Sponges in the class Demospongiae have growth forms that range from very simple crusts to elaborate branches and even Human-sized barrels. Although the growth form of a Sponge, whether massive, encrusting, tubular, or branched, can be a useful taxonomic character, shape is not always definitively diagnostic of a particular taxonomic group. Instead it is often essential to study microscopic characters to accurately identify species. It is common for Sponge species within a genus to have vastly different growth forms despite having similar skeletal arrangements and even molecular sequence identities. As filter feeders, the growth form of a particular Sponge can be influenced by the external environment, especially the flow regime of the surrounding water. The Sponge bodyform is optimised to efficiently remove as much suspended food as possible to maintain a balanced metabolic energy budget. This is evident in the North Atlantic where several species from different Sponge orders have convergent external morphologies. Some of the most striking, but often unrelated, Sponges in the North Atlantic form large fans and vases, creating complex three-dimensional underwater seascapes.
A variety of Sponge assemblages can be found throughout the world. Sponge grounds are dense aggregations of Sponges that form structurally complex habitats that enhance local benthic biodiversity. In Canadian waters, Sponge grounds formed by large, dense, often boulder-shaped Sponges are common in the North Atlantic, and elaborate Glass Sponge reefs occur near British Columbia which form habitat for diverse communities of Invertebrates and Fish. Less elaborate Sponge aggregations described as Sponge gardens have also been identified in Canada, including dense patches of bush-like Iophon Sponges in Frobisher Bay. Conversely, flabelliform (fan-shaped) Sponges (including leaf-like and vase shaped forms) are often solitary, though some species may aggregate in particular habitats. These fan-shaped sponges are conspicuous in underwater surveys and are associated with increased biodiversity. Flabelliform Sponges can constitute a large proportion of the benthic fauna and the three dimensional structure of a given benthic area, but do not necessarily form the majority of the benthic fauna as in the case of massive Geodia sp. Sponge grounds in the boreal Atlantic.
Recent efforts to identify Sponge fauna of the western Atlantic have focused primarily on characterising large charismatic sponges that could fit the criteria for vulnerable marine ecosystem designation. However, cryptic or less obvious species are often misidentified, and in many cases, specimens are placed in broadly defined taxa, which generates confusion. For example, fan-shaped Sponges found in photo and video surveys in the Flemish Pass and fan-shaped and stalked Sponges in a guide to Sponges of the North Atlantic Fisheries region were identified as Family Axinellidae. However, the same authors later questioned these classifications, recognizing that not all fan-shaped and lamellate Sponges in the region may belong to the family Axinellidae. Grouping sponges that are similar in appearance could lead to the mischaracterization of faunal associations with some Sponge species over others, or incorrect species distribution data.
In a paper published in the journal Zootaxa on 24 March 2020, Curtis Dinn of Fisheries and Oceans Canada, and the Department of Biological Sciences at the University of Alberta, Sally Leys, also of the Department of Biological Sciences at the University of Alberta, and Mylène Roussel and Denise Méthé, also of Fisheries and Oceans Canada, present the results of a study which aimed to provide a useful taxonomic guide for four species from eastern Canada that span different Sponge orders, but whose identities are often confused due to similar external morphology and overlapping geographic distribution. Three of the species discussed have not been formally described from Eastern Canada, and one species is discussed as a new combination of a little-known genus in the northern hemisphere.
Species were collected during several research surveys in eastern Canada. Arctic specimens were collected during two research cruises on the CCGS Amundsen (July 14–27, 2016; July 13–August 17, 2017). Gulf of St. Lawrence specimens were collected during three surveys: the 2018 snow crab survey aboard the Jean Mathieu (July 19 –September 16, 2018); the 2018 research vessel bottom-trawl survey of the southern Gulf of St. Lawrence aboard the CCGS Teleost (September 4–October 5, 2018); and the 2018 ground fish and shrimp multidisciplinary survey in the estuary and northern Gulf of St. Lawrence aboard the CCGS Teleost (August 4–September 1, 2018). Specimens were also collected during a cruise in the Gulf of St. Lawrence (August 23–30, 2017) using the ROV ROPOS aboard the CCGS Martha L. Black.
Map of the collection sites in Baffin Bay/Davis Strait (inset) and collections in the Gulf of St. Lawrence. Hollow diamonds, Axinella arctica; filled circles, Cladocroce spatula; filled squares, Semisuberites cribrosa; filled triangles, Plicatellopsis bowerbanki. Dinn et al. (2020).
The first species examined by Dinn et al. is Axinella arctica, a cup-like or flabellate shaped Axinellid Demosponge. The species often forms an inverted, hollow cone that ranges 5–25 cm in diameter, and has a solid stalk. The Sponge is pale yellow or buff to white in colour. The surface of the inner portions has many pin-hole sized openings; smaller openings are present on the outside surface. Dense longitudinal ribs extend from the stalk to the distal portions on the outside surface. The consistency is firm, and pieces will break off when bent more than 45°. The walls of the specimens are up to 0.5 cm thick. The distal lip is hispid from protruding spicules.
Axinella arctica: (A) Small specimen attached to pebble, thick stalk visible; (B) underside of collected fragments; (C) specimen amongst Duva sp. Soft Corals; (D) specimen from (B) in situ; (E) skeleton; (F) spicules, oxeas and styles. Dinn et al. (2020).
The megascleres (large spicules that function as the main support elements in the skeleton) of Axinella arctica are styles (spicules with one pointed end and one rounded one) 401–467–561 x 13–17–25 μm in lengths, and oxeas (spicules pointed at both ends) 329–386–443 x 13–18–24 μm long. No microscleres (small spicules which do not form part of the main supporting skeleton) are present. The skeleton consists of dense axial tracts of oxeas with loose extra-axial fibres consisting of styles which form long spicule brushes at the surface. Oxeas connect the extra-axial fibres.
Genetic testing of the collected specimens of Axinella arctica showed them to be identical to the species Axinella infundibuliformis, based on 539 of 584 nucleotides. However, they lack trichodragmata spicules (bundles of hair-like spicules), which are considered diagnostic of Axinella infundibuliformis, so until more comprehensive DNA–based distinctions in a less-conserved gene region can be made, the specimens collected from eastern Canada are considered by Dinnet al. to be Axinella arctica based on morphology.
Axinella arctica is found on rocky bottoms attached to hard substrates. In the North Labrador Sea and Baffin Bay the species was collected in deep water, at depths of greater than 412 m. The species has also been collected in the North Labrador Sea, and reported from the Barents Sea, Nordic Seas, and European Waters. Axinella arctica has a more northern distribution and was not collected in the Gulf of St. Lawrence.
The feature of this species that distinguishes it from other cup-shaped specimens in the region is the presence of oxeas and styles. The growth form and shape in situ is similar to Plicatellopsis bowerbanki, but Axinella arctica can be differentiated by its dense stalk and thicker tissue.
Axinella infundibuliformis was reportedly collected from the Gulf of St. Lawrence by Joseph Whiteaves in the 1870s, but no spicule measurements or description were given. It is probable that the specimen collected by Whiteaves was another flabelliform species because Axinella infundibuliformis is not otherwise recorded from the western Atlantic. Axinella infundibuliformis has styles 260–360 μm and oxeas 210–280 μm, considerably smaller than the specimens collected in the North Labrador Sea. Axinella infundibuliformis also has trichodragmata microscleres, though these may be scarce. The skeleton of Axinella infundibuliformis is also much more dense than Axinella arctica, with oxeas forming thick axial tracts and styles in multiple, tightly arranged extra-axial fibres. Interconnecting oxeas form a more noticeable reticulation in Axinella infundibuliformis and extra-axial fibres of styles ramify closer to the choanosome than in A. arctica, forming less obvious spicule brushes. Although A. arctica and Axinella infundibuliformis are difficult to distinguish from outer morphology, the size of the megascleres and skeletal arrangement are distinct. The genetic variation between the two species is, however, not discernable using the COI Folmer fragment gene region which was used in Dinn et al.'s study.
The second species examined by Dinn et al. is Cladocroce spatula, a stalked and spatulate Haplosclerid Demosponge with one or more flat, leaf-shaped lobes extending out from a central stalk. Specimens can reach 35 cm in height and more than 20 cm in width at the widest breadth of the fan. The Sponge is light brown in colour, though the Sponge holds a substantial amount of water which may drain after collection causing the Sponge to appear lighter in colour. Many large, round, raised oscula (large openings to the outside through which the current of water exits after passing through the spongocoel, having been absorbed through much smaller dermal pores) up to 0.3 cm in diameter run along the fan and stalk. There is sometimes a large visible osculum on the distal portion of a lobe in situ, but this collapses after collection. The consistency is elastic and soft. The axial skeleton beneath the ectosome forms dense, stringy fibres which result in a sponge that is difficult to tear. The stalk is more or less cylindrical and flares out at the base to form roots. The periphery of the lobes appear slightly hispid (bristly), but remains soft to the touch. The interior of the fan or lobes is hollow, though the sponge is always flattened. Epibionts and mud are common on the lower stalk.
Cladocroce spatula. (A)–(C) Collected specimens; (D) skeleton showing anisotrophic reticulation and axial fibres; (E) spicules. Thin oxeas are uncommon and may represent a developmental form. Dinn et al. (2020).
The spicules are oxeas 245–228–326 x 15–18–20 μm that are slightly bent. Some spicules may be thin, but these are less common and may represent developmental forms. The ends of the oxeas are sharply pointed near the distal portion, though in thinner spicules the point begins to taper closer to the centre. Stylote modifications and blunt ends are common. The skeleton is chalinid (forms a mesh), a mostly anisotropic reticulation of multiple primary axial tracts of 2–3 spicules that run towards the surface in a slightly recurved, rib-like manner. The primary tracts are joined by single spicules forming a mostly square mesh. Primary tracts and nodes where single spicules join the primary skeleton are joined by spongin. There is no obvious ectosomal skeleton (protusions above the surface).
Few specimens of Cladocroce spatula have been reported in the literature. The species is very common in the Gulf of St. Lawrence from 20–365 m in depth, while most specimens were collected above 100 m on shallow shelves in the southern Gulf of St. Lawrence. The species was observed during an remotely operated vehicle dive in the northern Laurentian Channel south of Anticosti Island. It was on soft sediment bottom attached to a rock. A single specimen was collected in the North Labrador Sea. A further record was identified from a preserved specimen from Scotland. The species was also reported from Korshavn, Norway, in 1938. The type specimen (specimen from which the species was originally described) was collected on the west Greenland Shelf, east of Cumberland Sound in the early 1900s.
This is the first record of this species from the Gulf of St. Lawrence. Although the species is not often reported, it was the most common sponge collected during the 2018 CCGS Teleost survey in the southern Gulf of St. Lawrence. The specimens fit the original description of the species made by William Lundbeck in 1902, except that spicules are slightly thicker. This species is most easily distinguished by the flabellate form with large oscula. The form described by Lundbeck was similar to some specimens collected in the Gulf of St. Lawrence, but other collected individuals grew into much larger elaborate fans or lobes. Cladocroce ventilabrum was apparently collected along the Scotian Shelf near Muscongus Bay, but no museum specimens exist from those collections. Cladocroce ventilabrum has slightly longer spicules that are 250 μm in length and the Sponge itself is described as being ventilabriform. The specimens collected in the Gulf of St. Lawrence resemble the congeneric Cladocroce kiska from the Aleutian Islands, but Cladocroce kiska has sigmas and longer oxeas. The consistency and arrangement of oscula in Cladocroce spatula is reminiscent of Haliclona (Haliclona) oculata, although Haliclona (Haliclona) oculata forms many long finger-like extensions rather than the spatulate form of this species, and Haliclona (Haliclona) oculata has smaller spicules. Cladocroce spatula is often confused with Isodictya palmata in the Gulf of St. Lawrence due to the similar shape and presence of many large oscula, although Isodictya palmata may grow much more pronounced, digitate branches. Cladocroce spatula differs from Isodictya palmata in lacking chelae, but the oxeas of the two species. overlap in size. It should be noted that the skeletal architecture of Cladocroce spatula is similar to that of Isodictya palmata, even though the sponges are classified in different sponge orders. Isodictya palmata was not identified from specimens collected from the Gulf of St. Lawrence in the survey year 2017–2018, but the species was collected in the North Labrador sea during the AQVIQ cruise of 2018. Isodictya palmata specimens were much thicker than the predominantly flat Cladocroce spatula specimens, and the chelae are very distinct in spicule preparations. Isodictya deichmannae was also collected from the Bay of Fundy, but that species has styles and chelae.
The third species considered is Semisuberites cribrosa, a Poecilosclerid Demosponge that is variable in size, but is often a trumpet shaped Sponge with a long stalk that forms a holdfast, sometimes with visible roots. The diameter of the cup can approach 35 cm and the Sponge can extend up to 25 cm in height. The surface is velvety with a very soft consistency. Larger specimens may have more elaborate forms where more than one stalk may intertwine and give rise to two vases. The distal lip of the Sponge is often frayed after collection. Specimens are white to brown or grey in colour. The stalk may have epibionts growing down its length, giving a dark brown or muddy appearance.
Semisuberites cribrosa. (A)–(C) Collected specimens showing long stalks; (D) spicules, styles of various sizes; (E) skeleton. Dinn et al. (2020).
Semisuberites cribrosa was mostly collected in the southern Gulf of St. Lawrence from depth of 34–289 m. It has also been reported from Arctic and northern boreal waters. It appears to grow mainly in soft sediment environments where the stalk is somewhat submerged beneath the substrate.
This is the first record of the species from the Gulf of St. Lawrence and it represents the southernmost extent of its known distribution in the North Atlantic. The species is often confused with, and sometimes reported as, Phakellia ventilabrum in eastern Canada. A description of Phakellia ventilabrum from the Gulf of St. Lawrence by Lawrence Lambe in 1896 does not include strongyle spicules, but rather flexuous styles. It is therefore probable that the specimens collected by Lambe do not belong to the genus Phakellia due to the lack of strongyles, but instead could be Semisuberites cribrosa; however Lambe’s specimens were not reviewed for Dinn et al.'s work.
The final species examined is Plicatellopsis bowerbanki, a Suberitid Demosponge, formerly placed in the genus Phakellia, but which Dinn et al. reassign to the genus Plicatellopsis on the basis of its skeletal structure and external morphology.
Plicatellopsis bowerbanki is generally a large, vase shaped Sponge that forms a funnel at the base. However, the Sponge is somewhat polymorphic, and a more fan shape form has been observed that grew to 30 cm wide. In some individuals, secondary vases may extend from the distal portions of the primary vase. The primary stalk attaches to a hard substrate, usually rock walls, but can grow on dead Coral skeletons and pebbles. The inner and outer surfaces have irregularly spaced pores less than 1 mm in diameter. In larger specimens, the outer surface is rippled, with ridges and depressions. In larger specimens the raised portions appear to form concentric rings or lateral ribs on the underside of the cup, but when viewed from above the ridges appear as depressions. The distal lip of the fan is smoothly curved but in an irregular scalloped manner, and is often frayed both in situ and after collection. Most specimens have large circular holes irregularly placed throughout the Sponge body which can include the stem. It is unknown if these holes form naturally or are caused by spongivorous predators. However, since no specimens appeared to have predators on or near the Sponge body in situ, or after collection, and because the holes are present on specimens throughout the large geographic area in which the Sponge was collected, it is suggested that the apertures occur naturally and are thus considered diagnostic of the species.
Plicatellopsis bowerbanki from eastern Canada. (A) Specimen in situ; (B) specimens growing on rock wall in Northern Gulf of St. Lawrence; (C) specimen showing secondary vase extending from single base; (D) specimen living amongst Keratoisis sp. Coral on the western Greenland Shelf; (E) specimen growing on dead Keratoisis sp. Coral fragment; (F) specimen from Pond Inlet, Baffin Island. Dinn et al. (2020).
The spicules of Plicatellopsis bowerbanki are stout, slightly bent tylostyles (pointed spicules with a knob at the blunt end) with very slight tyles (knobs). Most spicules look like styles at lower magnifications, but the tyles are evident upon closer inspection. The tylostyles can sometimes be separated into two thickness categories, but the thick and thin spicules are consistently similar lengths, and in some specimens the thickness categories are indistinguishable as the widths may overlap. Thinner tylostyles, if present, are not as common and are sometimes difficult to find in spicule preparations due to their inconsistent distribution throughout the sponge tissue. For this reason, thin spicule placement in the skeleton does not appear to be a useful taxonomic character of the species. The skeleton of the lectotype (specimen later selected to serve as the single type specimen for species, when an original holotype was not nominated or cannot be found) was examined by Dinn et al. and tylostyles with slight tyles were present, measuring 213–273–326 x 8–13–16.5 μm. Only two spicules that were less than 10 μm in thickness were measured from the lectotype. This suggests that the less common thin tylostyles are also present in the specimens from the original description made by Gualtherus Vosmaer in 1885, but a sufficient number could not be measured as they were not visible in the slide preparation. A specimen from Baffin Bay has tylostyles that are 258–317–359 x 14–18–21μm, and thinner spicules were not seen.
The skeletal architecture of the species consists of dense spicule tracts forming an axial skeleton, which branch out into extra-axial bundles towards the surface, terminating in spicule brushes. The axial and extra-axial spicule tracts consist of between 2–6 spicules. The axial skeleton of tight spicule bundles causes the sponge to have a fibrous texture when cut. When present, the thinner tylostyles are not localized in the skeleton, but rather appear to be loosely placed throughout the sponge tissues. In individuals that have a second category of thinner tylostyles, those spicules are not concentrated at the surface like the smaller spicules in other species of the genus, but rather they occur scattered in the choanosomal skeleton without a consistent distribution. The spicules outside of the axial and extra-axial skeletons form a loose, irregularly aligned reticulation of bundles and single spicules.
Plicatellopsis bowerbanki, spicules and skeleton. (A) Spicules; (B) detail of slight tyle; (C), (D) scanning electron microscope image of tylostyle I; (E), (F) scanning electron microscope image of tylostyle II; (G) skeleton of Baffin Bay specimen, ectosomal skeleton of spicule brushes are visible at the surface; (H) skeleton of specimen BMNH 1910.1.1.1477 (lectotype). Dinn et al. (2020).
Plicatellopsis bowerbanki appears to be ubiquitous on the east coast of Canada, extending from the northern tip of Baffin Bay to the northern portions of the Gulf of St. Lawrence. In Canadian waters it is found at depths ranging between 73 and 878 m. Plicatellopsis bowerbanki is abundant in the Banc-des-Américains Marine Protected Area and the Disko Fan Conservation Area which were recently closed to bottom contact fishing by the Department of Fisheries and Oceans, Canada. Plicatellopsis bowerbanki has been seen growing on varied substrates, but often attached to harder objects such as dead Coral, pebbles and rock walls in otherwise soft sediment environments. The species has also been collected in the waters off Norway, The Faroe Islands, Iceland, Greenland, and in the Barents Sea.
Although this is far from a complete review of all species in eastern Canada that may form a cup or vase shape, the four species described by Dinn et al. exemplify the difficulties associated with identifying Sponge specimens in situ or from gross morphology alone. Because the four species are similar in colour, size and general shape, in the past they have been often confused and given different species names. As a result, it is presently difficult to determine the correct distribution of flabelliform sponge species in eastern Canada. Efforts to monitor the health of marine Sponge assemblages, particularly in newly established marine refuges in eastern Canada, will benefit from species-level identifications to better understand the potential differences in ecosystem services and habitat functioning provided by similar-shaped, yet unrelated species. Considering the number of synonyms and the history of incorrect taxonomic placement of the species discussed here, it is apparent that close review of morphology, spicules, and molecular data are required for accurate species-level identification.
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