Order Halichondrida Gray, 1867

Abstract

Order Halichondrida Gray (Demospongiae) is here employed in the redefined sense of Van Soest et al. (1990; with subsequent published adjustments), and includes the former much more restricted Halichondrida and part of the former Axinellida as proposed in earlier classifications (e.g., Levi, 1973; Bergquist, 1978; Hartman, 1982). The order is defined to include sponges with smooth styles, oxeas or strongyles, occurring often in tandem within the same sponge, but usually without distinct localization, and lacking microscleres other than spined microxeas and raphides. Flexuous, sinuously curved, or contorted megascleres are relatively common. Skeletal architecture is basically plumoreticulate, but in various groups this is modified to dendritic columns of megascleres or confused bundles with many loose single megascleres. Ectosomal skeleton ranging from specialized, tangentially intercrossing or obliquely arranged palisades or bouquets of mega- and microscleres, to absent or undifferentiated, or replaced by thick organic skin, or rarely with sand grains. Five families are recognized, based on features of the ectosomal and choanosomal skeleton. Axinellidae, Bubaridae and Dictyonellidae lack a specialized ectosomal skeleton, whereas Halichondriidae and Desmoxyidae with few exceptions have an ectosomal skeleton consisting of spicules arranged tangentially or in the form of palisades/bouquets. Axinellidae differ from Bubaridae and Dictyonellidae in having a well-developed dense reticulation of megascleres, often showing axial and extra-axial differentiation, and their surface is velvety or microhispid due to projecting choanosomal spicule bundles. Bubaridae are thinly encrusting sponges with monactinal megascleres erect on a basal mass of bent or flexuous diactinal spicules. Dictyonellidae have a fleshy organic skin, and lack a neat reticulation, megascleres usually forming dendritic columns or a vague confused reticulation of styles, rarely oxeas. Desmoxyidae have spined microscleres or smaller megascleres at the surface, either tangentially or perpendicularly arranged. Halichondriidae have only smooth megascleres, and with few exceptions have a tangential crust of intercrossing oxeas, rarely styles. Where known most Halichondrida are oviparous, but Halichondria, Hymeniacidon (Halichondriidae), Scopalina and Svenzea (Dictyonellidae) are viviparous; the latter have very large ovoid larvae. Halichondrida are common inhabitants of all marine habitats in all oceans and seas.