Mendicula pygmaea ( Verrill & Bush, 1898 )

Mendicula pygmaea ( Verrill & Bush, 1898) View in CoL

Fig. 16 View Fig

Mendicula pygmaea View in CoL was described from off the coast of New England and appears in the European literature in Lande (1975), and Høisaeter (1986) recorded it from the coast of Norway between 60° and 63° N. Ockelmann’s unpublished map ( Fig. 16 View Fig ) shows a wide range but sparse distribution across the N Atlantic Ocean. It was listed from the Skagerrak by Wikander (1989) and later was cited from the North Sea oilfields by Oliver & Killeen (2002). Unfortunately, both Ockelmann and Oliver & Killeen had overlooked the fact that the material from the North Sea was not M. pygmaea View in CoL but was a neglected species of Adontorhina View in CoL published subsequently by Barry & McCormack (2007) as A. similis View in CoL . It is most unlike Ockelmann to have overlooked A. similis View in CoL , but without recourse to the SEM, he probably could not discern the denticulate dorsal margin typical of Adontorhina View in CoL . Oliver & Killeen (2002) did figure the denticulate margin, but failed to notice the significance taking the M. pygmaea View in CoL identification for granted.

Larval shells (prodissoconchs)

Ockelmann was mentored by Gunnar Thorson who postulated Thorson’s rule concerning the relationship of egg size and larval development to latitude. Egg size and larval development can be inferred in bivalves from their prodissoconch size and this was an area of interest for Ockelmann throughout his research. In his thyasirid archive, we find two very detailed plates of the drawings and sizes of prodissoconchs of 20 species of Thyasiridae ( Figs 17–18 View Fig View Fig ). In addition, there are scatter-charts of the length to height ratios of prodissoconchs ( Fig. 19), shell size plotted against depth ( Fig. 20 View Fig ) and bar charts of bathymetric ranges ( Fig. 21). All of these can be related to prodissoconch size and ultimately to Thorson’s rule. Ockelmann would also have been familiar with Rass’s rule which relates to the variation in larval development within a species affected by latitude or temperature ( Laptikhovsky 2006). Unfortunately, we only have the drawings and charts in the archive without any written commentary, so we cannot discern how Ockelmann was going to interpret his results.

The effort he made in compiling this data was impressive for the charts in Fig. 21 were compiled from 21 252 individual observations on 10 species. This data was probably compiled before 1976, as it includes T. “ frigida ” rather than T. dunbari . By contrast, the legend on his prodissoconch plate cites “ T. “ frigida ” n. sp. = T. dunbari ” so at least the legend was added after 1976. A selection of these images was sent to the author and published in Oliver & Killeen (2002). At the time we (Oliver & Killeen) were puzzled by the faint radial lines indicated on Ockelmann’s drawing of the prodissoconch of T. equalis ( Fig. 17 View Fig : no. 9) for we were unable to see any. In a forthcoming paper on NE Atlantic thyasirids, using scanning electron microscopy, I compare the prodissoconchs of a number of species and will demonstrate that T. equalis has a very distinct arrangement of raised ridges on the apex of the larval shell that I will describe as the “menorah” pattern, as they resemble to arms on the Jewish candlestick of that name. Furthermore, far from the smooth appearance inferred by Ockelmann’s drawings, the prodissoconch displays a variety of distinctive microsculptures that can be used to differentiate species ( Fig. 22 View Fig ). While Ockelmann’s drawings adequately indicate the sizes of the prodissoconchs, the details are missing, and this would have made it difficult to publish after the establishment of scanning electron microscopy.

The living animal

Ockelmann was what one terms as a “whole animal zoologist” and making observations on living animals in the laboratory was, I believe, a passion of his and is evidenced in other papers of his (e.g., Ockelmann 1964a, 1965; Ockelmann & Muus 1978). He made observations on the burrowing behaviour of at least 3 species of Thyasiridae , T. flexuosa , T. equalis and what he called M. pygmaea . The latter two were modified from rough sketches and reproduced in Oliver & Killeen (2002). The fully worked up reconstruction for T. flexuosa is reproduced below ( Fig. 23) but not in its original size that was portrait A3. This diagram shows the bivalve living at depth in the sediment with an anterior inhalant tube and the exhalant flow posteriorly into the surrounding sediment. Once buried in this position, the foot probes further into the sediment and makes a network of such tubes ( Dando & Southward 1986). I do not know if Ockelmann had an explanation for such behaviour, but he probably thought that pedal feeding was involved where particles of detrital food were brought into the mantle by the foot. It was not until Southward (1986) and Dando & Southward (1986) discovered that the foot was probing into the anoxic zone to supply its commensal chemoautotrophic bacteria with sulphides that the behaviour was truly recognised. This symbiosis of T. flexuosa with chemoautotrophic bacteria was discussed with Ockelmann by Paul Dando (pers. comm.) who recounts here: “Kurt described using India ink particles to study the flow of water through the pathways and noted that the inhalant tube flow was sometimes reversed, as noted by the dotted arrow line in Fig. 23. He also noted that the bivalve did not pump water from above and from below at the same time. This then made sense, since it avoided chemical oxidation of sulphide by oxygen before either reached the gills. The figure also shows that Ockelmann appreciated the oxidation of the sediment by water drawn down through the inhalant tube.” Eve Southward (1930– 2023) ( Dando et al. 2024), on 10 June 1986, wrote to Ockelmann asking for his comments on her interpretation of the burrowing behaviour, and urging him to publish his observations and to join a European consortium to apply for funds to further their research. There is no reply in Eve Southwards correspondence file, but Ockelmann did not publish his observations or join a research consortium.

This part of his studies included collecting and observing the larvae, their settlement and early life history. Paul Dando also recalls discussing such matters, including Ockelmann’s observations that the larvae of T. flexuosa and T. sarsii (Philippi, 1845) ( Philippi 1845b) were bentho-pelagic and that on settlement, the larvae chose organic-rich areas such as sunken mats of Zostera leaves. Ockelmann observed that, in his laboratory tanks, the young bivalves lived at the surface of the sediment for about a year before burrowing and taking up the life position seen in the above diagram. Paul Dando also recalls Kurt explaining to him how he studied the ciliary currents entering and exiting the mantle cavity by jamming open the valves so that he could see inside. Kathe Jensen (pers. comm.) also recalls Kurt’s enthusiasm for these observations in 1972. This study was aided by Ockelmann’s invention of a detritus sledge for collecting meiobenthos and newly settled larvae ( Ockelmann 1964b).

Ciliary currents

A routine part of the description of the function of a bivalve is to record the ciliary currents on the mantle and gill as pioneered by Ridewood (1903) and later by Atkins (e.g., 1937). Two diagrams ( Fig. 24A–B) illustrate ciliary currents in the mantle of what appears to be Thyasira flexuosa . The mantle ciliary currents are shown in great detail and would have taken very careful observation to record. This diagram is instructive as it indicates mantle apertures both fixed by tissue fusion and by ciliary junctions. The presence of two posterior apertures has been noted in some species by Payne & Allen (1991) and Zelaya (2009); the former suggested the second (ex a2 here) was a second inhalant aperture while the latter only recorded their presence. Ockelmann’s diagram clearly shows a small current exiting at ex a2 and I suggest this is expelling pseudofaeces. It also shows that only the upper exhalant aperture is created by mantle fusion (solid black line) while the other apertures are created by weaker ciliary junctions that most frequently are not preserved in preserved specimens.

Figure 24C shows glandular areas on the mantle in four species and that their extent is different in the four illustrated. The extent of the glandular areas has been shown to be a useful taxonomic character and was cited by Oliver & Killeen (2002).

Gross anatomy

One assumes that in preparation of the monograph that as each shell was illustrated, so too was the gross anatomy, but only six anatomical diagrams are present in the archive. These are a mixture of stipple drawing and ink, are morphologically exact but stylised interpretations that reflect Ockelmann’s knowledge of the anatomy and mastery of the pen ( Fig. 25).

Other shell drawings

• Thyasira tortuosa ( Jeffreys, 1881) View in CoL

Within the archive, besides the typical stipple drawings, is a rough sketch ( Fig. 26C) of the anatomy of T. tortuosa View in CoL made from a dried specimen from the French Travailleur expedition of 1880, dredge 10. It might be assumed that Ockelmann was sent this material, for there is no evidence that he visited the Paris museum where the bulk of this collection is kept. However, there are 2 lots in the Smithsonian, USNM 61983/61984, which are ex Travailleur in Jeffreys collection and it is likely it was one of these that he drew during his visit in 1959. Lot number USNM 61984 is listed as a lectotype, selected by Ockelmann, in the Smithsonian catalogue, but this was never formally published. By chance, one of these (USNM 61983) was illustrated by Payne & Allen (1991: fig. 45). Ockelmann indicates that the prodissoconch is sculptured and indeed it has a “menorah” pattern ( Fig. 26E) as seen in T. equalis View in CoL ( Fig. 22A View Fig ); this was not mentioned by Payne & Allen (1995). Also not mentioned by Payne & Allen (1995) and not commented upon by myself is Ockelmann’s observation that the uppermost part of the anterior adductor is separate from the remainder. It is indicated here as a “separate portion of ant add” and he also indicated in his anatomical diagram of T. dunbari View in CoL ( Fig. 25B), where it is annotated as “SP”.

• Thyasira ( Parathyasira) granulosa ( Monterosato, 1874) View in CoL ( Fig. 27A–C)

Here, Ockelmann illustrated the typical radially arranged microspicules on the shell surface ( Fig. 27B). These spicules can be seen on the supposed lectotype and paralectotype of Thyasira flexuosa var. rotunda View in CoL in the Smithsonian, USNM 61942 and 61942a. This selection was never formally published and is now superceded by the presence of type material from the type locality in the NHMUK.

• Thyasira succisa ( Jeffreys, 1876) View in CoL ( Fig. 27D–E)

• Axinopsida orbiculata (G.O. Sars, 1878) View in CoL ( Fig. 27F–G)

• Thyasira ( Axinulus) brevis ( Verrill & Bush, 1898) View in CoL ( Fig. 28A–B)

• Thyasira ( Axinulus) croulinensis ( Jeffreys, 1847) View in CoL ( Fig. 28C–D)

• Thyasira ( Genaxinus) eumyaria (M. Sars, 1870) View in CoL ( Fig. 28E)

• Mendicula ferruginosa ( Forbes, 1844) View in CoL ( Fig. 28F)

• Thyasira flexuosa ( Montagu, 1803) View in CoL ( Fig. 29A–C)

• Thyasira sarsii ( Philippi, 1845b) View in CoL ( Fig. 29D–E)

• Channelaxinus perplicatus ( Salas, 1996) View in CoL

The illustrated shell ( Fig. 30A View Fig ) is not identified, but is a typical example of what was known as Thyasira plicata ( Verrill, 1885) View in CoL but is now Channelaxinus perplicatus View in CoL .

• Axinus grandis (Verrill & Smith, 1885) The stipple drawing of the hinge of A. grandis is labelled “ holotype ” and is probably one of the shells in the Smithsonian under USNM 44824. There is a little variance in the literature, as Payne & Allen (1991) cite the holotype as being in the MCZ Harvard, the station data is the same for both USNM and MCZ shells.