Thrombus kittonii (Carter, 1874)

Figures 65 a–d

Corticium kittonii Carter, 1874: 253, Pl. XV figs 48a–c.

Thrombus kittoni; Van Soest & Stentoft 1988: 40, text-fig. 18.

Material examined. RMNH Por. 9995, Guyana, ‘ Snellius O.C.P.S. ’ Guyana Shelf Expedition, station H57, 7.595°N 56.8767°W, depth 94 m, bottom coarse sand shells, 11 May 1966.

Description. (Fig. 65 a) Dark brown (alcohol), massive specimen, riddled with vermetid shells. Size 7 x 5 x 4 cm. Surface optically smooth between the shells that are partially protruding. Microscopically irregular and rough to the touch. No oscules, apart from the many open shells. Consistency hard, crumbly.

Skeleton. The ectosomal skeleton consists of the upward directed cladi of the plagiotriaenes and larger trichotriaenes, in the choanosome all triaene types are strewn in the collagenous tissue between the vermetid shells. The latter form the main support of the sponge mass, which would otherwise likely be encrusting, because larger megascleres, which would provide structure, are lacking.

Spicules. (Figs b–d) Acanthotrichotriaenes, trichotriaenes, acanthoplagiotriaenes, plagiotriaenes.

Acanthotrichotriaenes (Figs 64 b–c), heavily spined on rhabdome and cladi, but with smooth areas at the centre where rhabdome and cladi meet; cladi trifid, branching almost in one plane; rhabdome and cladi endings bluntly rounded. There appear to be two overlapping categories, (1) rarer (n=6), smaller (Figs 65 b,b1,b2) with rhabdomes 40– 69 x 12–17 µm, primary cladi 13– 20 x 9–12 µm, secondary cladi 20– 29 x 10–16 µm, (2) more common, larger (Figs 64 c,c1) with rhabdomes 72– 91 –108 x 16 – 19.4 –21 µm, primary cladi 12– 18.3 – 30 x 12 – 15.6 –18 µm, secondary cladi 20– 23.8 – 26 x 13 – 14.6 –18 µm.

Trichotriaenes (Fig. 64 c2), smooth or very lightly spined, probably incipient acanthotrichotriaenes, rhabdomes 72– 98 x 4–10 µm, primary cladi 20– 33 x 4–9 µm, secondary cladi 21– 27 x 5–9 µm.

Acanthoplagiotriaenes (Figs 65 d), heavily spined with conical spines, with thick blunt-ending rhabdomes thicker than cladi; spines of the rhabdome may be verticillated; cladi are characteristically incurved near the base, but turn outwards at the apices; rhabdomes 144– 159 –171 x 21 – 30.4 –42 µm, cladi 78– 88 –108 x 21 – 26.6 –33 µm.

Plagiotriaenes (Fig. 65 d1), smooth or very lightly spined, presumably incipient acanthoplagiotriaenes as the cladi are also first incurved and then outcurved; rhabdomes 114– 121 –126 x 9 – 10.7 –12 µm, cladi 57– 69 – 75 x 6 – 7.7 –9 µm.

Distribution and ecology. Guyana Shelf, Panama, Barbados, coarse sand and shell bottom at 94–100 m depth.

Remarks. This is the first time a fullgrown specimen of Thrombus kittonii is reported. Carter’s (1874) type consisted of loose spicules in a sandy deposit near Colon, Panama, whereas Van Soest & Stentoft’s (1988) material was a thin encrustation. The latter was reexamined and was found to have closely similar spicules and sizes: acanthoplagiotriaenes were common, but not as abundant as in the present specimen, whereas the smaller category of acanthotrichotriaenes was the most abundant type. Some smooth triaenes were also observed. Both specimens lack the amphiaster microscleres characteristic for the type species and several other species of Thrombus.

Sollas (1886) already noted that T. kittonii is quite similar to the West Pacific T. challengeri Sollas, 1886, sharing most of the types and size categories of the acanthotriaenes. With the present specimen we can establish that also the habitus including the vermetid shells is very similar in the two species. The differences are found in the sizes, especially the rhabdomes, of the acanthoplagiotriaenes, which are only about half the length of those of T. kittonii. Other Thrombidae species from the Central West Atlantic region, T. jancai Lehnert, 1998 and Yucatania sphaeroidocladus (Hartman & Hubbard, 1999) (originally as Thrombus) differ clearly, o.a. by possessing amphiaster microscleres.

Cárdenas et al. (2011) in their molecular phylogenetic study of the Astrophorina found Thrombidae to be at the base of the Astrophorina tree, isolated and well away from mainstream astrophorine groups. This corresponds with the peculiar small-sized acanthose ‘triaenes’ of the group, which may not be homologous to the long-shafted smooth triaenes of the other Tetractinellida. Only in the thrombid genus Yucatania Gómez, 2006 there are longshafted spicules (termed demotriaenes), lacking in Thrombus, that may be considered homologous to normal triaenes, but for their heavy spination and small cladi. The group remains enigmatic.