen information: The holotype is in the Paris Museum, MNHN D.T. 271, labeled "136, Banyuls".


This is the only Western European species of the genus Acarnus, which is easily recognized by its possession of the cladotylotes, unique to the genus. Durán and Solórzano (1982) recorded A. tortilis from the Atlantic coast of North Spain. They reported only a single category of cladotylotes (74-103 µm), but re-examination of the material revealed there are two: viz. 65-94 x 1-2 µm and 108-145 x 3-4 µm; style sizes reported (217-331 µm) are outside the range of Mediterranean A. tortilis, but subsequent re-examination showed the majority of the styles to be between 295 and 345 µm) and the toxa III category, although not reported, is only up to 195 µm long. Style heads are frequently somewhat swollen and relatively heavily spined. If echinating acanthostyles would have been present, then the specimens would be similar to A. souriei and A levii (both known from the Mediterranean).
Topsent's (1904) Azores specimen largely conforms to Mediterranean specimens (even including the blunt spines on the cladotyles), but again the toxa III category reaches only 220 µm.
Cape Verde specimens (Topsent, 1928 and ZMA material) approach the specimens from north Spain in the relatively small cladotylote I size (123-174 µm), and again the toxas III are relatively short (up to 404 µm). It appears as if the Atlantic specimens share the short toxa III and possibly represent a separate taxon. However, in view of the adjacent distributions, the absence of further consistent differences and the large variability in many features, all Eastern Atlantic specimens are assigned to A. tortilis.
Indo-Pacific records of A. tortilis are referable to A. bergquistae Van Soest et al., 1991.
Synonymization of the sympatric Mediterranean A. polytylus is based on the argument that apart trom the polytylote condition of some of the megascleres of the single specimen of A. polytylus there are no other differences; moreover, quite a large proportion of the spicules in the specimen is "normal". Polytylote spicules occur in other sponges (e.g. (Clathria, Polymastia), and are likely to be an individual rather than a specific character.
This species differs from Brazilian A. toxeata, Caribbean A. deweerdtae and Indo-Pacific A. bergquistae in the straight modification of toxa III; many specimens especially from the Mediterranean show persistent occurrence of blunt spines on the cladotylotes, but Atlantic representatives have normal sharp spines.
Source: Van Soest et al., 1991.

Acarnus tortilis