Malacoceros Quatrefages, 1843; type-species: Spio vulgaris Johnston, 1827, designated by Pettibone, 1963. Colobranchus Schmarda, 1861; type-species: Colobranchus tetracerus Schmarda, 1861, by monotypy. Uncinia Quatrefages, 1865; type-species: Colobranchus ciliatus Keferstein, 1862 (= C. tetracerus Schmarda, 1861), by
monotypy.
Scolecolepis Malmgren, 1867; type-species: Spio vulgaris Johnston, 1827, by original designation. [Synonymy fide Blake & Kudenov, 1978]
Diagnosis. (after Delgado-Blas & Díaz-Díaz 2013, amended). Prostomium broad anteriorly, T-shaped, triangularshaped, bell-shaped; broadly rounded along anterior margin; occipital antenna absent. Eyes present, irregularly arranged or arranged in pairs, or eyes absent. Caruncle entire, trilobed or buttonlike. Nuchal organs as two small ciliated grooves posteriolaterally to the caruncle. Palps ventrally grooved. Peristomium reduced to moderately developed. Eversible, sac-like proboscis. Cirriform branchiae from chaetiger 1 to end or nearly end of body; basally fused or free to notopodial lamellae; branchiae usually overlapping at dorsal midline in anterior segments, reduced in length and thickness in middle and posterior segments. Dorsal ciliated organs present or absent. Transverse ciliated bands across the dorsum present. Parapodia 1– 3 may be shifted dorsally to subsequent segments. Chaetae include simple capillaries, scalpel chaetae, neuropodial uni-, bi-, tri- or quadridentate hooded hooks. Sabre chaetae present. Pygidium with 2, 4, 6, 6– 8, or 15–30 anal cirri or with two anal cirri and a rounded or spatuliform dorsal lobe. Eggs with complex thick egg membranes ornamented resembling honeycombs with numerous cortical alveoli (Blake & Arnofsky 1999). Male gametes are ect-aquasperm type (Guérin & Kerambrun 1984).
Remarks. The generic diagnosis provided by Delgado-Blas & Díaz-Díaz (2013) includes a number of characters which formerly had not been considered. Hence it can be regarded an appropriate attempt for the refinement of diagnosis. Many generic diagnoses are evidently not sufficient and the assignment of species is not reliable. This applies also to Malacoceros. The latest example is the publication by Sikorski (2013) in which the author proposes the transfer of Malacoceros jirkovi Sikorski, 1992 to Spio Fabricius, 1785. In the same publication the author considers that the morphologically similar M. indicus might also belong to Spio. Spio jirkovi (Sikorski, 1992) comb. nov. is established based on 1) the fixed number of four anal cirri opposed to a size-variable number of anal cirri in Malacoceros, 2) the absence of anterolateral horns and 3) the arrangement of teeth in a tandem pattern instead of a pair-wise arrangement. These arguments seem disputable. A fixed number of four anal cirri is not exclusive for Spio but quite widespread among Spionidae. None of the published generic diagnoses for Malacoceros state a size-variable number of anal cirri to be diagnostic. The term anterolateral horns might refer to morphologically similar but non-homologous structures within Spionidae (Rhynchospio, Glandulospio, Scolecolepides, Glyphochaeta, Pygospiopsis, Lindaspio, Atherospio, Spiophanes, Microspio, etc.) and a detailed review of this character is certainly required for all taxa (Meißner et al. 2014). However, the prostomial shape of S. jirkovi being subtriangular, seems rather unusual for Spio. The absence of paired teeth is not uncommon among Malacoceros species (e.g., M. jennicus Graff, Blake & Wishner, 2008, M. fuliginosus (Claparède, 1870), M. tetracerus (Schmarda, 1861), M. cariacoensis Delgado-Blas & Díaz-Díaz, 2010). On the other hand, M. indicus, the species Sikorski (2013) considers closely related to S. jirkovi, has paired apical teeth and hence contradicts Sikorski´s line of argument.
There are also good reasons why S. jirkovi is not in agreement with the generic diagnosis of Spio. For this species nuchal organs being small elongate grooves lateral to the posterior tip of the prostomium have been observed (Meißner et al. 2014). Metameric dorsal ciliated organs are not discernable (Meißner et al. 2014). The same applies to M. indicus (this paper and cited literature herein). For Spio nuchal organs with short median and long lateral ciliary bands, extending to chaetiger 2 or 3, are typical and metameric dorsal ciliated organs are usually present and easily detected under the light microscope after methyl green staining. Ventral epidermal glands are usually present in anterior and middle chaetigers of Spio species. Such glands could not be detected in M. jirkovi nor in M. indicus. Therefore we do not agree with the new combination proposed by Sikorski (2013) and regard Malacoceros jirkovi as the valid name for the species. Nethertheless we admit, that the generic diagnosis of Malacoceros (and of several other genera) might not be sufficient in its current version.