Chilostomellidae: Deep Forams

Holotype of Chilostomella serrata, from the Smithsonian National Museum of Natural History.

The specimen in the figure above is a fairly typical representative of the Chilostomellidae, a cosmopolitan family of forams known from the Jurassic to the present day. Members of this family have a translucent calcareous test with chambers arranged in a trochospiral (broad conical) or planispiral (flat spiral) pattern. The chambers of each spiral are expanded to cover over the prior spirals so only the outermost spiral is generally visible. The aperture of the test in the final chamber is a narrow slit along the margin with the underlying chamber (Loeblich & Tappan 1964).

Despite their long history and wide distribution, I get the general impression that chilostomellids are not usually abundant. They are generally restricted to deeper waters, more than 100 m below the surface (Cushman et al. 1954). Members of the genus Chilostomella, at least, have commonly been regarded as associated with low-oxygen environments. However, it has also been suggested that their favoured conditions are not so much a question of low oxygen as high organic flux (Jorissen 2002). Perhaps the best location to find chilostomellids would be around sites where dead animals and seaweeds have fallen to the deeper waters below.

REFERENCES

Cushman, J. A., R. Todd & R. J. Post. 1954. Recent Foraminifera of the Marshall Islands. Bikini and nearby atolls, part 2, oceanography (biologic). Geological Survey Professional Paper 260-H: 319–384, pls 82–93.

Jorissen, F. J. 2002. Benthic foraminiferal microhabitats below the sediment-water interface. In: Sen Gupta, B. K. (ed.) Modern Foraminifera pp. 161–179. Kluwer Academic Publishers: Dordrecht.

Loeblich, A. R., Jr & H. Tappan. 1964. Treatise on Invertebrate Paleontology pt C. Protista 2. Sarcodina: chiefly "thecamoebians" and Foraminiferida vol. 2. The Geological Society of America, and The University of Kansas Press.

Digging for Tellina

When I was a child, a large part of my extended family would gather over the Christmas period to park their tents and caravans alongside the estuary downhill from my great-grandparents' house (in the usual way of these things, my memory has these summer camping periods lasting for ages, but I don't think they could have been longer than a week or so). While we were there, I would spend a fair chunk of the day looking for the wildlife that inhabited the slightly muddy estuary beach. Among these were various bivalves whose shells could be found littering the shoreline, or which might be found by digging in the sand at low tide. Close to the surface were New Zealand cockles Austrovenus stutchburyi (not actually a direct relative of the English cockle but a member of the Veneridae family that has adopted a similar body form). A little deeper were pipis and tuatuas. And a little deeper again were the flat, slender shells of Tellina.

Thin tellin Tellina tenuis, copyright S. Rae.

I should note that Tellina species are not really deep burrowers in the grand scheme of things, generally only embedding themselves about one to three centimetres below the surface, but again I must ask that you make allowances for childhood memories. Their low profile and weakly inflated shells also make them fast diggers so they were probably able to elude most casual explorations. Like most subsurface bivalves, Tellina species are sediment feeders. Their usual aspect is lying horizontally beneath the sediment, extending their long, unfused siphons to the surface to gather detritus (Ujino & Matuskuma 2010; the shells of Tellina are usually twisted slightly to one side at the end to facilitate the siphons' passage). Even if you've not seen the Tellina animals themselves, you may have seen the radiating trails made by the siphons as they extend along the top of the sediment.

Sunrise tellin Tellina radiata, copyright James St. John.

Tellina is an extremely diverse genus, with species found worldwide and recognised through the entirety of the Mesozoic (Moore 1969). These species vary greatly in appearance, with shells varying from almost completely smooth to strongly ornamented, and from subcircular to quite elongate. It should therefore come as little surprise that numerous attempts have been made to divide Tellina between various subgenera and genera but issues such as homoeomorphy in Tellina's evolution (where distinct lineages have converged on similar body forms) have lead to disagreement over the best system to adopt. In 1934, the malacologist A. E. Salisbury complained that, "The number of genera, subgenera, and sections into which the Tellinidae has been cut up is getting somewhat appalling; the list of names is still increasing every year, and, if every variation of form is magnified, it is quite possible to go on until at last each species becomes the representative of a different genus and each variety that of a subgenus" (of course, as seems almost inevitable when one encounters complaints of this kind, Salisbury himself then proceeds to add to the tally of generic names in that same paper). Though I suspect most modern malacologists would probably disagree with the extremely broad concept of Tellina advocated by Salisbury, the question of how best to handle the genus taxonomically remains an open one.

REFERENCES

Moore, R. C. (ed.) 1969. Treatise on Invertebrate Paleontology pt N. Mollusca 6. Bivalvia vol. 2. The Geological Society of America, Inc., and The University of Kansas.

Salisbury, A. E. 1934. On the nomenclature of Tellinidae, with descriptions of new species and some remarks on distribution. Proceedings of the Malacological Society of London 21: 74–91.

Ujino, S., & A. Matsukuma. 2010. Inverse life positions of three species in the genus Cadella (Bivalvia: Tellinidae). Molluscan Research 30 (1): 25–28.

Agenioideus: Average Spider Hawks

I have commented in earlier posts on the challenges of identifying spider hawks of the family Pompilidae, resulting from this wasp family's combination of high species diversity with a mostly conservative body plan. As a result of this conservatism, pompilid classification has tended to drift towards a situation where the majority of species are included in a relatively small number of somewhat vaguely defined genera. Each of the species included in one of these genera can be associated with other species in the genus, and groups of species approach each other closely enough that clear lines cannot be settled upon, but identifying features shared by all members of the genus can prove difficult. A good example of one such genus is Agenioideus.

Female Agenioideus birkmanni, from the University of Texas at Austin.

Species assigned to Agenioideus can be found pretty much worldwide though the greatest diversity occurs in warmer parts of the Holarctic. Though there does not seem to be a great deal of disagreement over which species should be placed in this genus, it seems a little difficult to say exactly what makes an Agenioideus. If anything, Agenioideus species seem to be associated by how relentlessly average they are, possessing a unique combination of characters that are none of them individually unique. They have wings with three submarginal cells, a broad metapostnotum in front of the propodeum, and legs ending in a small arolium with a weak comb of setae between a pair of long claws, mostly with a single small ventral tooth (Krogmann & Austin 2012). If you don't know exactly what those terms mean, just know that they are all quite unspecialised features for pompilids. Males often have asymmetrical claws on the forelegs, with the inner claw strongly bent and bifid while the outer claw is like those on the other legs, and the pterostigma (the dark node at the front of the fore wings) is relatively large compared to other genera. Females often have a comb of longer spines on the inner margin of the fore tarsi. But these last, more derived, features may not be universally present across all species of the genus.

Female Agenioideus nigricornis with redback spider Latrodectus hasselti as prey, copyright Mark Newton.

As befits their unspecialised appearance, most Agenioideus species (as far as we know) are relatively unspecialised in their nesting behaviour (Shimizu 1997). Like other pompilids, they lay their eggs on paralysed spiders that will provide food for the larva when it hatches. Most Agenioideus species construct simple nests with a single brood cell containing a single spider for each nest. One European species, A. nubecula, is known to produce slightly more extensive nests with up to four cells. The nest may be made by digging in loose soil or by using a pre-existing cavity; whether the wasp is more likely to do one or the other is correlated with whether she possesses a well-developed tarsal comb. A Japanese species, A. ishikawai, is known to at least partially dig a nest before capturing a spider, completing construction after bringing it back. The most specialised provisioning behaviour known for the genus, however, is found in another European species, A. coronatus. This species hunts jumping spiders which she paralyses with her sting as is standard. The paralysis, however, is only temporary, lasting just a few minutes, just long enough for the female to deposit an egg near the base of the spider's abdomen where it cannot easily remove it. The spider is then freed to go about its business without being placed in a nest, until the wasp larva hatches and feeds on its host in the manner of a parasitoid.

REFERENCES

Krogmann, L., & A. D. Austin. 2012. Systematics of Australian Agenioideus Ashmead (Hymenoptera: Pompilidae) with the first record of a spider wasp parasitizing Latrodectus hasselti Thorell (redback spider). Australian Journal of Entomology 51: 166–174.

Shimizu, A. 1997. Taxonomic studies on the Pompilidae occurring in Japan north of the Ryukyus: the genus Agenioideus Ashmead (Hymenoptera). Japanese Journal of Entomology 65 (1): 143–167.